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What To Do When Your Cannabis Plants Grow Too Tall

Everybody loves big cannabis plants, but sometimes they can get too tall. This is bad news in a restricted space or for the stealth outdoor grower. Find out how to control your overeager marijuana plants with this informative blog from Royal Queen Seeds.

WHEN CANNABIS GROWS TOO TALL

When marijuana is exposed to prime growing conditions, it should fulfill its genetic potential. Sometimes, this means your plants will grow too tall. This can easily happen with sativas that stretch substantially or Kush varieties that tend to have generous distances between nodes. Some species of cannabis stretch notoriously when they enter the flowering phase, even doubling their vegetative height in some cases. Although the whole idea is to get the largest of plants with the heaviest yields of mature flowers possible, depending on the situation, plants that are too tall can be inconvenient.

Often, space is limited indoors and plants that are too tall can exceed the height of the grow space. If the grow room is a multi-strain grow, raising the lights to suit the taller plants can deprive shorter plants of sufficient lighting. Outdoors, especially in stealth grows, plants that are too tall can attract unwanted attention. Controlling the height of your marijuana plants is not very difficult and can be done in a number of ways. Here are a few techniques that can keep the height of your marijuana under control.

THINK AHEAD

If you are well aware that the strain you are growing is going to be tall, then height control starts in the early vegetative phase. Alternatively, the size of your grow space may require height control no matter the strain. Topping or fimming when plants are young encourages a shorter, but bushier plant. Grow room space still needs to be considered, however, as you are exchanging height for floor area.

Air circulation is a priority when plants become bushier in tight quarters. Controlling the height of your crop, only to suffer from mould or fungus due to lack of circulation is definitely a no-no. Lollipopping your plants will guarantee good air circulation below the canopy, preventing pathogens from taking hold. This also has the added bonus of chunkier and weightier colas come harvest, with less unwanted popcorn buds.

TRAINING

LST or low stress training is the art of gently tying the branches of your plants to grow more horizontally. Either the main stem is tied down, encouraging the side branches to be dominant, after which the side branches are also tied down; or the plants are topped a number of times and the new growth is tied down in the mainlining style. LST and mainlining have the benefit of exposing more of the plant to light, therefore encouraging weightier flowers all over.

What’s great about these techniques is an even canopy instead of one main cola. The extreme of this style is ScrOGging, in which the whole plant is encouraged to grow horizontally. A screen of netting is used to hold down all new growth until the plants form a mat. Appropriately called “screen of green,” one or two plants may fill an entire grow space.

PRE-FLOWER TOPPING

Letting your plants grow untouched during the vegetative phase, then topping just prior to flowering also controls height. When your plants are about to enter the flowering stage, top all branches. This has the effect of reining in the flower stretch. The plants are urged to produce new flower growth instead of focussing energy on stretching out.

Occasionally, pre-flower topping needs to be a full pre-flower pruning. If your plants fill the grow space during the vegetative phase, there is no way there will be enough room for flowering. It is time to get drastic. Aggressively prune the plants to half their size and switch lights to the 12/12 flower cycle. Don’t worry, cannabis is very hardy and can even recover from being reduced to a stalk with only one or two leaves remaining. Try not to beat yourself up about the loss of time and reduced final yields. All lessons are good lessons on the way to becoming a weed growing expert.

HUMIDITY

Cannabis plants transpire a substantial amount of water. Marijuana prefers a relative humidity (RH) of 50-65%. When they get too big, humidity can become a problem as the plants constantly release water vapor into the air. Water then gathers on leaves and other surfaces in the grow room, potentially causing mould and other issues. If humidity is becoming a problem, the use of a dehumidifier or increasing exhaust fan power can help. Defoliating can also increase airflow to control humidity. Remove lower and mid-level fan leaves only. This has the added benefit of increasing light penetration to the lower flowering branches.

TEMPERATURE

As with humidity, when plants get too big, temperature in the grow-op can rise. Reduced airflow and excessive height can drive up temperature and position plants too close to grow lights. Cannabis thrives when the temperature averages 25°C. When temps exceed this, plants are more likely to stretch. High temperatures can also affect leaf and flower formations, as well as final bud flavours and potency.

LIGHT INTENSITY

If your plants are getting too tall or too hot and you have the room, simply raise the grow lights as high as possible. Remember, lights that are too far from the tops of your plants can also cause stretching, so make sure to achieve the ideal light positioning.

Light intensity can be adjusted as a method for controlling growth speed and height. Some grow lights have adjustable outputs. If your plants are getting unruly, reduce the intensity. If you are using more than one lamp, simply turn some off.

During the flowering phase, growth can be controlled by reducing the photoperiod – but only if your plants have been flowering for at least six weeks. Reducing leaf production and stem stretching can be achieved by reducing the “lights-on” period from 12 hours to 11 or 10 hours.

CHOOSE WISELY

Appropriate strain choice can help avoid height issues before they even arise. If you have limited space, your dreams of cultivating a classic sativa will need to remain just that, dreams. Stout indicas and hybrids or autoflowering genetics can keep heights under control. Many autoflowering strains are ideal for indoor growing as they have relatively no vegetative period. Some strains will not grow taller than 50cm, but will still provide excellent yields per m².

DON’T WORRY, BE HAPPY

With a little bit of consideration and forethought, height need not be an issue when growing marijuana. Some research into strain characteristics is the first, very important step. Then, appropriate growing techniques will make sure you get lots of yummy buds with no drama. Happy growing!

Cannabis is a hardy plant that can grow too tall for your grow space if you just let it go. Sometimes you need to take charge to keep things under control.

What is the Optimum Final Height for Cannabis Plants?

Is there an Optimal Final Height for the plant?

Most everything I read says to continue the Vegetative Stage until the plant is about 1/2 its desired height, then flip to Flowering Stage, because plants about double in height on average.

Assuming you are keeping the canopy as flat as possible. Should you try for as tall as your tent/light will allow without burn? Or is there an optimal height found to give best yield/quality? Does the plant waste energy moving the nutrients up a taller plant?

I see amazing pictures of bountiful plants that appear to be under 36″, with lots of fat dense colas.

When it comes to training cannabis plants indoors, in the best-case scenario, you want your plants to be big enough to support as much bud as your grow light can produce. There’s no point in having a whole bunch of bud sites located below where light can reach because buds that don’t get any light tend not to fatten up. So, when growing indoors the optimal length of buds is heavily dependent on your grow light.

Optimal Plant Height Depends on Your Grow Light. Bigger Lights Can Support Bigger Plants and Longer Buds!

If you examine the structure of an indoor cannabis plant at harvest, you’ll often see that there are long fat “colas” at the top, and underneath there are smaller buds. The further down you get on the plant, the smaller the buds are. After a certain point, the buds are so small that they don’t really add any significant weight.

Big buds form on top, but as you get further from the light, the buds get smaller until there are almost none. If these plants had been allowed to get any taller (with everything else the same), they likely wouldn’t have produced much more bud than they did here. Any extra time spent in the vegetative stage likely would have been a waste of time.

This was the longest solid cola I’ve gotten from a 250W HPS grow light; it was about 12″ long. Below that point, the plant still made buds, but they were individual buds as opposed to a long cola. The final height of a plant should generally be about twice the height of your longest main cola. That tends to be the “sweet spot” for a lot of strains.

If nothing else were changed, the yields would not be that different whether the plant is 2 feet tall or 4 feet tall under a 250W HPS, because the light doesn’t go down that far into the plant. However, a bigger grow light could have supported a taller plant.

A too-tall plant isn’t a big deal if it fits in your grow space, but the extra lower growth that doesn’t produce bud is essentially a waste of electricity, time and money, since you potentially could have shaved weeks off your vegetative stage without sacrificing yields!

Under a 600W HPS, I haven’t ever seen a main cola that’s much longer than 2 feet even if light is getting down almost to the floor. So, I’m not sure how much benefit you would get by switching to 12/12 after the plant is 2 feet tall. These plants were switched at around 20″ tall under a 600W.

You can support even taller plants and longer colas under bigger grow lights!

Examples with Common Grow Lights

Although it’s true you want to flip to the flowering stage when your plant is about half the final desired height (since it will about double in size after the flip to the flowering stage), here are some general guidelines that have worked well for me:

Note: Always try to do an “autopsy” after you grow and take a hard look at your pictures to see if there’s anything you could have done to get even better results! I learn something new every grow!

  • CFLs, T5s and Other Fluorescents – Switch when plant is 6-12″ tall (unless you also have light from the sides, then it can be a bit taller as long as all the bud sites are getting light)
  • 250W HPS– Switch when plant is

12″ tall
400W HPS– Switch when plant is

17″ tall
600W HPS– Switch when plant is

21″ tall.
LEC or CMH Grow Lights– Follow general guidelines for HPS lights based on your wattage (a 315W is good for

15″ tall, and a 630W is good for

22″ tall)

  • LED Grow Lights– Unfortunately, this depends a lot on the model. It’s partially a matter of trial and error because there’s no “standard” with LEDs like there are with other grow lights. A good rule of thumb is to take note of the length of your main colas after the switch to 12/12 (from a previous grow), then try to initiate flowering with your new plants when they’re about that height. So, if your main colas end up around 12″ long, initiate flowering when the plant is 12″ tall, etc. But if you aren’t sure yet, I recommend just sticking to the “half the final desired height” rule with LEDs, because that ensures that at least the plant will fit in your grow space and you’ll be able to learn what to expect from your LED model.
  • Note: Defoliation (removing leaves to expose bud sites) lets you produce bigger buds further down into the plant! That means your light could potentially support slightly taller plants with longer buds if you use defoliation. This is why it’s important to always test your plants with your light and your setup, as everyone’s results will be a little different based on their strain, environment and personal growing techniques.

    “Tall” vs “Short” Strains

    Although plants typically double in height after the switch, some particularly tall and short strains can stretch more or less than average! Always pay attention to what the breeder tells you about your strain and plan accordingly!

    • If you have a “tall” strain, switch to the flowering stage earlier than recommended (1/3 the final desired height)
    • With a “short” strain you should wait until the plant is taller before the switch (3/4 the final desired height), to ensure the plant is tall enough after it’s done stretching.

    In the picture below, the grower could probably have switched to 12/12 earlier without hurting yields because the buds at the bottom are not adding much weight. Sativa plants like this can triple in height after the switch to the flowering stage, so it’s common to end up with a Sativa plant that’s far taller than expected!

    Optimum Cannabis Height Outdoors?

    Height restrictions are much different for outdoor plants that mature under the powerful light of the sun. Outdoors, plants can keep getting taller and taller as long as they get enough direct sunlight a day and have enough root space.

    Root space is more critical outside and with soil than indoors or with coco or hydro. Root space for outdoor plants is usually provided with big fabric pots (600+ gallon containers in some cases!) or with raised beds full of good soil.

    This plant got 12 feet tall in just one summer, with a 3+ foot cola at the top! It received 9+ hours of direct sunlight a day. Unlike an indoor grow light, the size of outdoor plants is limited by the root space and the number of hours of direct sunlight a day!

    So What’s the Best Plant Height?

    Ultimately, there is no “right” or “best” height for a cannabis plant, and like most things when it comes to growing, it depends a lot on your setup. I’ve given you some general ideas of what to aim for and what to expect, but it’s also very simplified. Unfortunately, there’s no single formula that fits every growers setup. But the following pictures will hopefully help give you some ideas!

    Check Out Examples of Cannabis Plants That Are Too Tall, Too Short, or Just Right!

    This marijuana plant got excellent yields for its size under a 600W grow light, but notice how the buds are thick all the way down. This is a clue that this plant likely would have produced more if it’d been allowed to get a little bigger before switching to the flowering stage – you can see the buds “want” to go down further than the bottom of the plant. Additionally, there is empty space behind and to the left side of the plant. If the plant had been trained to grow wider in the vegetative stage, those empty spots would be full of colas, too!

    In this example, the plant was “lollipopped” (the lower part of the plant was stripped of leaves and bud sites before being switched to the flowering stage). However, on the parts of the plant that weren’t stripped, you can see significant buds going a lot further down. This plant likely would have yielded even more if it’d been allowed to keep more bud sites. The grower could have still stripped all the leaves, but they may have gotten better results if they’d left bud sites for at least a few more inches if not all the way to the bottom.

    Notice how small the buds are at the bottom of this next plant. If it had been allowed to get much taller it likely wouldn’t have produced significantly more yields, but it would have taken extra time in the vegetative stage. This is an example of what the plant should generally look at harvest like if it’s the proper height – about twice the height of the big buds on top, with significant but small buds at the bottom! It could have been a little shorter and probably not lost any yield, but definitely an example of a good final height!

    Here’s another example of a plant that was a good height at the end. You can see there isn’t a whole lot of extra growth at the bottom with small buds. However, you can see the buds end where the thick layer of leaves begins. This grower could have used some light defoliation to expose more bud sites lower down, and produced buds further down into the plant. In that case, their grow light could possibly have supported a plant that was even taller!

    Another example of plants that were a great height at harvest!

    This plant was under a 1000W light and has huge, thick, arm-sized buds that go basically to the bottom of the plant. These buds are so thick at the bottom where they end that it’s good evidence this plant would have produced quite a bit more bud if it had been allowed to get taller in the vegetative stage. Those colas would have been longer, and there would have been many chunky buds underneath. This plant should probably have been about twice the height (twice the size of the longest cola) to have really produced what it was capable of under this grow light. But the grower still had a whole lot of bud to console himself with 🙂

    What to Remember About Plant Height

    • The optimal plant height before flowering is about the length of your “main” (solid) colas. For your first couple of grows, you’ll have to guess (and I gave some guidelines above to help get you started), but after some experience with your setup you’ll be able to dial it in perfectly every time as long as you do strain research first.
    • Increasing overall plant size and number of bud sites in the vegetative stage (while staying the proper height) will increase number of colas and ultimately yields.
    • Training the canopy to be flat, yet wide enough to fill the entire space under the light ensures all cola are a good distance to the grow light in the flowering stage.
    • Letting plants get taller than your light can support will result in wasted time in the vegetative stage!
    • Defoliation lets you produce bigger buds further down into the plant by exposing more bud sites to light!
    • Again, always try to do an “autopsy” after you grow and take a hard look at your pictures to see if there’s anything you could have done to get even better results! I learn something new every grow!

    Now that you have a better idea of the proper plant size and its relation to your grow light (and how to diagnose after the fact whether you should have let the plant get taller or shorter), I’m hoping that some of you will be able to either increase your yields by letting your plants get to the right size, or save time in the vegetative stage by switching to flowering before your plants get bigger than necessary! 🙂

    P.S. One Last Thing About Plant Size…

    In addition to the height, the overall size/mass of the plant has a big effect on final yields. A bigger plant can simply support more and bigger colas. Because of that, it’s good to build up overall plant size as opposed to just height to maximize yields. You want plants that are wide and flat like a table, not tall and skinny!

    Many indoor growers let their plants get bigger horizontally while restricting the plants’ ability to grow taller than the grow light can support. This lets you keep adding more and more bud sites without letting the plants get too tall.

    In addition to making sure your plants are the proper height, you also want to train your plant canopy to be flat and wide before switching to the flowering stage. The best yields are achieved by filling the entire grow space front-to-back and left-to-right with a flat canopy because it lets all the buds be close to the grow light.

    Maximize your yields by filling your grow space with colas!

    Too-tall cannabis plants waste time and electricity, but too-short plants end up hurting yields! Learn how to find that perfect sweet spot for plant height!