Hemp Update #9

Longest Day – What Does it Mean For Hemp?

Since hemp is day length sensitive, for long-day varieties, the summer solstice (or shortly thereafter as days get shorter) triggers a new cycle in the plants life -flowering.  Here’s a good observation from NC State this year:


additionally, this is selected from: https://www.hempbasics.com/hhusb/hh4bot.htm .  Sections removed for brevity.  See the link for full text.

Hemp HusbandryRobert A. NelsonInternet Edition , Copyright 2000

Chapter 4,   Botany & Breeding

4.8 ~ Light

Cannabis’ rate of growth is proportional to the intensity of the light it receives, and is inversely proportional to the length of the photoperiod.

Cannabis responds to light in accordance with the intensity, wavelength, and photoperiod. Cannabis is a “short-day” species: it flowers when the photoperiod decreases to about 8 hours. The plant requires at least 3 hours of light daily just to survive, and at least 8 hours daily to thrive. While the plant is young, up to 3 months old, it responds vigorously to increasingly longer periods of light (up to 16 hours). Daily photoperiods of 16 hours or more will cause cannabis to grow indefinitely in a vegetative phase. The plant will grow about 25% faster under 24-hour lighting. Nutrient consumption increases proportionately. (54)

The photoperiod must be shortened to less than 10 hours to induce flowering and complete the growth cycle. Cannabis flowers quickest with a photoperiod of 8 hours. Thus, mature plants will develop flowers within 2 weeks of short-day treatment. Immature plants require up to one month of long nights to induce flowering. A short light period usually will bring cannabis into bloom within a month after emerging from the ground, but of course the plants will be very small. Short photoperiods inhibit the growth of stems and foliage, leaves produce fewer serrations in the margins. Flowering is hastened. The number of serrations correlates well with the degree of lighttime treatment.

Erratic lighting will confuse cannabis. V. Sofinskaya studied the conditioning of hemp with lighttime, and observed the following effects:

“The decrease in day length favored the acceleration of light stage completion but was unfavorable to plant growth. A prolonged short-day treatment resulted in a greater growth delay and in stunted plants, especially when plants were grown under short-day conditions since their emergence. Sharp changes of light conditions during the light stage resulted in various morphological alterations and in the appearance of hemp forms widely differing in habitat. Changes in light conditions during the light stage caused transgression in the normal course of the stadial plant development, resulting in considerable morphological changes of infloresence development as well as in the shape and size of leaves.” (55)

Cannabis must not be disturbed during its night; unscheduled illumination during the dark period will inhibit flowering. Total darkness is required. The flowering response of hemp is controlled by the length of the dark night, not by the length of daylight. As little as 0.03 footcandles (FC) of red light interrupting the dark period will inhibit the anthesis of hemp. A long night thus becomes two short nights separated by an extremely short day, such as 1 minute of illumination.

Very long nights cause hemp flowers to ripen more quickly. This technique is most effective after the 4th week of the flowering phase. Far red light (supplied by incandescent spotlights) can reduce the time required for the flowering phase by about one week.

Cannabis will grow with as little as 800 FC of light, but the growth will not be vigorous. A minimum of 1500 FC is required for a healthy crop. When grown in a short-day regime under low-intensity light, cannabis becomes starved for photons. The hypocotl elongates excessively during the first 2 weeks after the plant emerges. It may reach a height of 6 inches before any internode leaves develop in the plumule. If the illumination is intensified, the plants may survive, and they will develop a clockwise spiral twist in the cotyledon.

Low light levels also produce smaller, thinner leaves, elongated internodes, reduced concentrations of chlorophyll, and less dry weight. High levels of light shorten the growth period, stimulate branching and budding, and increase the production of red anthocyanin pigments. Excessive light causes dessication, bleaching due to photodestruction of chlorophyll, and then necrosis.

Laser light has similar effects. G. Krustev, et al., used a He-Ne laser (632.8 nm/15 & 30 minutes) and a nitrogen laser.  The sowing qualities of the seed are improved, the phases of plant development are shortened, the plants are more vigorous, and the yield of seeds and stems. (83)

Rejuvenation — The growth cycle of cannabis usually lasts about 16 weeks. When cultivated indoors, however, cannabis can be rejuvenated after it has bloomed and begins to go into senile decline. Some varieties are very amenable to rejuvenation after their flowers have been harvested. The plants should be cut back to the second branching node. Let as many leaves as possible remain, and a few buds. Give the plants at least 18 hours of light daily. New meristems will develop within three weeks. Extra nutrients (especially N) must be supplied at this time, or the new flowers will be male. The process can be augmented with foliar sprays of Indole Acetic Acid (IAA) or Napthalene-AA. The soil should also be treated with the hormones. Hemp can be rejuvenated repeatedly with such treatment, thus living several times longer than usual. Even without continuous-light rejuvenation, female hemp may live several months longer after flowering if the plant remains unpollinated. If female plants become senile between rejuvenations, then sex-reversals usually occur, especially under the influence of short-day photoperiods after the continuous-light treatment. In such a case, about 90% of the females reverse to male or hermaphroditic intergrades. (56-60)

Rejuvenated cannabis blossoms from the terminal bud or from lateral buds below the infloresence. Usually the first few leaves on rejuvenated plants are entire (smooth edged). After several such leaves have developed, subsequent leaves again have the usual serrations. When grown under continuous light, the phyllotaxy of the branches changes from opposite to alternate at some point after the seventh node. Plants grown with normal long-days do not change their phyllotaxy until 12 internodes have developed. Rejuvenated plants are very sensitive to tobacco smoke and can be killed by it.

D. Kohler researched the effects of short and long days on hemp morphology, and found another way to rejuvenate cannabis, based on its response to light:

“In short-day and long-day hemp the first leaves are simple and comparatively broad, the later are divided, their leaflets being comparatively narrow. The size of the leaves following one another is continuously increased. Plants begin to flower (qualitatively reacting short-day hemp in short-day only). The shape of the leaves produced in the infloresence is determined in the first days of flowering: they become more and more simple and their leaflets comparatively broader. The leaf size is influenced by the length of day. The leaves of plants kept in flower-inducing daylength grow less and less due to competition between reproductive and vegetative organs, whilst the leaves of flowering plants, which are transferred into longer day, grow larger and larger. In this case the latest leaves are of the same size and shape as the earliest one; a second life-cycle starts, whilst the plants in the original daylight are dying. Considering the photoperiodic response of hemp, leaf-size is a measure of the physiological age. With monoecious hemp a certain leaf size is necessary for the formation of male flowers. If female plants are put into longer day during blossom, they do imitate the male habit.” (61)

Ocra Wilton found a correlation of cambial activity with cannabis’ flowering and regeneration:

“A study was made of cross-sections of all the internodes from the tips to the bases of the stems… When Cannabis sativa has reached an advanced stage of reproductiveness, the meristematic tissue of the stem tends to become entirely differentiated into xylem and phloem elements. This anatomical condition is a possible explanation of the death of such plants at the close of one reproductive cycle. The cessation or decline of cambial activity which accompanies the production of flowers in C. sativa progresses from the region of the infloresence toward the base of the plant, which it may or may not reach depending on the degree of reproductiveness which the plant attains. Vigorously vegetative plants have an active cambium throughout their stems… a certain amount of at least potentially meristematic tissue is necessary for a renewal of vegetative growth in stems.”

Photoperiodism —- Photoperiodic control can be very useful to the cannabis breeder. If yield is not important (as is often the case in the early stages of a breeding program), the time required for the life cycle can be greatly reduced by using short photoperiods. Thus, several generations of plants can be produced each year. Under such conditions, cannabis will flower when it is only a few inches tall. (62)

Photoperiodic control makes it possible to synchronize the flowering dates of male and female plants, thus making possible their cross-breeding. Most importantly, photoperiodic control enables breeders to stimulate the production of male flowers on female plants. Self-pollination can be accomplished only by means of such flowers. Male flowers on female hemp do not contain the Y (male) chromosome; they produce only female pollen. When this is used to fertilize female flowers on female plants, they will produce purely female seeds. The pollen from male flowers is of two kinds, and usually produces a ratio of males 1:1 females. A few viable seeds can be obtained from female flowers produced on male plants and self-pollinated, but such seeds are only weakly fertile and produce mostly female plants.

Report: Edible Hemp Foliar Sampling Project


What is Sensemilla?

This is another cannabis-related word heard from time-to-time.  It translates as “without seed”.  The word was developed as the marijuana culture learned that unpollinated flowers produced more THC (or that, in reality, pollination reduces THC).  Flower without seeds then became more valuable because oi this.  “Sensemilla” as a tem became more of a brand for marijuana, suggesting higher potency.  Some confuse the term Sensemilla for a variety.  It is not, it just refers to a growing practice of seedless flowers by quarantining females from males.

Upcoming Events for Hemp Producers:

JUL 2    8:30 AM

2019 Seed Growers Field Day

A field tour that will highlight Cornell research on small grains, forages, biofuel species, and industrial hemp. Topics to include breeding, varieties, pathogens and pests, seed production, and…

Location: New York Seed Improvement Project (NYSIP) Foundation Seed Barn, 791 Dryden Road (Rt. 366), Ithaca NY

 Event type: Field Day

JUL 31

Greenhouse IPM In-depth Hands-on Workshop

The latest in pest management methods for commercial greenhouse producers. Topics will include best practices for beneficial insects, greenhouse sanitation to prevent pests, diseases and weeds and…

Location: Morrison Hall, Cornell Campus, Ithaca

 Event type: Conference/Workshop

Empire Farm Days

August 6-8 – Tuesday & Wednesday 9-5, Thursday 9-4

The Empire State Potato Growers held their first agricultural show in 1931. Over the years, the event became Empire Farm Days. Rodman Lott & Son Farms in Seneca Falls has hosted the event since 1988.

The event is the largest outdoor agricultural trade show in the Northeastern U.S., showcasing all the latest tractors, farm implements, dairy industry innovations alongside working demonstrations, live animal seminars, and more than 600 exhibits loaded with the latest agricultural information for successful farming.