14 
Colorado Experiment Station 
grow directly in the seep, but around the moist edges and back 
to the extremely dry land. 
Fig. 10 — A single pod showing large numbers 
of seeds escaping. 
In regions where the infestation is comparatively new the 
milkweed is found along the ditch banks in isolated, heavy 
patches from a few square feet in area to a quarter of an acre 
or more in extent (Fig. 13) . No other plant grows where the milk- 
weed is well established. Below a ditch bank near Paonia, an 
area of foxtail (Hordeum jubatum) half an acre in extent was 
driven out by whorled milkweed in three years. In oat fields, 
where there were patches of milkweed and the field was plowed in 
the spring, no oats grew in the areas occupied by the stands of 
milkweed, despite the fact that the surrounding crop was very 
heavy. 
This species has been found on several types of soil, from a 
very light sandy loam to a heavy clay adobe, and from fine soil to 
a soil composed largely of gravel and rocks. It does best, how- 
