Spectrum:
The spectrum of the light source has an effect on how plants grow and develop, using several light pigments to control how and where photosynthate energy is to be used. Blue light inhibits cell expansion, and is important for cannabis during vegetation. Blue light keeps cannabis short and compact, allowing easier management of height and better distribution of branching characteristics. Around 30% blue, such as that with metal halide, are good amounts to use during veg. But should not be used for flowering.
Light levels:
Light intensity has a direct proportional effect on elongation. And also has a direct effect on overall plant growth. Increasing the total light by using more powerful light sources, can increase overall plant growth as you are providing more energy to the plants. Causing excessing branching and elongation. For sativa dominant strains, light levels around 20,000-40,000 lux (250 - 400umols) is a good range to limit stretching. Where as indica dominant strains can go right up to 80,000 lux (1000umols)
Temperature Differential:
Another factor that affects stretching is temperature differences between day and night (or lights on/lights off). When daytime temperatures within our grow space are much higher than those at night, plants are induced to produce more gibberellin, causing them to elongate considerably. By reducing the temperature difference you can downregulate the production of gibberillin and reduce elongation. You can take it even further by creating a negative difference where night temperatures are higher than day temperatures. But where it can be avoided, a temperature difference between 5-10°F is best as plants have been shown to grow best with these levels.
Nutrients:
Nutrients can have an effect on plant growth behavior. during elongation plants consume large amounts of macro nutrients, specifically nitrogen and phosphorus. So by reducing nutrients available to plants you could potentially reduce stretching. Nitrogen limitation is the main factor for this, as it has a large responsibility towards many physiological cellular developments related.
Air Movement:
Air movement can limit stretching by focusing energy on the strengthening of branches and cell walls. Making sure there is always good stimulated plant movement through air movement is beneficial for many factors including elongation.
Genetics:
Different genetics display varied characteristics with branch and stem development. Sativa dominant plants are known to be taller due to their evolutionary conditionings where they origonate from. Where as indica dominate are known to be shorter and more manipulative with training techniques as a result. But even different genetics within the same dominant family, can have completely different characteristics from eachother. So researching your strains growing characteristics may help to prepare for any height issues you may experience.
Flowering Stretch Timing:
The reason for the flowering stretch has less to do with genetic reasons and the flowering process and more to do with the fact that the light cycle has decreased and so less blue is provided to plants overall. Plants use several key light spectrums to generate morphological changes to adapt to the ever changing environment.
Blue light is responsible for many morphological functions including the elongation of petioles and stems. Where blue is used to determine whether plants are shaded or saturated with light. The decrease in overall daily light from switching to flower cycles, sends signals that generate these morphological characteristics, which increase stretching.
Timing when to flower is importent to prevent excessive elongation and overgrown plants. Too early however and the plants will not develop enough to fill the required space. Too late and plants will tend to be excessively tall with more foliage than required.
Training/Pruning:
Training and pruning are techniques that can be used to reduce the effects of stretching by manipulating the plant into a shape of your choosing. Topping is one good example where terminal apical dominance is shifted to multiple branches that compete for apical dominance. Supercropping is a great emergency technique for lowering your overall plant height imediately. If you find you are absolutely stuck and need to reduce the height of your plant quickley, it is a good idea to look into supercropping. Scropping can also increase yield if there is unoccupied horizontal space which has not been filled in.
As always please comment your thoughts or any questions on the subject.
@drphoton hey. Wasn’t sure where to put this. Have a very sativa leaning plant in my room. Getting ready to flip and was wondering what to do. I need her to grow out a little (need more branching ect but not so much height) would keeping MH in for first 2 weeks after flip? Then let the last week of stretch go full blast? Kinda stuck here lol. I need both sides. Need her to not get much taller but I need her to thicken out with branching.
She’s total of 35 height. Also, I hve an extra light after having to toss 3 plants. Would I gain anything putting my extra light in middle of the canopy and leaving the MH bulb in for the whole grow??
Hey @Drphoton have another one for you lol. 2 of the plants in room are noticeably smaller than the rest. I’m keeping the room under MH for the first 2 weeks to limit stretch. These two plants could use 2 weeks under hps and not 1 I think. If I had one lamp hps will it affect the rest of room? Itd hit parts of a couple plants but nothing major just maybe 2ft overlap or so? I can add pictures once I get home to give you a better idea of situation.
@Drphoton Hi, hope all is well with you.
Im currently running one 315w c.m.h. and due to an accident with the second c.m..h. i have had to pop in a 400w HID was on the m.h. and now just switched in the hps bulb, they are hanging around 14 inches from the plants, do you think i should keep them both at same height? two on the right seem to be less stretchy and theyre all related as in came from same mother plant as seed.
Also do you think that the infra red light on a wifi cam if left on for night vision purposes, would be distracting to the plants when lights are off.?
Thank you for your thoughts and help.
💚🖖
I didn't see this noted and thought it would be useful here. Many indoor growers will start Sativa varieties under a 12/12 photoperiod right from the point of germination, as often this is the only way to keep these fast-growing and hugely vigorous plants to a manageable size and grow them within the restrictions of an indoor growing space.
To avoid any potential problems with the height of the plants in our indoor grow, or to grow plants that have a considerable stretch at the onset of bloom, we can begin the flowering phase with a photoperiod that uses fewer hours of light (11/13 for example), so the plants will stretch less during the preflower stage. Once the plant stops its vertical growth, usually within the first two weeks, we can change back to 12/12 hours for greater yields, even to 13/11 if we don't mind lengthening the flowering period for a few days more.
Great information. I'm using the 12-1 method from Neilson's Greenhouse Management book, still haven't given up on it or felt it's hindered at all, especially learning about DLI here and around. I started as an outdoor grower avoiding the legal issues and HPS days and always just plant my seedlings in full sun outside and they usually do as good or better.
Using 12-1 I do 350-400 safely. I think I could do 500 or so with 12-1. 18-6 a 300par is similar to a 450par on 12-1 but a much longer duration to endure and about half the resting dark time. I find if the first Cotyledons get a nice blast of sun the roots and plant adjust right away I guess. They are really thick durable leaves. 12-1 and keeping a fairly high DLI with 350-700 par with an 800+ hardening peak is enough to create a low and thick canopy. I find they don't stretch much and little need for training mostly.
I did mess up a grow using the Photon par App until I got used to a quirk with it reading 100 par less if I held it horizontally instead of vertically parallel with the longer lengths of the panel or bars, which is strange but evident. Also having 4 100w panels in a 3x3 and having to use your arm to get into the back corner lights, ends up blocking a lot of cross lighting, and if you combine the two problems you get bleached and stressed out plants during the more tender final stretch new growth moments?
I notice with an 18-6 schedule you come from a lower 500 or so max par DLI but want to go up to 800+ for flower on 12 preferably to prevent stretching and match your DLI, which if done too fast or right away will also burn even mature leaves, so a flowering switch from 12-1 (12on 5.5off 1on 5.5off) to 12-12 (or 8on 16off as directed)* will allow a seamless light level switch, allowing me to have a really high par of 800-850 during final stretch, which worked for most plants, but not my 3x3 incident lol...
The default 18-6 to 12-12 DLI curve most commercial growers recommend avoids a high par during final stretch I guess, or is that to the contrary? And do you think 12-1 may be better to deal with plant stretching during flip to flower? I'm thinking so but I have never tried and 18-6 vegetation period yet myself.
