Found this article on stretching.
Stretch - what is it and how you can minimise it.
What is stretch?
'Stretch' is a term for the natural vertical growth spurt in early flowering. Cannabis will grow in height throughout most of its flowering phase, but the first 2 to 3 weeks of flowering will show the most dramatic change in height and internodal distance. Stretch can also occur in the vegetative growth phase, naturally and artificially induced.
Some varieties will natrually stretch a lot more than others. Tropical and land race pure Sativas will stretch a lot more than typical landrace Indicas. Hybrids of the two will display characteristics somewhere between the two extremes and many pure Sativas are crossed with true breeding line Indica's for the specific reason to control height and improve yield. It's rare to find a Sativa hybrid with no Indica in it these days.
Many growers when growing a strain for the first time will use the 60/40 theory for predicting harvest time. This is done by regularly measuring the plant through the primary 'stretch period' which is usually at the switch to 12/12. Once this main 'stretch' period is over and the growth slows dramatically (usually after about 2-3 weeks of switching to 12/12) then 40% of the plant ripening time has passed, so there's 60% more time to go till harvest. eg The plant starts rapid stretch at day 8 of switching to 12/12 and this stretch stops or slows considerably at day 26, the plant will be ready for harvest around day 65.
Stretch can, in some situations be beneficial, some growers prefer a little stretch to allow budsites to fill in correctly, but buds with perfect density can be grown by controlling stretch. Encouraging it 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.
Why can stretch be a problem?
Because in indoor grows, as opposed to outdoors, it can severely reduce yield and this stems from the limitations of artificial light and the plants adaptations to it. Stretch can also produce 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 versus internodal distance
Single point light source light intensity diminishes exponentially to the square of the distance travelled - this is the inverse square law. Light Intensity = Intensity (divided by) distance squared. Sunlight is not affected by this. Cannabis requires high light levels (between 30-50w per square foot for HIDs and 2,500-3,000 lumens for vegetative growth and 10,000 lumens for flowering for both HID's and CFL's ) 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 grow 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.
So we know stretch is a fairly undesirable trait to have, but what can we do about minimising it? There are a number of techniques we can employ and moving the light closer is only one of the cures and the causes!
To mimimise all the causes of stretch we need to employ most if not all of the techniques described below and it should be noted that not all these techniques apply to all stages of growth and some only apply to the first 2-3 weeks of flowering.
Grow a compact strain
As mentioned earlier some strains will naturally stretch less and have tighter internode spacing than others. Indicas tend to have minimal stretch growing characteristics in flower compared to tall-growing Sativas, and Indica/Sativa hybrids will stretch somwhere between Sativas and Indicas depending on how much of each variety is in the hybrid, also whether the stretch and long internodal distances were kept in as a dominant trait by the breeder.
Keep light at the optimum distance
Low light levels will cause seedlings and plants to stretch and produce poorly.
“….High light inhibits plant growth - plants tend to grow taller when there is less light."
Use adequate spacing between plants
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.
"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. Pruning in SOG systems is not recommended, as this promotes bushiness.
Use blue-spectrum light
Using predominantly blue spectrum lights (5,000k - 6,500k) as found in MH lights and CFL's exclusively during vegetative and early flower phases will help keep internodal length shorter. 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 Infra-red that encourages stretching. The best mixture of light for flowering is probably 3:1 or 2:1, HPS to MH.
Minimize night/day temperature fluctuations
According to Cannabis Culture:
"Plant internodal length is directly related to the difference between day and night temperatures (or lights on and off) – 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.
Use supplemental side lighting (vertical bulbs)
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.”
Grower's tips
Use blue spectrum 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. 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
Males will typically stretch much more than females and are easily identified and pre-sexxed by their extra height.
Based on an original FAQ which appeared on the old Overgrow site.
I'm pretty psyched by all the bud sites I see. They are getting so thick I'm constantly tucking fan leaves under them so they get the light! 
Thats still less than what they recommend on the label though. I feed the bombs half strength and the pineapples quarter strength. I could probably feed the bombs more as they have never showed any signs of burning. 
