Stretch is a term for the natural vertical growth spurt in early flowering. Marijuana will grow in height throughout its flowering phase, but the first 2 weeks of flowering will show the most dramatic change in height and internodal distance. Stretch can also occur in the vegetative phase.
Stretch is generally considered an unavoidable evil, but some gardeners prefer a bit of stretch to allow budsites to fill in adequately. Buds with perfect density can be grown by controlling stretch.
Encouraging stretch can be a good technique to prevent budrot in susceptible strains and massive colas. Elongated buds are less dense, and the humidity within buds is reduced.
Note: Males will typically stretch much more than females and are easily identified and pre-sexxed by their extra height.
Why is stretch a problem?
Stretch is a problem primarily in indoor grows because it greatly reduces yield. The problem stems from the limitations of artificial light and the plants adaptations to low light.
Stretch also produces tall plants with weak and spindly stems. The weight of the buds later in flower will require staking to avoid damage and falling over. Stretch may be a sudden problem for growers with limited ceiling height.
Light intensity vs internodal distance
Light intensity diminishes exponentially with distance under artificial light (the inverse squared law). Sunlight is not affected by distance and can penetrate deep into a canopy.
Marijuana requires high light levels (At least 30w/m2, preferably 50 w/m2) to thrive and produce high yields. A plant is at its optimum flowering height when all its bud sites are bathed in intense light (ideally the entire plant is equally lit, producing dense bud from top to bottom).
Stretched plants are tall and their lower bud sites receive exponentially lower lumens than top colas. Budsites that are not receiving intense light will not produce well, and often the light and extra plant height is wasted. Tall plants require more energy to pump water to its leaves and transport photosynthate from its leaves. This extra energy could have gone into flowering. Stretched plants can yield 30-50% less.
Lower bud sites that are not illuminated sufficiently will produce under-developed buds: airy or popcorn bud. Popcorn buds have lower dry weight, take much longer to trim, and often have less bag appeal.
How can I minimize stretch?
Note: Some of these techniques only apply to the stretch phase.
Use a compact strain
Indicas tend to have minimal stretch growing characteristics in flower compared to tall-growing Sativas. Hybrids may grow unevenly, or react to stress unexpectedly.
Keep light at the optimum distance
Low light levels will cause seedlings and plants to stretch and produce poorly.
from:
http://www.hrt.msu.edu/course/HRT100/lec8.htm
.High light inhibits plant growth - plants tend to grow taller when there is less light."
Use adequate spacing
Very high planting densities will encourage stretch. As the plant grows, its leaves will quickly overlap its neighbors. Eventually the sub-canopy will darken and lower portions of the plant will experience significant shading.
from:
http://ohioline.osu.edu/sc154/sc154_17.html
"Overlapping of plant parts reduces light intensity and changes light quality, resulting in light below plant canopies that is richer in near-infrared and far-red which encourages plant stretching."
Pruning, FIM'ing and topping
Pruning plants in veg and early flower can effect the flow of auxins within the plant. Re-distributing natural growth hormones from terminal shoots to lateral shoots is a good way to slow height growth.
Note: Pruning in SOG systems is not recommended, as this promotes bushiness.
Use blue-spectrum light
Using MH light exclusively during vegetative and early flower phases will help keep internodal length shorter.
From: What do all of those lighting terms mean?
The light produced by metal halide lamps is in the white-blue spectrum, which encourages vegetative growth and "bushiness" while discouraging upward growth. This is the bulb to use in the first, vegetative phase of plant growth.
HPS bulbs are the preferred lighting source for flowering. However, most HPS bulbs have a harsh and limited light spectrum concentrated in the far red and IR that encourages stretching.
Growers have a variety of bulb options to increase blue light during the vegetative and early flowering phase:
Supplemental fluorescents
MH bulbs, or a mix of HPS to MH (3:1 recommended)
Conversion bulbs (MH bulbs that work with HPS ballasts)
Enhanced HPS
Minimize night/day temperature fluctuations
According to Cannabis Culture : (http://www.cannabisculture.com/articles/1536.html)
"Plant internodal length is directly related to the difference between day and night temperatures the warmer your day cycle is as compared to your night cycle, the greater your internode length will be. The opposite also holds true; the closer your day and night temperatures, the shorter your internodes will be."
Minimize stress
Stretching can be a physiological reaction to a variety of environmental stresses (Low light, high humidity, low/high temperatures, transplant shock, leaf loss, etc). Maintaining optimum growing conditions is your best defense against stretch.
Anti-stretch additives
SuperBud is a popular additive to help minimize stretch and initiate flowering faster. See faq: What is Superbud?
Bonza Bud (contains antigibberillins) shortens internode length while helping the predominance of female flowers.
Growth retardants:
http://www.ext.vt.edu/pubs/greenhouse/430-102/430-102.html#L4
Avoid hormones
Some foliar sprays and additives can encourage stretch. Spraying Growth Plus (containing cytokines and vitamins) during early flower may encourage stretch. "Cytokinine - activates cell stretching..."
Use side lighting (vertical bulbs)
Untested. The Phototropic effects of side lighting may limit the amount of stretch: dark side of the stem grows more than the lit side of the stem. (http://www.hrt.msu.edu/course/HRT100/lec8.htm)
Grower's tricks
Use MH lights during vegging and the first 2 weeks of flowering (during the stretch phase) to minimize stretch.
Avoid high levels of Nitrogen during the stretch. Use a transition fertilizer ratio (2-2-2) during early flowering to maintain a medium level of Nitrogen.
Note: Switching immediately to a flowering ratio from veg can result in premature yellowing. Remember to flush soil out/change the reservoir to remove any high-N fertilizers.
Use oscillating fans to help strengthen stems
OverGrow Links:
why do my seedlings stretch?
What distance should my light be from my plants?
Vegetative growth>Training/Pruning folder
What do hormones/growth regulators contain?
What are hormones and what do they do?