Treacle mustard Erysimum repandum
IDENTIFICATION — The seeds of this annual sprout in fall and are green all winter. By early spring you’ll spot a low rosette of leaves about 6 in. across. As the weather warms, a stem stretches to become 2 ft. tall. From midspring to early summer the sparsely branched stem in the illustration is tipped with clusters of 1/4-in.-wide yellow flowers. Each flower has just four petals and is soon replaced with a slender 2- to 4-in.-long seedpod. Being an annual means treacle mustard, sometimes called “bushy wallflower,” spreads only through reseeding.
FAVORITE CONDITIONS — While treacle mustard prefers full sun, you may find it in part shade. It tolerates a wide variety of soil conditions, but grows best in slightly moist loam. And it’s mostly found in areas where the soil is disturbed frequently, such as vegetable gardens and flower beds.
CONTROL — Hoe or pull out young plants before they have a chance to flower and set seed. You can pull mature plants. However, a stout taproot can make it difficult. Treacle mustard is rarely found in lawns, but if you do find it, use selective herbicides that won’t harm the grass.