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For the average indoor grower, it is often suggested that shorter, stockier varieties are the best ones to go for, usually due to the low vertical height that is a common issue in grow-rooms, what with that big hot light hanging around from the ceiling and all! There are other important factors to consider: tall lanky plants give more shade towards the bottom, thus leading to weak scrawny flowers in the lower half. You can trim off those puny little buds before they have a chance to develop and rob the juicy ones up top of their strength, but then your plant has wasted a lot of energy growing so tall when it could have used that energy to pack out your flowers instead. So how can we control the height of a plant whose genetics are constantly pushing it to be taller? You may have seen various mysterious bottles of PGR (Plant Growth Regulators) in your local grow shop.

These chemicals are synthetic hormones that tinker with the growth structure of the plant, but it is not known if they are safe for consumable crops and personally I would rather not use them on anything I plan to ingest. PGRs are uses extensively in the commercial flower business, but nobody is eating, vaping or smoking carnations! There are several ways to influence height without resorting to chemicals, so let’s explore these today instead. If you position your grow lamps too high above the plant canopy then you will see stretching happen as the plants race upwards to where the light is strongest. In general your lights should be as close to the tops of the plants as possible without being at all uncomfortable for them – if your hand is cool and comfortable at the level of your plants, then all is well. Be careful not to go the opposite way and have the lights too close, otherwise burning of the plant tops can occur, leaving dead tissue which is unsightly but also an invitation to bugs and mildews. But it’s not that simple, as the difference in day and night temperature is also a factor. The wider the difference in temperature, the longer the distance between floral internodes (the length of stem between bud sites).

In an ideal world you should be aiming for approximately 4 degrees C difference between your plant’s day and night temperatures. If you can get 22 degrees C during the day and 18 degrees C at night, your plant can perform at it’s peak. This seems to be most important to the plant in weeks 2-3 of flowering when the plant is finishing off it’s flowering stretch. I’ve heard of some growers getting extremely short internodes by actually reversing the temperature differential, so the plant is warmer at night. Good air movement around the plants is hugely important for so many reasons, and it also helps keep height down too. Circulating air fast enough to move the plants during vegetative growth is one of the best things you can do. As soon as the seed sprouts or your clone has rooted and can take a breeze without wilting it should be moving around in the path of an oscillating fan. This strengthens the stem, encouraging lateral growth as opposed to vertical, almost as if the plant “knows” it will be blown over if it gets too tall. Now we have our environment sorted, let’s look at some more creative ways we can control our plants height. This involves pinching out the very newest growing tip, thus stunting the growth up there while the lower branches catch up. This happens due to hormones called Auxins in every actively growing tip. The Auxins restrict lateral growth behind the tip on each branch, so when we pull the growing tip off, the lower shoots can develop without being restricted by the hormonal messages coming from above, and they will all race to become the next lead tip. In time the growth tip you removed will be caught up by the two shoots below it, so you will now have two active growth tips where there was one. There is a variant of this technique know as “FIM” (“F**k I Missed!”) legend has it that this was discovered by accident, hence the acronym. FIM involves not picking the growing tip off cleanly below the bulbous growth shoot, but a little higher so you actually leave a tiny portion of the growing tip attached. This still restricts the Auxin as usual, but you may find that three or even four new growth tips will appear from the site. Only use this technique during vegetative growth, as the aim is to get the shape correct before you trigger flowering, thus avoiding plant stress at the crucial budding phase. Low Stress Training (“LST”) is a great technique that can work on its own, or in conjunction with the Topping/FIM described above. You can get super creative with it, and with care make your plant fill whatever space you have.

It involves tying the top shoots of your plant down. The bending of the stem restricts the flow of Auxins in the same manner as Topping, thus making the lower shoots race to be the new leading stem.

After a week or so you can untie the branches and let the plant carry on unrestricted. You can begin LST on a small plant to get the structure tight from the start, thus ending up with incredibly short bushy plants. Go slowly, some sturdy branches will need to be tied down a little further every day over the course of 3 or 4 days to prevent snapping.


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