Wednesday wildflower: Indian mustard.

Chance can be an important aspect of biological discovery, even very minor discoveries such as I'm describing today, but a prepared mind is an essential complementary aspect to it.  Yesterday I went home from work by a new route, governed by two objectives: to get some eye bolts from the hardware store to support the earthquake-proofing at home, and to collect Veronica hulkeana flowers from the University grounds to photograph.  So I was walking an unusual route between the hardware store and the University when I saw a mustard that looked unfamiliar, growing in a small front yard of a cottage, right up against the house.  

Brassica juncea, Vivian Street Wellington city.

I had a good look over the fence, but I didn't want to trespass, and this morning I went past again, wondering if I should knock on the door and ask permission to collect.  Then I noticed there were half a dozen plants of it, and one or two branches were poking outside the boundary over the footpath: fair game!  So I quickly grabbed a sample and brought it with me to work and spent a happy half-hour keying it out to Indian mustard, Brassica juncea.

Flowering branch, about to be made into a herbarium specimen.

Indian mustard is an Asian Brassica that has been collected only occasionally in New Zealand.  The Flora of New Zealand Vol. 4 describes its distribution as Northland, and the NZPCN website shows several additional records from Auckland City.  These are vigorous plants, up to 80 cm tall.  They're hairless, and have stout stems with wide pith in the centre.  

Brassica juncea: upper leaf

The leaves are bright green; the stem leaves coarsely toothed, the upper ones becoming simple and linear.  The technical details needed for identification include: 

sepals erecto-patent; 

petals bright yellow; stamens 6, the outer spreading (a bit); 

ovary with one vein on each valve, seeds in one row, gynophore absent; style about 4 mm long at this early fruiting stage; stigma capitate.

These plants have coarsely toothed leaves, matching the cultivated variety mizuna.  There seems to be some confusion whether mizuna is B. rapa or B. juncea, and maybe forms of both species are grown under that name.  These ones though are not B. rapa, which, along with B. oleracea and B. napus, has distinctive glaucous and stem-clasping upper leaves.  It's a plant that has become popular in supermarket salads, so I expect more people are growing it these days, and so it's more likely to escape into the wild.  That's good for wild food foragers.

When plant distributions spread southwards, it's tempting to think it might be something to do with global warming.  But weed distributions are governed by all sorts of things, not least by chance, such as the accident of their place of introduction.  It'll be interesting to see if this Brassica spreads further in Wellington and if it's already established in other parts of the city.