What are perennial alyssum, sweet alyssum, and Iberis?

Alyssum is a general term for plants belonging to the genus Alyssum, and the most common species cultivated in horticulture are Alyssum montanum and Alyssum wulfenianum.

Perennial alyssum is a general term for a group of plants closely related to the genus Aurinia, whose roots survive the winter and which are perennial. In addition to the aforementioned genus Aurinia, it also includes Aurinia saxatilis, which was formerly included in the genus Aurinia but has been separated into the genus Aurinia.

Lobularia maritima, also known as sweet alyssum, is often simply called alyssum. Previously classified under the genus Lobularia, it has now been separated and is classified under the genus Lobularia. Native to the Mediterranean coast of Europe and North Africa, it is an annual or subshrub cultivated worldwide for ornamental purposes (RBG Kew, 2026). While primarily cultivated for ornamental purposes in Japan, it is also used as food globally.

Iberis sempervirens, also known as Tokiwa-magaribana, Tokiwa-nazuna, Iberis, or Iberis sempervirens, is commonly known as Tokiwa-magaribana in Japanese, although some sources refer to it as Tokiwa-ro-magaribana (Fuchida et al., 2014). Native to the Mediterranean coast of Europe (Spain, France, Italy, Romania, and the Balkan Peninsula), the Middle East (Turkey and Syria), and North Africa (Morocco and Algeria), it is an evergreen subshrub cultivated in Japan for ornamental purposes (RBG Kew, 2026).

All of these plants belong to the Brassicaceae family. Their leaves creep and spread horizontally, and while they have the characteristic cruciate flowers of the Brassicaceae family, they consist of four petals that are considerably smaller than typical Brassicaceae flowers. They are cute and hardy, so they are frequently cultivated as ornamental plants.

However, there are many closely related species and terms, and from a classification standpoint, the situation is complex, with academic definitions and common horticultural names intertwined.

When you say "Alyssum," there are three possible interpretations!?

If you simply refer to it as "Alyssum," there are three possible patterns.

• This is a general term for the genus Alyssum, which are scientifically recognized as alyssum species, and refers specifically to species such as Alyssum montanum and Alyssum wulfenianum.
• This is an abbreviation for sweet alyssum, and refers to Lobularia maritima.
• This is an abbreviation for perennial alyssum, and refers to a general term that includes not only the genus Aurinia but also species such as Aurinia saxatilis.

In actual conversations in Japan, "allyssum" almost always refers to sweet alyssum. However, since there is already another plant called alyssum, I think it's necessary to distinguish it by calling it "garden alyssum" or at least "sweet alyssum."

What are the differences between perennial alyssum, sweet alyssum, and Iberis?

Distinguishing between perennial alyssum, sweet alyssum, and Iberis requires observing various characteristics, but sweet alyssum and Iberis have very distinctive flowers, so they can be easily distinguished if you can see the flowers.

What are the differences between perennial alyssum (sweet alyssum) and Iberis?

Distinguishing between perennial alyssum (sweet alyssum) and Iberis (evergreen barringtonia) is easy.

Regarding the flowers, perennial alyssum and sweet alyssum have four petals of a cruciform flower that are all almost the same shape, whereas in Iberis, the two inner petals are small, while the two outer petals are large and protrude significantly.

Regarding the leaves, perennial alyssum and sweet alyssum have soft, bright green leaves because they wither in winter, while Iberis leaves are dark green and tough because they are evergreen.

What is the difference between perennial alyssum and sweet alyssum?

Distinguishing between perennial alyssum and sweet alyssum is also relatively easy.

Regarding their life cycle, perennial alyssum is a perennial plant whose above-ground parts die back in winter but whose roots survive, allowing it to live on into the following years. In contrast, the original sweet alyssum species is not very cold-hardy and may only grow as an annual. However, the sweet alyssum cultivated recently is a cultivated variety called "super alyssum," which is a perennial that retains its leaves and blooms even in winter.

Regarding the flowers, perennial alyssum almost always has yellow, relatively large petals, while sweet alyssum has smaller petals in a variety of colors, including white, red, purple, and orange.

Regarding the inflorescence, perennial alyssum has flowers that bloom sparsely, while sweet alyssum has flowers that bloom densely.

Therefore, a major characteristic of sweet alyssum is that, combined with its smaller petals, the entire inflorescence is hemispherical in shape.

Regarding the leaves, perennial alyssum has spatulate leaf blades with rounded tips, while sweet alyssum has lanceolate leaf blades with pointed tips.

What is the difference between true alyssum and perennial alyssum?

Perennial alyssum includes true alyssum, which is scientifically classified under the genus Alyssum, and Aurinia saxatilis, which was formerly classified under the genus Alyssum but has been separated into the genus Aurinia.

In practical terms, true alyssum does not grow very upright and does not have stellate hairs (hairs that look like stars) on its leaves, whereas Aurinia saxatiris grows slightly upright and has stellate hairs on its leaves.

From a taxonomic standpoint, true alyssum species in the genus Alyssum have one or two ovules per ovary, while those in the genus Alyssum have 4 to 40 ovules per ovary. However, confirming this difference would be difficult for the average person (Flora of North America Editorial Committee, 2010).

What is the difference between Alyssum montanam and Alyssum wulfenianum, both of which are true alyssum species?

True alyssum includes several other varieties.

However, in horticulture, Alyssum montanum and Alyssum wulfenianum are the two most representative species, and the cultivar 'Golden Spring' of Alyssum wulfenianum is the most common.

However, I couldn't find any information on the internet, even internationally, that showed the difference between Alyssum montanam and Alyssum wulfenianum.

An old German atlas shows the difference (Röhling et al., 1833).

According to the study, although they are very similar, Alyssum montanum is densely covered with stellate hairs all over the plant body, whereas Alyssum wulfenianum has sparse stellate hairs all over the plant body, and may even be hairless.

From what I can see in the photos, stellate hairs can also be found in Alyssum wulfenianum, so caution is advised.

Are there any other similar species?

Although there are many closely related species around the world, I believe that the species listed here cover almost all of the species found in Japan.

Although shepherd's purse is the species from which the Japanese name "Niwanazuna" (garden shepherd's purse) originates, they hardly resemble each other.

What are the differences between Shepherd's Purse, Lepidium ternatum, Thaliana japonica, and Arabidopsis thaliana? An explanation of how to distinguish between similar species – Ecological Notes Web

Shepherd's purse, Bean-leaf lacecap, Dog's purse, and Arabidopsis thaliana all belong to the Brassicaceae family, and among them, the one with the name "shepherd's purse" is a representative weed of Japan. It is characterized by flowering in early spring and quickly producing flat fruits. However, it is not well known.

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References

Flora of North America Editorial Committee. (2010). Flora of North America (Vol. 7 Magnoliophyta: Salicaceae to Brassicaceae). Oxford University Press. ISBN: 9780195318227

Fuchida, S., Yamamoto, S., Sawada, Y., Fujiwara, M., Oyabu, T., & Nose, K. (2014). Special Feature: "Invasive Species and Vegetation Management" - Creation of a Name Correspondence Table between Invasive Species and Horticultural Plants. Journal of the Japanese Society of Landscape Architecture, 40(2), 352-364. https://doi.org/10.7211/jjsrt.40.352

RBG Kew. (2026). The International Plant Names Index and World Checklist of Vascular Plants. Plants of the World Online. http://www.ipni.org and https://powo.science.kew.org/

Röhling, JC, Mertens, FCK, & WDJ (1833). Deutschlands Flora (Vol. 4). Wilmans. ISBN: 9781332453641, https://www.google.co.jp/books/edition/Deutschlands_Flora/LUg-AAAAcAAJ

What are Kakinegarashi and Inukakinegarashi?

Sisymbrium officinale, also known as hedge mustard, is native to Europe and became naturalized throughout Japan in the late Meiji period. It is an annual to biennial plant that grows in sunny roadsides and wastelands.

Sisymbrium orientale, also known as dog fence mustard, is native to the Mediterranean coast and became naturalized throughout Japan in the early Showa period. It is an annual plant that grows along roadsides in urban areas and in wastelands.

Both belong to the genus Arabis in the family Brassicaceae, and are introduced weeds that can grow even in urban areas with low levels of natural environment, such as cracks in concrete. The Japanese name is said to come from the fact that when the branches spread horizontally and they grow in clusters, they look like a hedge.

At first glance, the flowers, which are characteristic of the Brassicaceae family and are called "cruciate flowers," are yellow and resemble those of common Brassica napus or mustard greens. However, they can be distinguished by the fact that their stems and leaves do not clasp the stem, and their leaf blades are divided.

What are the differences between mustard greens, rapeseed, European rapeseed, and black mustard? How do their uses differ? Why did they evolve to be spicy? Brassica plants, essential for Japanese mustard and salad oil, have increased fruit production thanks to insect visits!

Brassica napus, rapeseed, European rapeseed, and black mustard are four species of yellow flowers that bloom in spring and belong to the Brassicaceae family. Their flower shapes are almost identical, and their habitats are also nearly the same. Furthermore, there are many closely related species, making identification extremely confusing. This complexity is such that even researchers sometimes misidentify them. Recent research has revealed...

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From a taxonomic standpoint, the difference lies in the fact that the tip of the silique (a slender fruit characteristic of the Brassicaceae family) is not covered by a beak, but it is quite difficult to confirm this point.

Because they share so many similarities, you might find it difficult to distinguish between *Calanthe discolor* and *Calanthe japonica*, which belong to the same genus.

What is the difference between Kakinegashi and Inukakinegashi?

Distinguishing between *Calanthe discolor* and *Calanthe japonica* is relatively easy with careful observation.

The most reliable way to distinguish them is to look at the fruit (silique). In *Cassia japonica*, the fruit stalk is very short, and the fruit is less than 15 mm long and pressed against the stem (parallel to the stem), whereas in *Cassia japonica*, the fruit stalk is clearly present, and the fruit is more than 5 cm long and separates from the stem (forming an acute angle with the stem).

Also, the petals of *Inukakinegarashi* are larger than those of *Kakinegara*.

There is no reliable way to distinguish between species due to the large individual variations in the shape of divided leaves, but in *Rorippa indica*, the base of the leaf blade of the apical stem leaves tends to protrude in a lanceolate shape, so this point may be helpful.

Are there any other similar species?

Several other species of the genus Arabis have also become naturalized, although they are rarer than the two species mentioned above.

Sisymbrium irio is a perennial herb whose stems and leaves are usually undivided.

Sisymbrium altissimum resembles Sisymbrium moniliforme, but unlike Sisymbrium moniliforme, the base of the leaf blades of the apical stem leaves does not protrude in a lance-like shape, and its sepals are hairless.

Sisymbrium irio exhibits a unique characteristic: after flowering, its young fruits rapidly elongate, extending beyond the flower at the top and surrounding it.

References

Kanagawa Prefecture Flora Survey Association. 2018. Kanagawa Prefecture Flora 2018 (Electronic Edition). Kanagawa Prefecture Flora Survey Association, Odawara. 1803pp. ISBN: 9784991053726

What are sea radish, flower radish (flower radish), and large-flowered radish (large-flowered radish)?

The beach radish , Raphanus sativus var. hortensis f. raphanistroides, is distributed throughout Japan and is a biennial plant that grows on coastlines and riverbanks (Kanagawa Prefecture Flora Survey Association, 2018). There are several theories about its origin; one is that cultivated radish, Raphanus sativus var. hortensis, escaped (ran away) and became naturalized. However, in recent years, the theory that wild populations that were distributed across the Eurasian continent in an earlier period (after the Yayoi period) arrived in Japan along with grains and became a population that has fully adapted to the wild environment is gaining traction (Fujita, 2004; Ban et al., 2009). Unlike the radish, the root is not commonly used except in the San'in region, and the immature stems, leaves, and flower spikes are sometimes eaten (Ban et al., 2009).

Hesperis matronalis, also known as flower radish, Western flower radish, or sweet rocket, is a biennial plant (though it cannot survive winter in Japan, so it is considered an annual) distributed along the Mediterranean coast of Europe, the Caucasus, and Turkey. It is cultivated worldwide, including in Japan, for ornamental purposes and for its edible leaves (herb), and has escaped cultivation in disturbed areas. In Japan, it has escaped cultivation in various locations, and has become naturalized, especially in Hokkaido.

Orychophragmus violaceus, also known as rhododendron, large purple radish, broad-leaved radish, or purple flowering radish, is an annual to biennial plant native to China that was introduced to Japan during the Edo period and has since become widely cultivated and naturalized (Kanagawa Prefecture Flora Survey Association, 2018). While primarily used for ornamental purposes, its young leaves and flower buds are edible, and it is cultivated as a vegetable in China. Oil can also be extracted from its seeds, similar to rapeseed, and it was cultivated for this purpose after World War II. The name "Zhuge Cai" (諸葛菜) is interesting, and is said to originate from a legend that Zhuge Liang, the military strategist of the Shu Han kingdom during the Three Kingdoms period of China, sowed the seeds of this flower first when he set up camp.

All of these belong to the Brassicaceae family, and their names sometimes include "radish." They are herbaceous plants that are cultivated for either ornamental or edible purposes, and have also become naturalized in the wild.

Not only are the flowers of the Brassicaceae family arranged in a cross shape with four petals in pairs, a characteristic feature of the family, but the flowers are also purple in color, and the leaves are soft and have serrated edges, making them very similar in morphology.

Furthermore, the term "hanadaikon" can refer to either Hesperis matronalis or Orychophragmus violaceus, which can be quite confusing.

Therefore, you may often find yourself unsure of what to do.

Since "Hanadaikon" is ambiguous, we will use the terms Hamadaikon, Hanasuzushiro, and Shokatsusai consistently in the following explanation, following the most reliable list of correspondences between scientific and Japanese names , 'Ylist'.

What are the differences between sea radish, flower radish, and Chinese cabbage?

From the perspective of their uses, sea radish is only found as a wild species on coastlines and riverbanks, whereas wild radishes and Chinese cabbage, which are cultivated for ornamental or edible purposes, have become naturalized.

Therefore, the situations in which they meet will likely differ.

Let's also consider the morphological differences.

As a fundamental point, these three species belong to completely different genera, as can be seen from their scientific names. Therefore, there are significant differences in their body structures.

The biggest difference is evident in the fruit. All three species belong to the Brassicaceae family, so their fruits are siliques, which are divided into two pericarps called "valves" (developed carpels) and seeds, with multiple seeds contained within each valve.

In wild radish, the siliques are distinctly thick, do not split open when ripe, and contain large seeds, usually fewer than four. In contrast, in Japanese radish and Chinese cabbage, the siliques are distinctly thin and linear, split open when ripe, and contain numerous seeds.

Wild radish (Raphanus sativus) spreads its habitat through "water current dispersal," where its siliques float on water and are carried away by the current (Han et al., 2015). This is why it grows on riverbanks and coastlines. On the other hand, Japanese radish (Raphanus sativus) and Chinese cabbage (Raphanus scoparius) disperse through "automatic dispersal," where their siliques split open.

Regarding *Hypochaeris radicata* and *Chinese cabbage*, the difference is that *Hypochaeris radicata* has four thick ridges (pointed corners) on its silique, while *Chinese cabbage* has no ridges and is rounded.

However, you might occasionally find individuals like the three species during periods when there are no fruits. In those cases, the only option is to examine the leaves and stems.

In the Brassicaceae family, the basal leaves (leaves that grow from the root) and stem leaves (leaves that grow from the stem) often have different shapes. Regarding the stem leaves, in wild radish, the leaf tips are rounded, while in Japanese radish and Chinese cabbage, the leaf tips are pointed like a tail.

Furthermore, the stems and leaves of *Hanasuzushiro* are hairy, while those of *Shokatsusai* are hairless.

The flower colors vary; in sea radish, the tips of the petals are partially purple; in daisy-like flowers, they are uniformly purple or white; and in Chinese radish, they are mostly pale purple with a white base. However, there is considerable individual variation, so this should only be used as a general guide.

References

Takuya Ban, Nobuo Kobayashi, Hiroshi Motoya, Masayuki Kadowaki, and Shingo Matsumoto. 2009. On the cultivation and utilization of wild radish. Japanese Journal of Horticultural Science 8(4): 413-417. https://doi.org/10.2503/hrj.8.413

Fujita, Satoshi. 2004. Cultural History of Keisen Vegetables (1) Radish. Bulletin of the Institute for Horticultural Culture, Keisen Women's University. Horticultural Culture 1: 54-60. ISSN: 1882-5044, https://keisen.repo.nii.ac.jp/records/733

Han, Q., Higashi, H., Mitsui, Y., & Setoguchi, H. 2015. Distinct phylogeographic structures of wild radish (Raphanus sativus L. var. raphanistroides Makino) in Japan. PloS One 10(8): e0135132. https://doi.org/10.1371/journal.pone.0135132

Kanagawa Prefecture Flora Survey Association. 2018. Kanagawa Prefecture Flora 2018 (Electronic Edition). Kanagawa Prefecture Flora Survey Association, Odawara. 1803pp. ISBN: 9784991053726

What are Shepherd's Purse, Bean-Eating Shepherd's Purse, Dog Shepherd's Purse, and White Shepherd's Purse?

Shepherd's purse (Capsella bursa-pastoris var. triangularis), also known as penpengusa or shamisengusa, is a biennial plant widely distributed throughout Japan (Hokkaido, Honshu, Shikoku, Kyushu, and Ryukyu Islands) and the Northern Hemisphere, growing along roadsides and in fields.

Lepidium virginicum, also known as bean-legged shepherd's purse, is a biennial plant native to North America that grows in wastelands and roadsides. It was introduced to Japan during the Meiji era and is now found throughout the country.

Draba nemorosa, also known as dog's toad lily, is a biennial herb found in roadsides and fields, distributed throughout Hokkaido, Honshu, Shikoku, and Kyushu in Japan, as well as the Northern Hemisphere (Eurasia excluding North America, Southeast Asia, South Asia, and the Arabian Peninsula). It does not grow in urban areas.

Arabidopsis thaliana, also known as white dog thaliana, is a biennial herb distributed across a wide area of Japan (Hokkaido, Honshu, Shikoku, and Kyushu), Eurasia, and Africa, growing in coastal and lowland grasslands and roadsides.

Both belong to the Brassicaceae family, and among them, the plant known as "nazuna" is a representative weed of Japan. It is characterized by its early flowering in early spring and its early production of flat fruits.

With shepherd's purse, you pull the fruit stalk (the thin part attached to the fruit) downwards, letting the fruit hang down, and then rotate the central axis (the base of all the fruit stalks) with your fingers to make a "pen-pen" sound—a game that almost every Japanese person has probably played at least once. It is one of the seven spring herbs, and has been used in seven-herb rice porridge since ancient times, with the young, rosette-shaped seedlings, which do not have upright stems, being eaten.

Arabidopsis thaliana possesses many characteristics that make it an excellent model organism, such as its small genome size, short generation time of about two months, ease of cultivation indoors, ability to produce a large number of seeds, lack of self-incompatibility, and ease of transformation. It is widely used in genetics research, including population genetics, and in plant science research related to plant evolution and development, and is always featured in textbooks at the high school level and above.

However, despite their fame, some people may not be able to distinguish between these four species. In particular, shepherd's purse, lacecap shepherd's purse, and Arabidopsis thaliana can grow even in urban areas, making identification difficult.

What are the differences between Shepherd's Purse, Bean-Leaf Shepherd's Purse, Dog Shepherd's Purse, and White Shepherd's Purse?

The easiest way to distinguish between these four species is to examine their fruits (Kanagawa Prefecture Flora Survey Association, 2018).

All four species belong to the Brassicaceae family, and therefore produce a special type of fruit called a "silique," which has two carpels and consists of two chambers. When ripe, it splits open lengthwise, releasing the seeds inside.

The shape of the siliques is clearly different in all four species.

Specifically, the differences are that shepherd's purse is heart-shaped, lacecap shepherd's purse is round, dog shepherd's purse is a slender, elongated oval, and Arabidopsis thaliana is a slender, rod-shaped.

You won't mistake these differences, so you shouldn't get confused.

Another point of reference is that while the flowers of shepherd's purse, lacecap shepherd's purse, and Arabidopsis thaliana are white, the flowers of dog shepherd's purse are yellow.

Also, among these, the shepherd's purse is clearly the largest. The dog shepherd's purse is not usually found in urban areas.

From a taxonomic standpoint, they belong to the genera *Capsella*, *Capsella*, *Capsella thaliana*, and *Arabidopsis*, respectively. Although their names are similar, you can see that there are significant differences between them.

Are there any other similar species?

The four species listed above are representative of the genera *Capsella*, *Capsella*, *Capsella bursa-pastoris*, and *Arabidopsis thaliana*, respectively. However, there are similar species within each genera, so caution is sometimes necessary.

Other known species in the genus Capsella include Capsella bursa-pastoris var. bursa-pastoris, in which the length of the fruit along its long axis is clearly longer than its width; Capsella grandiflora, in which the petals exceed 5 mm in length and are more than twice as long as the sepals; and Capsella rubella, in which the petals are slightly longer than or almost the same length as the sepals, and the outer edge of the fruit is slightly curved inward.

Nine species of the genus *Philadelphia* have naturalized in Japan, and while it's impossible to list them all here, their numbers are significantly smaller than *Philadelphia japonica*. Generally, if the fruit is nearly perfectly round and the leaves are not pinnately divided, you can assume it is *Philadelphia japonica* itself.

The genus Draba includes a species called Draba verna, whose petals are not two-lobed. Draba nambuensis, a high-altitude plant, is sometimes cultivated as a wild mountain plant.

There are only three species of the Arabidopsis genus in Japan, and the other two are alpine plants, so you are unlikely to mistake them in the wild.

References

Kanagawa Prefecture Flora Survey Association. 2018. Kanagawa Prefecture Flora 2018 (Electronic Edition). Kanagawa Prefecture Flora Survey Association, Odawara. 1803pp. ISBN: 9784991053726

Four species of yellow Brassicaceae flowers that bloom in spring, even researchers sometimes confuse.

Brassica juncea, also known as mustard greens, is an annual or biennial plant (Tsuda et al., 2016). Because wild Brassica juncea species still grow in the Middle East, it is widely believed to originate from West Asia. Currently, it is widely distributed across Asia, Europe, North Africa, Australia, and North and South America, found in coastal lowlands, sandy beaches, plateaus, and mountainous areas at altitudes of 1,150m or higher, and grows wild along sandy roadsides, in wastelands, and in fields. In Japan, it was introduced before the Heian period and became naturalized after the Meiji era, distributing across Hokkaido, Honshu, Shikoku, Kyushu, and the Ryukyu Islands (extent unknown), forming large colonies in some areas, such as on embankments. The wild type that naturalized after the Meiji era is sometimes called European mustard greens, but the scientific name remains unchanged, and unless specific differences are discovered, this is considered inappropriate.

Brassica rapa var. oleifera is an annual or biennial plant (Tsuda et al., 2016). Native to Europe, it is now widely distributed in temperate regions of North Africa, Asia, Oceania, and North America. It was introduced to Japan as a vegetable during the Nara period and has naturalized, growing along roadsides, cultivated fields, docks, in urban areas, wastelands, and riverbanks. In Japan, it is said to be distributed in parts of Honshu and Kyushu. However, its actual distribution is often confused with rapeseed and is not well understood. In Japan, it is said to grow along roadsides, in grasslands, and on embankments, but recent research suggests that it is more common in riverine environments (Nakayama et al., 2022).

Brassica napus, also known as European rapeseed, is an annual or biennial plant (Tsuda et al., 2016). Native to Europe, it is now widely distributed in temperate regions of Europe, Asia, Africa, Oceania, and North and South America, growing wild along riverbanks and railway lines. In Japan, it is said to be distributed in Hokkaido, Honshu, Shikoku, and Kyushu. Recent research suggests it is common along roads (Nakayama et al., 2022). It was introduced from Europe in the early Meiji period and cultivated for the extraction of rapeseed oil from its seeds.

Black mustard (Brassica nigra) is an annual plant. It is native to Europe and West Asia and grows in barren areas. It has also become naturalized in Japan.

All of these are yellow flowers of the Brassica genus in the Brassicaceae family that bloom in spring, and their flower shapes are almost identical. Their habitats are also nearly the same. Moreover, there are many closely related species, making identification extremely confusing. The complexity is such that even researchers sometimes misidentify them (Nakayama et al., 2022).

What are the differences between mustard greens, rapeseed, European rapeseed, and black mustard?

However, recent research has finally revealed how to distinguish between them (Kanagawa Prefecture Flora Survey Association, 2018; Nakayama et al., 2022).

The most important point is how the stem and leaves clasp the stem; this allows us to classify them in broad terms.

In mustard and black mustard, the base of the stem and leaves narrows and does not clasp the stem, whereas in rapeseed and European rapeseed, the base of the stem and leaves clearly clasps the stem.

Furthermore, compared to rapeseed, rapeseed has a slightly to moderately wide base that clasps the stem (often earlobe-shaped) and a relatively narrow leaf blade, while rapeseed has a moderately wide base that completely clasps the stem (though sometimes it does not) and a broad leaf blade.

It is (probably) impossible to distinguish between mustard greens and black mustard at the base of the stems and leaves.

Distinguishing between mustard greens (Brassica napus) and rapeseed (Brassica rapa) requires examining different parts of the plant.

The main difference between mustard greens (Brassica juncea) and black mustard (Brassica oleracea) is that in mustard greens, the pedicels are widely separated from the pedicel (mid-axis), the entire inflorescence appears open, and the fruits are long, over 30 mm in length. In contrast, in black mustard, the pedicels are closely attached to the pedicel (mid-axis) at a narrow angle, the entire inflorescence appears close together, and the fruits are short, 12-20 mm in length. Checking the length of the fruits is the quickest way to tell them apart.

Another difference between rapeseed and European rapeseed is that in rapeseed, the leaves are often covered with a waxy substance, making the underside appear whitish (the waxy substance on the upper surface easily falls off, and the waxy substance on the underside can be removed by rubbing with your fingers), whereas in European rapeseed, there is little to no waxy substance.

I've listed some of the most common differences between Brassica napus and Brassica rapa, but there are also differences in the inflorescence, sepals, pod formation, and siliques. If you're unsure, please refer to the original paper (Nakayama et al., 2022).

It should be noted that "nanohana" (rapeseed flower) is a general term for flowers of the Brassica genus, and is also used as an alternative name for Brassica napus or Brassica rapa in particular, but this name is not appropriate in a biological context.

What are the differences in how mustard greens, rapeseed, European rapeseed, and black mustard are used?

How do human usage methods differ?

As its name suggests, mustard greens (Brassica juncea) are used for their seeds, which are used to make Japanese mustard (Oriental mustard). They are also used as vegetables and medicinal herbs, and their form varies depending on their use. Sabellica var. sabellica has round leaves with crinkled tips and is used raw as a garnish for dishes or in pickles. In Japan, takana var. integrifolia is eaten as takana pickles, which are fermented. Zha cai var. tumida has swollen, knob-like stems that are pickled and used in Chinese cuisine. Setsuriko var. multiceps resembles takana pickles and is pickled and used in Chinese cuisine. Other known varieties include ninsuka var. multisecta (silver thread mustard) and nekarashina var. napiformis.

Western mustard is produced from the seeds of the white mustard plant (Sinapis alba) and is said to be milder in spiciness than Japanese mustard.

While rapeseed seeds are sometimes used to produce rapeseed oil and canola oil, it is primarily used as a vegetable, and numerous varieties have been developed through repeated selective breeding around the world. In Japan, well-known varieties include turnip var. rapa, Chinese cabbage var. glabra 'Pe-tsai', komatsuna var. perviridis (native to Komatsugawa, Edogawa Ward, Tokyo), mizuna var. nipposinica (native to southwestern Kyoto), nozawana var. hakabura (also said to be native to Nozawa Onsen Village, Nagano Prefecture), and taisai var. chinensis (native to China). Hinona var. akana (Hino vegetable) is a traditional vegetable native to Hino Town, Shiga Prefecture, and is similar to the turnip. Kisaragina var. narinosa (Kisaragi vegetable, taisai) is a Chinese winter vegetable characterized by its dark green, thick leaves with crinkled wrinkles. Sugukina var. neosuguki (sour stem vegetable) is one of Kyoto's traditional vegetables and is eaten as fermented suguki pickles. Santosai var. pekinensis 'Dentata' (mountain cabbage) is said to originate from Shandong Province in China and is mainly cultivated in Saitama Prefecture and eaten as Santosai pickles. Akizaki rapeseed var. dichotoma has seeds that are used for oil production. Kanzaki flower var. amplexicaulis is grown for ornamental purposes.

The main uses of rapeseed are for producing rapeseed oil, canola oil (rapeseed oil made from canola seeds that do not contain erucic acid and glucosinolates), and salad oil (canola oil blended with other plant oils). Rapeseed meal, the residue left over after pressing rapeseed oil, is a high-protein animal feed comparable to soybeans and is used for cattle and pigs. Rapeseed oil is also used for biodiesel in addition to being a food source. As a vegetable, the Swedish turnip var. napobrassica is well-known.

Black mustard was once cultivated for its seeds used as a spice, and it is also used as a leafy vegetable, but there has been little variety development. Compared to other species, its use seems to be declining. The reason for this is said to be that its tall stature and easily dropped seeds make it unsuitable for mechanical harvesting, thus preventing industrialization.

Why did mustard evolve to be spicy? The effects in nature.

Brassicaceae vegetables are generally known to contain mustard oil glycosides (glucosinolates) as characteristic allelopathic substances (Tsuda et al., 2016). Allelopathy refers to the effects of fungicides, insecticides, and inhibits the growth of other plants.

Cells naturally contain a volatile substance called isothiocyanate, and when cell tissue is damaged, the enzyme myrosinase undergoes a hydrolysis reaction to produce glucosinolate.

This glucosinolate is an important chemical component in cruciferous vegetables that affects their taste for humans, but it is also known to have strong physiological activity, such as antibacterial and antiparasitic activity. In nature, it is likely used to combat parasitic fungi, bacteria, and insects.

Furthermore, among isothiocyanates, allyl isothiocyanate in particular is found in black mustard and mustard greens, and is known as the main component responsible for their spiciness.

In other words, spiciness evolved to fight against other fungi and plants.

In fact, allyl isothiocyanate has been reported to inhibit the growth of Fusarium oxysporum, the fungus that causes wilting in spinach, and to exhibit fungicidal effects.

Furthermore, studies have shown that incorporating fresh mustard leaves into the field may have an effect in suppressing the germination of weed seeds.

It's not yet fully understood why only black mustard and mustard greens evolved to be spicy, but it's possible that for some reason, their ancestors faced particularly fierce battles against parasites in their environment.

Why is the classification of the Brassica genus so complicated? The complex evolutionary history of mustard and rapeseed.

Why is the classification of the Brassica genus so complicated?

This has a complex evolutionary history known as the "U's triangle" (Tsuda et al., 2016; Nakayama et al., 2022). Originally, the only Brassica species in the world, from Europe to the Middle East, were rapeseed, mustard, and cabbage (the ancestors of cabbage and broccoli).

However, the hybridization of rapeseed and black mustard in the Middle East resulted in Brassica juncea, the hybridization of rapeseed and calyx in Europe resulted in Brassica napus, and the hybridization of black mustard and calyx resulted in Abyssinian mustard (Brassica carinata).

In other words, the three existing species crossbred to create three new species. This was inferred from the genome composition and has been confirmed by genetic studies.

Generally, it is difficult for animals to reproduce and pass on their genes through crossbreeding, but plants can sometimes do so.

Mustard greens have inherited the characteristics of black mustard, so they are spicy.

Brassica napus inherits characteristics from cabbage, such as thick leaves and a waxy substance. This is easier to understand if you imagine cabbage.

The similarities between mustard greens, rapeseed, European rapeseed, and black mustard are to be expected, as they are closely related species, and some hybrids have resulted in species with intermediate characteristics.

Can we see different characteristics in cruciform flowers when viewed with an ultraviolet camera compared to what the human eye sees?

Brassica napus, rapeseed, and European rapeseed all bloom from March to April. The flowering period for black mustard in Japan is unknown, but it is thought that all of them bloom in spring. All of them are yellow and have the characteristic cross shape of the Brassicaceae family. This is called a cruciform flower. What kinds of insects visit these flowers?

Honeybees are the primary visitors to mustard plants, but other species such as bumblebees, syrphidae, and flies have also been observed (Tsuda et al., 2016).

European honeybees (Apis mellifera) and bumblebees (Bombus spp.) have been observed visiting rapeseed plants and are considered to be major pollinators responsible for long-distance pollen dispersal (Tsuda et al., 2016).

Generally, because cruciate flowers are flat, they seem to be most important for bees, but many insects can utilize them, suggesting they may have evolved to be generalists.

However, you might feel that this simple flower doesn't have any other distinguishing features.

However, it has been found that when rapeseed is photographed with a camera that can see ultraviolet light, the center of the flower turns black (Yamaoka, 2009; Tanaka, 2009). The center becomes "ultraviolet-colored," which is visible to insects, and they seem to be attracted to it. Mustard, rapeseed, and black mustard may also have similar coloration. To humans, they are just yellow flowers, but to insects, they may be surprisingly colorful.

While mustard greens can self-pollinate, cross-pollination by insects is essential!

By the way, since rapeseed is self-incompatible, cross-pollination is essential (Tsuda et al., 2016). Therefore, as mentioned above, insect pollination is essential.

On the other hand, mustard greens and rapeseed are self-compatible and can self-pollinate. Therefore, insect pollination is not essential. For a long time, it was thought that insects were unnecessary for mustard greens.

However, subsequent Japanese studies using Nozawana (a variety of rapeseed) and Brassica juncea have shown that this is not the case (Osawa and Ikui, 1987). In this study, researchers created environments with and without the presence of the insect *Eristalis tenax*, and experimented to see how the fruiting rate changed when *Eristalis tenax* visited the flowers.

As a result, in environments without the striped hoverfly, the fruiting rate was 0% for *Chinese nutmeg* and 56% for *Chinese mustard*, whereas in environments with the striped hoverfly, it rose to 75% for *Chinese nutmeg* and 90% for *Chinese mustard*.

These results indicate that even in mustard plants capable of self-pollination, the presence of insects increases the likelihood of fruit set. It can be concluded that wild insects were essential for increasing the yield of mustard plants.

It is believed that, under field conditions, cross-pollination occurs in approximately 30% of cases for rapeseed plants through physical contact between adjacent individuals and pollinating insects.

This kind of unknowing benefit we receive from wildlife is called "ecosystem services." If you enjoy Japanese mustard or cooking with salad oil, please also pay attention to nature conservation that protects the habitats of such insects.

The fruit is a silique and is automatically dispersed.

All fruits are capsules, and those of the Brassicaceae family are specifically called siliques.

Although the length of the silique varies depending on the species, it is composed of two carpels with a septum in between, and the carpels detach as valves, causing it to dehisce. When ripe, stimulation causes the pericarp to instantly separate into two parts and curl up, automatically dispersing the seeds (Kobayashi, 2007).

References

Kanagawa Prefecture Flora Survey Association. 2018. Kanagawa Prefecture Flora 2018 (Electronic Edition). Kanagawa Prefecture Flora Survey Association, Odawara. 1803pp. ISBN: 9784991053726

Kobayashi, Masaaki. 2007. From Flower to Seed: The Science of Seed Dispersal. National Rural Education Association, Tokyo. 247pp. ISBN: 9784881371251

Nakayama, Yuichiro; Gohara, Masashi; and Asai, Motoaki. 2022. Identification of Brassica species growing naturally in riverine environments. Weed Research 67(1): 31-43. https://doi.org/10.3719/weed.67.31

Osawa, R. & Ikui, H. 1987. The role of pollinating insects in the pollination and fruiting of Brassica campestris and Brassica juncea. Journal of Plant Breeding 37(4): 453-463. ISSN: 0536-3683, https://doi.org/10.1270/jsbbs1951.37.453

Tanaka, Hajime. 2009. Handbook of Flowers Attracting Insects. Bun-ichi Sogo Shuppan, Tokyo. 80pp. ISBN: 9784829901397

Tsuda, Mai; Tabei, Yutaka; Osawa, Ryo; Shimono, Ayako; Yoshida, Yasuko; and Yoshimura, Yasuyuki. 2016. A collection of biological information on Brassica juncea, Brassica rapa, and Brassica napus necessary for assessing the biodiversity impact of genetically modified Brassica napus. Bulletin of the National Institute for Agro-Environmental Sciences 36: 1-45. ISSN: 0911-9450, https://doi.org/10.24514/00003021

Yamaoka, K. 2009. Improvement of Biology Teaching Materials for Humanities Students (4) Nectar Guides of Angiosperms (Part 2) Pseudo-ultraviolet Color Images of Nectar Guides. Toyo University Bulletin, Natural Sciences Series 53: 69-87. ISSN: 1346-8987, http://id.nii.ac.jp/1060/00006086

Source

This article is a significantly expanded version of a piece originally published in the following book.