🌾 Tansymustard

What it looks like

Tansymustard is not one plant. It is a genus — Descurainia, in the mustard family (Brassicaceae) — of plants known commonly as the tansymustards, and the genus name commemorates François Descurain, a French botanist and herbalist of 1658 to 1749. Forty-seven species are accepted in the genus, and most of them share the same basic look: a hairy, heavily branched, mustardlike annual or biennial, with leaves that are the whole identification story — highly lobed or divided, pinnately compound, feathery in outline, often covered in fine hairs that give the foliage a greyish-green cast. If the leaf looks like a tiny fern that got lost on its way to a mustard patch, you are probably looking at a Descurainia.

The flowers are a masterclass in mustard minimalism: tiny, four-petaled, yellow, arranged in elongating racemes that keep blooming at the tip while fruit forms below. The fruit is where the genus separates itself from its mustard cousins. Where tall tumble mustard (Sisymbrium altissimum) makes long, slender pods 5 to 10 centimetres long, tansymustards make short pods — siliques or silicles, often club-shaped, usually well under two centimetres, on threadlike stalks. That short-pod, feathery-leaf combination is the genus signature, and it holds across species that otherwise vary wildly in height, hairiness, and habitat, from desert floors to mountain slopes. The plants reproduce only from seed, which means their whole strategy is to make a lot of seed, scatter it widely, and trust disturbed soil to do the rest.

For butterflies, tansymustards are host plants, not nectar showpieces. In the butterfly guides the genus appears as a host plant for the spring white and the western white, two whites whose caterpillars feed on mustards. The spring white guide lists tansymustards (Descurainia) among its caterpillar hosts, alongside rock cresses, and the western white guide lists tansymustards among its mountain-mustard hosts. That is the ecological job this genus does best: it grows where little else will, in dry, disturbed, nutrient-poor ground, and it feeds the caterpillars of butterflies that specialize in exactly those harsh places. A patch of tansymustard on a dry bank is not a garden failure. It is a nursery.

This genus guide does not cover any single species on its own. The pinnate tansymustard species guide covers Descurainia pinnata — the western tansymustard, the species named in Becker’s white and the desert orangetip — and that species guide claims Descurainia pinnata only. This guide claims the genus: the tansymustards as a group, the plants the spring white and western white guides name when they write “tansymustards” without picking a species. If a butterfly guide names a single species, it belongs to that species’ guide. If it names the genus, it belongs here. That distinction is the whole point of having both guides, and it is worth keeping straight, because the mustard family punishes sloppy filing more than almost any other plant family.

How to grow

Here is the honest framing: tansymustards are not plants most gardeners need to plant. They plant themselves. If you have open, dry, disturbed, sandy, or gravelly ground in their range, they will very likely arrive on their own, and the gardener’s job is management, not cultivation. If you want to encourage them deliberately — for a butterfly patch, a dry meadow, or a native-plant corner — the method is gloriously simple: scatter seed in fall or early spring onto bare, well-drained, sunny ground, press it lightly into the surface, and do not bury it deep, fertilize it, or water it much. They are winter annuals in warm climates and spring annuals in cold ones, germinating with cool-season moisture, growing fast, flowering in spring, and setting seed before summer heat arrives. Do not amend the soil richly. Rich soil and regular water produce rank, floppy growth and invite competitors that crowd them out. Poor soil is not a handicap here. It is the habitat.

Management, for those who have them and want to keep them without letting them take over, is equally simple. Let a few plants set seed each year if you want them to return, and pull or mow the rest before seed ripens if you do not. Because they reproduce only by seed, preventing seed set is complete control — there are no rhizomes sneaking under the fence. Thin volunteers to 6 to 12 inches apart if you want sturdy, well-branched plants rather than a crowded thicket, and leave some bare ground each year: tansymustards are disturbance plants, and a patch that is never disturbed eventually gets shaded out by perennials and grasses. A little bare dirt, refreshed annually, is the whole cultivation program.

Florida note: Some tansymustards do occur in Florida, and in Florida they behave as cool-season annuals of sandy, disturbed ground — a winter and early-spring wildflower of open sand and roadsides, gone by the time summer heat arrives. Gardeners in Florida who want them for a butterfly patch should treat them as seasonal wildflowers to encourage in a sunny, sandy, unamended corner, not as perennial border plants, and should expect them to vanish in summer and return with cool weather if seed is allowed to set. One caution: flixweed (Descurainia sophia), the introduced Eurasian member of the genus, is reported as absent from Florida in some treatments, and Florida gardeners should favor native tansymustards over any introduced species in any case.

Water & feeding

Water needs are low. Tansymustards are adapted to rapidly drying soils and need moisture mainly to establish their short taproots and germinate; after that, they thrive on neglect in dry ground. In a garden setting, water only to establish seedlings in a dry spell, and then stop. Overwatering produces soft growth and invites the fungal problems that dry-land mustards normally escape. Well-drained soil is the rule, and sandy or gravelly ground is ideal.

Feeding: do not feed them. These are plants of nutrient-poor, disturbed soils, and fertilizer helps their competitors more than it helps the tansymustards. The soil science note is the genus’s whole story: as mustards, they are fast-growing, seed-heavy colonists that turn bare, poor ground into caterpillar food and then into seed, and their biomass, left in place, returns organic matter to ground that had little. They are not crops to feed. They are pioneers to let work.

Common problems

• They spread by seed — a lot of seed. A single plant produces many small seeds that persist in the soil for years. If you do not want them everywhere, pull or mow plants before seed ripens. Because they have no rhizomes, seed control is total control.
• Toxic to grazing animals in large quantities. This is the serious warning, and it belongs to the genus’s best-known members. Tansymustard contains nitrates and thiocyanates and is toxic to livestock in large amounts, though nutritious in smaller amounts. Do not plant tansymustards deliberately in or near pasture, and do not let them dominate grazing ground.
• Confusion with other mustards — and with each other. The mustard family is full of small yellow-flowered lookalikes, and the Descurainia species themselves are famously variable and hard to separate without checking pods, hairs, and leaf divisions together. Tall tumble mustard has long pods; tansymustards have short ones. That single check separates the genera. Separating species within Descurainia is a job for a flora key, not a glance, and this guide does not pretend otherwise.
• They look weedy — because they are. Tansymustards are native weeds in the ecological sense: fast colonists of disturbed ground. In a formal border they will look out of place. Grow them in a meadow, a dry bank, or a wild corner, where scruffy is a style.
• They disappear in summer. As cool-season annuals, they finish, set seed, and die before summer heat. That is not failure. That is the life cycle, and the seed bank is next year’s patch.
• Variable appearance. Species and subspecies differ, and plants look different in different climates. Do not expect every plant to match a photo exactly; check leaves, flowers, and fruit together.

Harvest

Harvest, for the adventurous, is about seed and leaves. The seeds of the best-known tansymustards are said to taste somewhat like black mustard and were used as food by Native peoples, and the small leaves are edible as a mustard green — bitter, peppery, and best used sparingly, like other wild mustards. This guide does not present tansymustards as a food crop: they are wild plants with a toxicity warning for livestock, and wild-mustard foraging belongs to people who can identify the genus — and ideally the species — confidently and understand the nitrate caution. The harvest this guide recommends without reservation is the ecological one: let a patch flower for the spring white and the western white, let it set seed for next year and for seed-eating birds, and cut or pull the spent plants once seed has scattered. A patch managed that way gives you butterflies in spring and a self-renewing patch next year, which is a better yield than a salad leaf.

Saving seeds

Genus-level honesty first: tansymustard is a whole genus of feathery, feral mustards — Descurainia, dozens of species across two continents — and seed needs are shared across most of it. The seed is easy: let the pods ripen on the plant until they are dry and brown, then collect them into a paper bag before they split and scatter. No cold treatment is needed; sow in fall or spring in moist soil, barely covered, and germination usually comes in one to two weeks.

It self-seeds freely, which is the whole point of a feral mustard: a patch you planted becomes a patch that plants itself. Deadhead if you want to keep it in bounds, or let it go and pull the volunteers you do not want. Save from the plants you liked best and dry the seed thoroughly before storing; it keeps several years in a cool, dry jar.

Tansymustard feeds white and orangetip caterpillars, which is why it earns its keep in a butterfly garden even though it looks like a weed. A patch you planted on purpose is a host-plant garden; a patch you ignored is a weed problem, and the difference is only whether you meant it.

In Florida, tansymustards are winter annuals of open ground and roadsides, widespread across North America and not zone-limited by hardiness. They colonize bare dirt, feed caterpillars, and then die back when the heat comes — a mustard with a job to do and no interest in staying past it.

Species in this group

Pinnate tansymustard (Descurainia pinnata) — the western tansymustard, with feathery, pinnately divided leaves and tiny yellow flowers; covered by its own species guide, which claims Descurainia pinnata only.

Flixweed, or herb sophia (Descurainia sophia) — the type species of the genus, an introduced Eurasian annual or biennial, also called tansy mustard; a weedy mustard of disturbed ground across North America.

Mountain tansymustard (Descurainia incana) — a widespread native species of western North America, with lobed leaflets and pods indented around the seeds.

Desert tansymustard (Descurainia obtusa) — a native species of the western United States, with hairy pods when young and leaflets usually with rounded tips.

Sierra tansymustard (Descurainia californica) — a native species of the Sierra Nevada and the West, named for its mountain range.

Tierra del Fuego tansymustard (Descurainia antarctica) — a southern South American species, proof that the genus ranges far beyond the North American roadside.

Other tansymustards (Descurainia spp.) — the genus holds 47 accepted species, and this guide claims the genus as a group, not any single species beyond those named above; a butterfly guide naming a single Descurainia species belongs to that species’ guide.