Waterhyssops are annual, aquatic broadleaf weeds that grow in shallow water or mud and are common in rice fields, ditches, ponds, and reservoirs. Eisen waterhyssop, B. eisenii, a native Californian plant, is found in the Central Valley, southern central-western regions, and Sierra Nevada to 330 feet (100 m) on the west side of the Sierra Nevada and to 3900 feet (1200 m) on the east side. Disk waterhyssop, B. rotundifolia, (shown in these images) is not native to California and grows in the Central Valley to an elevation of 330 feet (100 m). Seeds of both species germinate with rice field flooding in the spring.
Seedling
Cotyledons (seed leaves) are 1-1/2 to 2 times longer than they are broad. The oval or round first leaves are produced on stems and lack stalks.
Mature Plant
Mature waterhyssops have branched, fleshy, hollow stems that are slightly constricted at the joints where they may grow roots. Circular or wedge-shaped leaves are opposite to one another along the stem. Leaf veins radiate from a common point.
Flowers
Plants produce one or two five-petaled, white and yellow flowers at the base of leaves or on narrow stalks. Flowers of Eisen waterhyssop bloom from June through September.
Fruits
Mature fruits, which contain two to four seed capsules, are submerged.
