This section is from the book "A Manual Of Weeds", by Ada E. Georgia. Also available from Amazon: A Manual Of Weeds.
Native. Annual and winter annual. Propagates by seeds. Time of bloom: June to October. Seed-time: July to November.
Range: Throughout North America except the extreme North. Habitat: Grain fields, meadows, pastures, and waste places.
From this plant is distilled the volatile oil of fleabane, used in making "mosquito dope" for the use of persons who have occasion to go where mosquitoes are troublesome; it is also used in medicine, and the dried plants are quoted at five to six cents a pound in the drug market; its juices are resinous and bitter, and grazing animals will not touch the weed. The hands should be protected when pulling or collecting the plants, for the oily and acrid juices are sometimes very irritating to the skin, producing an eruption which resembles that caused by the touch of Poison Ivy.
In good soil the stem may attain to a height of ten feet, and, again, it will adapt itself to hard conditions and bloom when less than six inches tall; it is erect, finely grooved, bristly with short hairs, simple or branching from the base; when cut it stools freely, hastening to develop new fruiting branches. Lower leaves spatulate or sometimes cut-lobed, tapering to petioles; upper ones usually entire, lance-shaped to linear, finely hairy, much crowded on the stalks. Heads in panicled clusters, very small and very numerous, each about a sixth of an inch broad, with smooth, cylindric involucre, nearly concealing the very small, white rays. Seeds many and small, with yellowish brown pappus. (Fig. 305.)
Where not too abundant to make the task impracticable, hand-pull the weeds and remove them from the ground, for the woody stalks contain enough nutriment to mature the first-opened flowers. Burn over stubbles on infested grain fields for the purpose of destroying the seeds on the ground. Meadows badly "run to Horseweed" should be put to a well-tilled hoed crop before reseeding. Plants of roadside and waste places should be pulled or cut in early bloom or before, for the protection of adjacent property.
Fig. 305.- Canada Fleabane (Erigeron canadensis). X 1/4