The habitat of Ventura marsh milk-vetch is poorly understood. Historically, it had been associated with cienegas (freshwater alkaline wet meadows), seashore meadows and salt marshes. The California Natural Diversity Database describes the habitat for this species as coastal salt marsh salt marsh
Salt marshes are found in tidal areas near the coast, where freshwater mixes with saltwater.

Learn more about salt marsh that is protected by barrier beaches or is near the high tide. However, this description is based on the habitat of the closely related species, coastal marsh milk-vetch (Astragalus pycnostachyus var. pycnostachyus). None of the remaining populations occur under entirely natural conditions; the only unplanted population exists in a heavily modified oil waste disposal site maintained via supplemental watering and the control of competing plants. The remaining populations are all planted with the seeds from the rediscovered site and exist in coastal wetland habitats. Plans for recovery of the species include identifying appropriate coastal wetland habitat with proper conditions. As such, more research is needed to understand the habitat of this species.  

Ventura marsh milk-vetch (Astragalus pycnostachyus var. lanosissimus) is a short-lived perennial shrub that can reach up to 2 meters in height. During winter, it produces leafy stems from a woody root crown that is visible on some individuals. Flowering in the summer, this species produces about 31 to 40 flowers per inflorescence (flower cluster). The fruit it produces is relatively small for the genus and is slightly curved. The leaves of this species are compound leaves and are composed of 27 to 39 leaflets per leaf.  

Measurements: 

Stem height: 106 cm to 170cm (42 inches to 67 inches) 

The leaves of the species are a muted sage green color. During the summer, it produces flowers that are pale yellow with possible tinges of green. New stems are green, while stems exposed to direct sunlight have a dark maroon color. Individuals in shaded areas have a green stem color. Additionally, stems that are partly shaded beneath other plants will be green, while portions of the stem that are exposed to the sun will be maroon. 

During the winter, this species produces leafy stems. Buds begin to appear on inflorescences, flower clusters, on these stems starting in mid-June, with flowers opening as early as late June and as late as early September. However, peak flowering occurs during mid-July. By early September, most of the flowers mature into fruits. The stems die as winter approaches, and it is common for some fruits to remain attached to these stems into the following year. The fruits usually disperse near the parent plant. Germination occurs following periods of heavy rainfall. If successful germination does occur, seedlings will emerge as early as 16 days and can emerge as late as 42 days, with most emerging before the 24-day mark.  

There are two varieties of Astragalus pycnostachyus. These varieties are allopatric; existing in two separate non-overlapping areas. The northern variety, coastal marsh milk-vetch (Astragalus pycnostachyus var. pycnostachyus), occurs in Marin, San Mateo and Humboldt counties while Ventura marsh milk-vetch historically occurred in Los Angeles and Ventura counties. Both varieties are associated with coastal wetlands.