Orychophragmus violaceus (大紫羅欄花, オオアラセイトウ)

This one is truly a weed, but a very pretty one for sure. It's found in all kinds of ruderal places, from street sides to grassy fields. The plant is introduced from China, and the Japanese don't seem to know what to call it. It has at least 5 common names here. I've chosen to use the longest of them, since it seems to be used most often: 大紫羅欄花 (オオアラセイトウ), which is read "ooaraseitou". The name means something like "great-column-of-purple-silk flower". Not even Japanese would be able to write that name in Kanji (i.e. the Chinese characters used to write Japanese). Other names include 紫花菜 (ムラサキハナナ, "murasakihanana"), meaning "purple-flower rape" (rape as in rapeseed of course), 紫金草 (シキンソウ, "shikinsou"), meaning "purple-gold plant" and 諸葛菜 (ショカツサイ, "shokatsusai"), which is apparently the original Chinese name. It's also called 花大根（ハナダイコン, "hanadaikon"), which means "flower daikon" (a daikon being a huge radish for those of you not up on your Japanese veggies), but only because it resembles the related Hesperis matronalis, which is apparently the true hanadaikon, and it is also introduced. Oh, and it doesn't have a common name in English


So if you haven't guessed after all the names referencing radishes and rapeseed, the plant is in the radish and rapeseed family, more often called the cabbage family, or the crucifers (Brassicaceae). Orychophragmus violaceus (L.) O. E. Schulz is native to China and Korea, but was introduced to Japan some time ago. One source I looked at says that the plant was introduced sometime during the Edo period (江戸時代), which ran from 1603 to 1868, apparently as an ornamental. As is often the case, it escaped from cultivation and spread all over. The flora of China says that it is common in "roadsides, gardens, forests, fields, thickets, valleys, hillsides, and sunny slopes", which seems more or less accurate for Japan too. It's one of two species in its genus, with the other, O. limprichtianus, being endemic to China.

The plants are relatively big, averaging a half a metre tall, though some grow up to just shy of a metre. One of the reasons that they are so successful may stem from the fact that each ovary has 40 to 70 ovules, and given that each plant is loaded with flowers, individuals are producing seeds in the thousands. Not that you would see the other species here, but the great-column-of-purple-silk flower (I don't think that name will catch on abroad...) has auriculate leaves, meaning that they have ear-like projections that clasp the stem and stick out on the opposite side, while the other species does not. Our topical species also has petals with very distinct claws (i.e. the stalk-like part of the petal), which the other species lacks. The plant didn't really look like a crucifer to me at first, probably because I don't recall seeing many violet-flowered crucifers before, but it's got all the typical features, from the flowers to the siliques (i.e. the fruit common among crucifers), although the shape of the anthers seems perhaps a bit odd for the family. Oh, and the flower colour ranges from a light blue to a fairly deep purple, even within the same stand. A nice weed to have around!

Here are some useful links with more information
Flora of China entry
Flora of China illustration
Account from the "Japanese Plant Watcher's Page"