The Bold Brassicaceae

The other day while walking to my mailbox I saw a lot of this on the ground:


It’s Cardamine hirsuta (hairy bittercress), a particularly annoying lawn and garden weed. (Weed = a plant growing where it isn’t wanted.) C. hirsuta is a winter annual: the seeds germinate in autumn, and by winter produce these basal rosettes of leaves. In a few months the plants will send up slender flowering stalks, with tiny white flowers that are kind of pretty, if you’re a botanerd, anyway. But then those flowers will set seed, and at the slightest disturbance (like trying to pull them from the garden), the seeds will pop out explosively, in a surprisingly large radius.

I have to admit up front that the mustard family is not my favorite. There are many weedy-looking brassica species in the Maryland piedmont, but at the same time some lovely early spring wildflowers.

Brassicas are mostly herbaceous annuals or perennials, often with a basal rosette of leaves. Stem leaves are usually alternate and are often pinnately compound (as are the basal leaves), though sometimes palmately compound. The older family name, Cruciferae, is a reference to the flower shape: cruciform, like a cross, with four petals. There are usually four sepals (sometimes three) as well, but they fall off early, so you might not see them when looking at the flowers, which are borne on racemes or corymbs. (A corymb is a type of inflorescence where each pedicel is arranged along the stem alternately, with the lowest flowers having the longest pedicels and the highest flowers having the shortest, with the result that the flowers are borne more or less in one plane. At first glance, corymbs look rather like umbels, but in umbels all the pedicels originate from the same point on the stem.)

The fruits of brassicas are called siliques. Siliques are long and narrow, dry, and split open lengthwise at maturity, revealing a lengthwise septum (partitioning tissue).

Almost all brassicas contain pungent compounds called glucosinolates, which give the plants a characteristic pepperiness (like the flavor of watercress). Possibly this is an evolutionary response to herbivory. I have read that all brassica species are edible, but for many people that may depend on the definition of edible. None are outright poisonous, anyway. Many of the brassicas also contain isothiocyantes, aka mustard oils.

Worldwide there are a little over four thousand brassica species, in 370-some genera. North America has about 650 native species, making the Brassicaceae the sixth-largest flowering plant family on this continent. And in Maryland there are over 80 species, about half of which can be found in the piedmont, and more than 50 of which are alien. Four of the natives are on the watchlist (S3) and four more are ranked S1/highly state rare.

Brassicas are important economically as ornamentals and food crops. Common garden plants include alyssums or alisons (Alyssum species), candytufts (Iberis species), dame’s rocket (Hesperis matronalis), and wallfowers (Erysimum species).

Brassica food crops include radish, horseradish, the various mustards, cresses, and wasabi. A surprisingly large number of our culinary vegetables are actually cultivars of a single species, Brassica oleracea: broccoli, Brussels sprouts, cabbage, cauliflower, Chinese broccoli, collards, kale, kohlrabi, Romanesco, and more.

next time: early spring brassicas in the piedmont

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