Descurainia pinnata / western tansy mustard

Adjectives: annual, disturbed, drought tolerant, native, nutrient poor soils, seed bank, weed

Also known as: western tansymustard, green tansymustard, pinnate tansymustard, Menzies’ tansymustard, Nelson’s tansymustard, Payson’s tansymustard

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Western tansy mustard is a weedy, albeit native, winter annual. It spreads only by seed but the seeds can persist in the soil for several years. They are also small enough and sticky enough that then can be transported far afield by wind, water, birds and mammals, but more usually, babies are found near the mom. Once they grow up, they can be up to 3 feet tall.

Western tansy mustard flowers are teeny (ca. 1/8 inch across), yellow and generally like all the rest of the brassicas. As far as I am concerned, this makes them of little use in identifying the plants, other than to the family level. The inflorescences are flat-topped but so small you have to look closely to see that. They are at the branch tips, and at least that much is clear. Each even-smaller flower has four green-ish yellow (occasionally pink, they say) sepals and four yellow to cream petals, giving the overall appearance of eight “somethings”. Both the six stamens and the stigma extend a bit beyond the petals.

Like other mustards, tansey mustard is cross-pollinated with selfing possible. The flowers are said to be attractive to butterflies but also a variety of other small insects.

The fruit is a green sort of fat-ish silique, around half an inch long.

The stems of western tansy mustard are erect and coarse, and branch sparingly. Both stems and flowering stalks are glandular, and more or less pubescent, depending on the subspecies, of which there are at least ten. The photos in the gallery here are of a plant at Mail Cabin Creek trail and have little if any pubescence. The leaves are deeply lobed, “feathery” and sometimes have a light covering of hairs, especially on the bottom surface. Those “up” the stem are apetiolate. i.e. lacking petioles. The tips of the leaves and lobes are pointed.

Tansey mustard is well adapted to rapidly drying soils, but needs moisture to establish its short taproot. It is not particular about nutrient availability and is fine with disturbed, barren sites and gravelly, eroded soils. It tends to dominate quickly after fires.

Germination is facilitated by the seedcoat which, when wetted, forms a layer of mucilage that retains water. This provides good, local conditions for germination.