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Statewide IPM Program, University of California

Pale smartweed  (Polygonum lapathifolium)

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Life stages of Pale smartweed fruit achenes seedling flowers mature plant swollen stem nodes and characteristic shelth

Pale smartweed is a summer annual broadleaf plant that typically inhabits areas in or around water. It is found throughout California to about 4900 feet (1500 m) and grows in or on the edges of marshes, lakes, ponds, and streams. Their leaves provide protection for wildlife and their seeds are an important source of food for many waterfowl, songbirds, and mammals. Although considered desirable members of natural areas, pale smartweed can invade agricultural land and other disturbed areas. In fact, stands of pale smartweed can reduce the flow of water in irrigation ditches, canals, and drainage areas.


Margins of water bodies, pastures, orchards, irrigated crop fields, and rice fields.


Cotyledons are narrowly football shaped to oblong, have a mostly hairless surface sparsely covered with minute, flat, glandular dots, are 1/10 to 1/2 an inch (3–12 mm) long, and have short stalks that are fused at the base. The first and later leaves are football shaped and are alternate to one another along the stem.

Mature plant

Plants may be erect or creeping with weak, ascending stems, and reach 5 feet (1.5 m) in length. Stems are coarse, often reddish, swollen at the nodes (joints), and typically root at the lower nodes. Leaves are long, narrow, and taper at the tip. They have whitish undersides, range from roughly 1 to 8 inches (2.5–20 cm), and are alternate to one another along the stem. The lower leaf surface is dotted with tiny sunken glands and generally covered with short hairs. The upper leaf surface is mostly smooth, except for minute stiff hairs found over the veins.


Flowering takes place from June through October. Small whitish green to pale pink flowers cluster densely along the flowering stem forming cylindrical-shaped spikes that are about 1-1/5 to 3 inches (3–8 cm) long and droop from the stalk. Individual flowers never open. Flowers of the closely related ladysthumb, P. persicaria, open, are brighter pink, and cluster on a shorter, more erect flowering spike.


Fruits are tiny, egg shaped, less than 1/12 of an inch (2 mm) long, brown to black, glossy, and contain a single seed.


Reproduces by seed.

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