DAY LENGTH AND FLOWERING - HEMP

New practices that can be used in breeding hemp have resulted from recent
photoperiodic studies. Hemp is a short-day plant. It flowers promptly
when day lengths are less than 14 hours and very slowly or not at all when
day lengths are greater than 14 hours. Under average field conditions
about half of the plants are female and half are male. Under some
circumstances, however, female plants, each of which may produce a very
large number of female flowers, will also produce a few typical male
flowers. The opposite condition occurs in the males; that is, a male plant
will form an occasional female flower in addition to a great number of male
flowers. This tendency for hemp plants to produce these extra flowers of
the opposite sex is increased by subjecting them to photoperiods of 14
hours or less and to cool nights when flowers are being formed.

Most rapid progress in breeding plants can be made with those that can be
self-pollinated. Hemp, obviously, can be self-pollinated only when it
produces these occasional additional flowers of the opposite sex. The
advantage of being able to increase their tendency to form these flowers is
apparent.

Dr. Hugh C. McPhee of the Department made use of this behavior of hemp
several years ago. He used the pollen of the male flowers that were
produced in small numbers on certain of the female plants to self-pollinate
those plants. When he grew the resulting seeds he made the important
discovery that all of the plants were female. We have recently produced
several thousand seeds in this way and have not obtained a single male
plant, thus thoroughly confirming Dr. McPhee's observations.

Under greenhouse conditions, with proper control of temperature and day
length, a very high percentage of female plants produced enough male
flowers so that self-pollination could be effected, and in certain
experimental lots of female plants produced out-of-doors in late summer
when days were short and nights were cool, enough male flowers were formed
so that natural pollination occurred and a quantity of pure "female" seed
was produced. These results suggest that a locality can be found in which
the conditions are favorable to the formation of these intersex male
flowers on female plants in sufficient quantity that a good crop of seed
could be obtained. If this could be done, a means would be at hand to
produce commercial quantities of "female" seed, thereby enabling growers to
produce a pure stand of female plants. Such pure stands would result in a
more uniform fiber crop and eliminate certain harvest problems. The basis
for developing such a procedure lies in finding in nature a combination of
environmental factors similar to that which, under experimental conditions,
has resulted in formation of abundant male flowers.

Checked-by: Richard Lake