Tumble mustard (Sisymbrium altissimum)
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Tumble mustard, also known as Jim Hill mustard, is a broadleaf winter or summer annual and sometimes biennial plant with a highly branched top. Tumble mustard is found throughout California up to an elevation of 8200 feet (2500 m) and inhabits agricultural land and other disturbed sites.
Orchards, vineyards, gardens, agronomic and vegetable crop fields, landscaped areas, roadsides, and other disturbed, unmanaged sites.
The seedling has very small cotyledons (seed leaves) that are egg shaped to oblong, about 1/12 to 1/5 of an inch (2–6 mm) long and about 1/25 to 1/12 of an inch (1–2 mm) wide. They have no apparent stem. The first true leaf is slightly larger than the cotyledons, dull green, coarsely toothed, has an elongate football shape, and a few hairs along the edge and on the upper surface. Later leaves are similar to the first, but grow increasing larger and have shallow-toothed margins. Leaves are alternate to one another along the stem.
Tumble mustard exists as a rosette until the flower stem develops at maturity. Leaves at the base of young winter rosettes are coarse and deeply lobed. Leaves near the top are smaller and deeply cut to form threadlike divisions. Although the rosette leaves of London rocket, Sisymbrium irio, and tumble mustard have similarities; those of tumble mustard have terminal lobes that are more triangular, while those of London rocket are egg shaped.
Stems are erect and branched. The plant can reach a height of 5 feet (1.5 m) tall. The related London rocket only reaches 1-3/5 feet (0.5 m) in height. Leaves at the base of tumble mustard (rosette leaves) are coarse and deeply divided, or lobed to compound (fully divided into separate leaflets), have stalks, and are about 6 inches (15 cm) long.
Upper leaves are stalkless, smaller than the lower leaves, deeply cut to form threadlike divisions, and do not clasp the stem. Long unbranched hairs sparsely cover leaves. In contrast, London rocket’s upper leaves are oblong and have two spreading lobes at the base. Flixweed, Descurainia sophia, can be distinguished by its branched hairs compared to the unbranched hairs of tumble mustard. Also flixweed has very finely divided leaves whose leaflets are often finely divided once or twice again (tumble mustard leaves are divided only once). Russian thistle, Salsola tragus, has leaves that graduate into rigid, spine-tipped scalelike structures (bracts) in the flower head, while in tumble mustard, upper leaves remain threadlike.
Flowers bloom from April through September. Tiny, pale yellow (rarely white) flowers with four petals cluster on thick, spreading stalks.
The fruit, produced on a short stalk, is a one-ribbed, narrow pod about 1/24 of an inch (1 mm) wide and 2 to 4 inches (5–10 cm) long. Pods point somewhat upwards.
Seeds are oblong, roughly 1/24 of an inch (1 mm) long, half as wide, and brownish yellow to dull orange.
Reproduces by seed. Mature plants tend to break off at the base and tumble in the wind, which shakes loose and spreads seed.