Perennial pepperweed (Tall whitetop) (Lepidium latifolium)
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Perennial pepperweed, an erect perennial broadleaf plant, is highly aggressive and frequently forms dense stands that outcompete and exclude native plants and wildlife. It grows to about 7200 feet (2200 m) throughout California except in the deserts and northern North Coast and neighboring mountains. Goats seem to tolerate feeding on large quantities of fresh perennial pepperweed. However, there have been reports of horses becoming ill after feeding on hay contaminated with it. In California and several other western states it is a state-listed noxious weed.
Perennial pepperweed is found in crops such as alfalfa, vineyards, orchards, and irrigated pastures; ornamental plantings; noncrop areas such as wetlands, meadows, salt marshes, vernal pools, sand dunes, flood plains, riparian areas, roadsides and irrigation ditches; typically found on moist or seasonally wet sites but can tolerate alkaline and saline conditions.
Cotyledons (seed leaves) are egg shaped to oblong, about 1/8 to 1/3 inch (3–8 mm) long, hairless, with a rounded tip, and a base that tapers into a short stalk, about 1/12 to 1/8 inch (2–3 mm) long.
First leaves are egg shaped to oblong, about 1/6 to 1/2 inch (4–12 mm) long, hairless, with a somewhat rounded tip and a wedge shaped base. The edges of the leaves are smooth to slightly wavy and have a stalk about 1/5 inch (5 mm) long.
Later leaves resemble the first leaves but are progressively larger.
Young plants form a basal rosette and can grow from perennial pepperweed rootstock.
The mature plant grows rigidly erect to about 6-1/2 feet (2 m) tall with multiple branches. The leaves range from lance to football shaped to oblong and are hairless, green to gray green, and alternate along the stem. The stem leaves are reduced, stalkless, or nearly stalkless, with a tapered base, and a smooth to a weakly toothed edged. Basal leaves are larger and wider than the stem leaves up to about 12 inches (30 cm) long and 3-1/5 inches (8 cm) wide with toothed edges on a stalk about 1-1/5 to 6 inches (3–15 cm) long. Its lower stems and crown are somewhat woody and it has extensive creeping roots.
Flowers bloom from May through September. Small, white flowers form dense clusters (inflorescence) arranged in branches at the tip of each stem. The top of the inflorescence is pyramid shaped to round. The individual flowers are small, with 4 white spoon-shaped petals about 1/16 inch (1.5 mm) long.
Fruit are small, round to somewhat egg-shaped, slightly compressed pods commonly covered in long hairs, about 1/12 inch (2 mm). Each pod has two chambers with one seed per chamber.
Seeds are egg shaped, reddish brown, somewhat flattened, with a weak groove on each side, and about 1/25 inch (1 mm) long and 1/51 inch (0.5 mm) wide. Seeds drop from the pods throughout the winter and some remain in the pods until the next season. There is one seed per fruit pod chamber.
Perennial pepperweed reproduces from seeds and from root fragments and rootstock and creeping roots. Seeds and root fragments disperse in water, with soil, and agricultural and other human activities. Seeds can cling to animals, tires, shoes, and can contaminate crops, hay, or pasture seed.
Related or similar plants
- Field pepperweed, Lepidium campestre
- Hairy whitetop, Cardaria pubescens
- Hoary cress, Cardaria draba
- Lens-podded whitetop, Cardaria chalepensis