HEMP AS WEED CONTROL

D. P. West, Ph.D.

Weed control is a recalcitrant issue in crops grown for
organic certification. One approach is the prior use of a
competitive crop. In his textbook, Modern Weed Control, A. S.
Crafts cites as potential weed smothering crops: millet, Sudan
grass, sweet clover, sunflower, rape, barley, rye, reed canary
grass, crested wheatgrass, sorghums, buckwheat, soybeans,
alfalfa, cowpeas, clovers, hemp, Jerusalem artichoke, and
ensilage corn. Of these only one, hemp, can be taken seriously as
an adequate weed controlling mechanism.
The historical testimonials to hemp's ability to control
weeds are numerous. For example:

"...it is certain that hemp contributes more than any other crop
towards repairing the damage done by its own growth through the
return of the leaves to the soil, besides other matters while it is
undergoing the process of retting. Hemp is an admirable weed
killer and in flax countries is sometimes employed as a crop in
rotation, to precede flax because it puts the soil in so good
condition."
-Charles Dodge, Director, Office of Fiber Investigation, 1890.

"There will be little trouble with weeds if the first crop is
well destroyed by the spring plowing, for hemp generally occupies
all the ground giving weeds but little chance to intrude....In proof of
this, a North River farmer a few years ago made the statement that
thistles heretofore had mastered him in a certain field, but after
sowing it with hemp not a thistle survived, and while ridding his
land of this pest the hemp yielded him nearly $60 per acre where
previously nothing valuable could be produced."
-C. Dodge, Hemp Culture, USDA Yearbook of Agriculture, 1895

"Hemp prevents the growth of weeds and other vegetation
which would be found on such soils in most other crops or after
others are laid by, and its cultivation also seems to make the soil
more uniform in character."
-Lyster Dewey, The Hemp Industry in the United States, USDA
Yearbook of Agriculture, 1901


"Very few of the common weeds troublesome on the farm
can survive the dense shade of a good crop of hemp...In one 4-
acre field in Vernon County, Wis., where Canada thistles were very
thick, fully 95 per cent of the thistles were killed...." Lyster Dewey,
Hemp. USDA Yearbook of Agriculture, 1913.

"Hemp has been demonstrated to be the best smother crop
for assisting in the eradication of quack grass and Canada
thistles....At Waupon in 1911 the hemp was grown on land badly
infested with quack grass, and in spite of an unfavorable season a
yield of 2,100 pounds of fiber to the acre was obtained and the quack
grass was practically destroyed." -Andrew Wright, Wisconsin's
Hemp Industry, 1918.

"Hemp has been recommended as a weed control crop. Its dense,
tall growth helps to kill out many common weeds. The
noxious bindweed, a member of the morning glory family is checked
to some extent by hemp."-B. B. Robinson, Hemp, USDA Agric Bull
#1453, 1943

"Among the species studied, the hemp species proved itself to be the
best in fiber production. This plant was all the more interesting
owing to its low fertilization requirements, and its ability to grow
without being irrigated and without chemicals, whether it be for
weed or pest control." Barriere, et al. 19941

"Hemp grows quickly, soon covers the ground and chokes out
the weeds. So weed control is not necessary." -Eddy A. A. de
Maeyer. 19941

In Holland, Lotz, et al. tested hemp's superior weed
suppressing ability (Figure 1) against four other cropping
situations in a controlled experimental setting. The target weed
was yellow nutgrass (Cyperus esculentus), a weed also common
in the US, which propagates by tubers and is difficult to control.
The authors conclude, "...hemp was the most competitive crop in
this study. Selecting this crop in a rotation will cause the
strongest population reduction of C. esculentus on infested
farmland. This control option of hemp against harmful weeds as
C. esculentus is an attendant benefit of the introduction of hemp
as a commercial crop."2

1From papers delivered at the Conference on Alternative Oilseed and Fiber Crops for the
Cool and Wet Regions of Europe, Wageningen, The Netherlands, April 7-8, 1994.

2Lotz, L. A., P. R. M. W. Groeneveld, B. Habekotte, and H. van Oene. 1991. Reduction of
growth and reproduction of Cyperus esculentus by specific crops. Weed Research
31:153-160.