Time of bloom: June to September.
Seed-time: July to October.
Range: Illinois to the Northwest Territory, southward to Texas, Mexico, and California. Habitat: Rich prairie soil; invades all crops.
This is a much harder weed to combat than its annual relatives, for one must have a care in cultivation not to break up and spread abroad the creeping rootstocks and thus increase the plague.
The plant looks very like the smaller Ragweed, but is stouter and grows two to six feet high. Leaves once or twice pinnatifid, with lobes usually acute, thick and bristly instead of thin and soft. Male flowers very abundant, on numerous long racemes, the involucres deeply cup-shaped; fertile flowers mostly solitary, the small, brown achene-like fruits obovoid, hairy, short-pointed, with fewer tubercles than the preceding species or sometimes none at all; they are often found in grass and clover seed and in baled hay. (Fig. 319.)
Newly infested areas, if not so large as to make the method impracticable, should have prompt treatment with a strong herbicide - caustic soda or hot brine - the soil being cleaned of all plant-growth for a season rather than allow the pest to gain a foothold. Large areas can be finally suppressed by putting the ground under cultivation, plowing deeply during very dry weather, and exposing the root-stocks as much as possible; after this summer fallowing put in a hoed crop, and give such persistent and careful tillage as to kill surviving rootstocks by depriving them of leaf growth.
Fig. 319. - Perennial Ragweed (Ambrosia psilostachya). X1/6.