28 
Colorado Experiment Station 
salt could not be washed away. Also the plot is removed from 
an irrigation ditch and consequently is not kept moist. It is not 
known whether or not this method would prove so successful un- 
der humid conditions; but where the summer season is usually 
very dry it should prove rather efficient, tho somewhat expensive- 
Thirty pounds of salt were used on this area, containing a little 
over 75 square feet. 
Plot II. Smothering with Tar Paper — For this experiment 
a patch of milkweed almost identical with that used for Plot I 
was selected (Fig. 22). This plot, however, was somewhat larg- 
er, containing about 100 square feet. The tops were first cut off 
when about two or three inches high on June 15, 1918. Tar pa- 
per was applied on the same day. The strips of paper were laid 
fiat on the ground and lapped about 1 1-2 inches to insure no en- 
trance of air. The sun effectively sealed these laps. The out- 
side edges of the tar paper were covered over with dirt. 
On June 15, 1919, one-half of this tar paper was removed 
and during the season of 1919, no milkweed growth appeared 
above ground on this part of the plot. The remainder of this plot 
was left covered. 
This method is recommended for use only on small patches 
of milkweed on smooth ground where stock do not run and where 
the ground is not in use. Its advantage is that it needs to be 
attended to at only one time and can then be forgotten. Care 
must be taken that the tar paper extends beyond the outer lim- 
its of the patch or underground growth will creep out from the 
main patch. 
An interesting modification of this method of treatment was 
tried out by a farmer near Hotchkiss, Colorado. Wet, heavy 
manure was piled onto the patch and packed solidly to a depth 
of from 8 inches to 1 foot. In this way the air was excluded 
from the patch as effectively as with tar paper and as good re- 
sults were obtained. Success with this method as with any other 
method of eradication, depends entirely upon the thoroughness 
with which the job is done. Slipshod work is ineffective. In this 
method, the object sought is to exclude air from the plants and 
consequently an airtight cover must be put on the patch. 
Grubbing on Dry Land Away from Ditches. — This 
plot is about 15 feet in diameter and at the beginning of the ex- 
periment was covered solidly with the milkweed. The first grub- 
bing was given on July 8, 1918, when the plants were just begin- 
ning to bloom. The tool used was a heavy, broad-bladed mat- 
tock. A trench was first dug at one side of the patch to a depth 
