7 Problems with Tall Marijuana Plants

Rambo February 13, 2012 20
7 Problems with Tall Marijuana Plants

Let’s talk about plant height in your indoor garden. If you want to play in the NBA, being a few inches taller can have its advantages. With indoor marijuana plants, this is rarely the case. Excessive plant height in a grow room causes serious problems. These are all problems you will know how to prevent, if you diligently read to the end of this article.

I am going to take it easy on you, and lead with the really obvious problems first.

  1. Grow rooms, by their very nature, have a ceiling. Most of us only have about eight feet to work with. Somewhere between the top of your growing medium and the bottom of your light is the maximum height of the plant. In most cases, this is not more than about four feet.
  2. Taller plants have a higher center of gravity. If a two foot plant in a five gallon pot puts on a few pounds of wet buds, it is just a nice plant. That same weight on six foot plant can tip the whole plant and pot right over. This is an even  bigger problem if you allow your growing medium to dry out. Now think about dominoes.
  3. Plants that grow tall quickly often have flimsy stems. Once the buds put on some weight, this is going to mean a lot of work staking, caging and tying, or a lot of bent and broken branches
  4. The taller the plant, the more stem you find between the roots and the buds, presenting two problems. The plant has used a ton of energy to grow that long stalk that it could have used for grow big fat buds instead. Additionally, the plant must spend further energy pumping water and nutrients against gravity to a higher elevation than necessary.
  5. CO2 is heavier than air and settles in grow rooms close to the floor. This means the roots are receiving much more CO2 than the leaves and flowers, when it should be the other way around.
  6. Tall plants cast longer shadows and decrease the available light to their neighbors.
  7. When plants grow to be tall, the distance between the light and the lower branches increases. According to light intensity principles and the inverse square law, as the distance between leaves and light source increases, the lumens reaching those leaves rapidly diminish. On tall plants, the higher leaves usually shade the lower branches preventing direct light. The only light that can penetrate the thick foliage to the lower branches is far red light between about 760 and 800 nanometers. Far red light activates phytochrome far red (PFR) which signals the shaded branches to stretch to compete for more light, exacerbating the problem further.

Six Ways to Limit Plant Height and Internode Length

Here are five proven ways to limit marijuana plant height and keep the internode length nice and short. Combined, these methods give you complete control of a marijuana plant’s stature.

Light Intensity

Plants stretch because gibberellins (plant hormones) promote longer and quicker cell growth between the nodes when plants are exposed to inadequate light. To prevent this, make sure you have adequate light for the area in which you are growing. Keep the lights as close to the the tops of the plants as possible without burning them. Maintain proper plant spacing so that all of the branches are under direct intense light. To be sure you have enough light, you might consider purchasing a light meter.

Air Movement

Oscillating fans in your grow room simulate a breeze moving the leaves and stems. This movement disrupts plant cell elongation, causing the cells to exchange length for thicker cell walls. The plant is making natural adjustments to stand up to the breeze so that it is not flattened by the first big wind. As an added benefit, oscillating fans help stir CO2  up off the floor, and reduce pest problems.

Negative Differential (Temperature)

The growth activity of many plants, including marijuana, is affected by temperature differences. If the temperature during the day (lights on) is lower than at night (lights out), this  is called negative differential or neg. diff. You can use this to keep your plants short and stocky by increasing the temperature during the dark to a few degrees warmer than the light period. Be careful not to overdo it: a five degree difference is plenty. Some strains will respond to neg. diff. more than others, depending on the climate to which the landrace is accustomed. You need to be keeping track of temperature and humidity anyway so make sure you have a good thermometer and humidistrat.


Topping marijuana plants helps to keep them short for two reasons. First because you have removed at least some of their physical height. It is not efficient to let your plants grow tall and then cut the tops off to make them short again, but it does work. The better reason to top your plants is because auxins (plant hormones) collect in the tips of the leading shoots and drive continued growth. When you pinch these tips off, the concentration of auxins is redistributed to the tips of the other branches. By dividing these auxins among many branches you can  promote shorter, bushier growth.


With some practice you can learn to take a tall plant and train it into a shorter, bushier plant. Branches and tops can be pulled down and forced to grow horizontally either with trellis netting or cages. Low stress training is not a skill you will learn overnight. You will likely break some branches before you master the skill. Not all garden setups easily accommodate heavy training. If it is the only method you use to control a height problem, you have your work cut out for you.


The ag chemical industry has developed several plant growth regulators (PGR’s) to help control the height of various ornamental crops. I am not aware of any that are approved for food production. There are several products targeted towards the hydroponic market such as Super Bud, Bushmaster and Top Load that contain some kind of undisclosed PGR’s to help control plant height and increase branching. I have personally used Bushmaster and found it frighteningly effective in very low doses. Unfortunately these products do not list their ingredients. I suspect that they may simply be watered down versions of the commercially available PGR’s like A-Rest, B-Nine, Bonzi, Cycocel, Florel, and Sumagic. If this is the case, I question if they are safe to use on your marijuana.

When growing a crop meant for consumption, it is your ethical duty to guarantee that your harvest is safe. We live in a world where Monsanto’s lab engineered GMO and chemically treated foods can be found in most every food product. I question how seriously they take their responsibility for our safety. This is not an excuse for the rest of us. Never forget that you are growing medicine that may be used by some very sick people. Always do your best to reduce or eliminate the need for potentially harmful chemicals by mastering good gardening practices and the other five methods of controlling plant height.

The sooner you accept the fact that you will never play in the NBA the better. Instead, master the skills of managing plant height. These skills are easily achievable, will limit your problems, and help maximize your indoor harvests.

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7 Problems with Tall Marijuana Plants, 4.2 out of 5 based on 43 ratings



  1. Ronda July 6, 2013 at 6:27 am -

    Newbie to hydroponic growing. I am three weeks into flowering and two carmelicious plant are about 32 inches, just fine for my space. I have a medjuiana plant that is 48 inches tall and causing lighting problems. Just would like someone’s opinion.

    Thanks in advance.

    • kram July 7, 2013 at 11:14 am -

      Hi Ronda, I suggest you ask this question in the forum where moderators can more easily answer your questions. Also, if you have pictures that you can post that would be helpful.

  2. Rosebud October 2, 2013 at 12:36 am -

    I have heard keeping your seedling in a smaller container throughout the entire grow will help limit the size. I hear the roots will be fine as long as it’s temporary.

    • Rambo October 2, 2013 at 7:52 pm -

      smaller containers will mean watering more often “throughout the entire grow” and “Temporary” are kind of contradictions. I guess the real questions is what exactly do you mean by smaller.

  3. James March 15, 2014 at 5:01 am -

    I have a plant that is about 8-10″ and I’m wondering what I should do next but from what I see my plants tops are turning yellowish but its not in the flowering stage . My plant is about 2-2 1/2 months old and I’m a first time grower. This is actually my 2nd attempt n I’m more guarding this round but I think I might have took to long to fix the problem. So I did some research n found out it is an iron def. I’m doin everything naturally cuz no equipment jus yet . What should I do next ?

  4. budz 420 March 23, 2014 at 3:56 am -

    Hey how do u germinate the seeds?

  5. mitch April 6, 2014 at 12:35 pm -

    I have a single sensi kush plant that I harvested two weeks ago. I cut off the fan leaves and hung it upside down. . anyway,the buds are full of seeds , so it appears somehow I stressed the plant and it is now a hermaphrodite. I have seven girl scout cookie plants in the flowering room, do I have to worry about them being pollinatedalso???Are the seeds viable??? can I use them. Thanks

    • Jesse April 24, 2014 at 11:51 am -

      As far as the germination of your other plants I would worry in fact, I would keep your harvested plant well away . You really don’t want that pollen any where near your girls. Take a close look at all of your plants make sure you don’t have a male hiding out amidst your girls.

  6. Gabe April 13, 2014 at 4:06 pm -

    I have 6 germinated plants that I keep close(2 to 3 inches) away from a ballast cfl as well as a blue plant light. I am attempting to limit their immediate height growth due to skinny stalk. They seem to keep growing, even into the lightbulb. How do I stop this? I am considering putting the stalks back in soil and causing growth again(leaving leaves out). Idk what to do. I currently give them twelve hours light/dark rotation.

  7. Jesse April 24, 2014 at 11:46 am -

    Hey guys new to the site. I have a bubbleberry that is kinda making me worry. She is indoor and about 6ft. I finally topped her cause she just wanted to keep heading for the lights. Now my problem is that I do not believe her pistils are giving me a accurate indication of her window. Her trichomes are cloudy and some are even turning amber as they should. She is a strawberry/diesel and is just so top heavy I’ve hade to both stake and tie her. Please give me any info or knowledge I can use to promote the most sensible harvest.

  8. Jesse April 24, 2014 at 11:47 am -

    Correction the plant I’m worried for is a strawberry diesel cross.

  9. Hortense June 7, 2014 at 2:37 pm -

    Undeniably believe that which you said. Your favorite reason seemed to be on the internet the simplest thing to be aware of.

    I say to you, I certainly get annoyed while people think about worries that they just don’t know about.
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    • jimboslice June 30, 2014 at 7:06 am -


  10. Chase July 10, 2014 at 1:06 pm -

    I got 3 plants there about a week old when do I too them?

  11. Chase July 10, 2014 at 1:06 pm -

    Top them

  12. Ames Nowell July 27, 2014 at 6:34 am -

    If you use a tent you must have good airflow.My weed plants first grow are doing so well I have 4 Dwarf autoflower 1 big bud 2 New York Diesel —My NY Diesel is 4 ft+ good strong branches I use a 1000 watt LED from Growace It works so well I just started 12/12 started on the 20th of july

  13. Ames Nowell July 27, 2014 at 6:36 am -

    everybody knows my lady she gives, she gives ,,,,Head

  14. Ames Nowell July 27, 2014 at 6:39 am -

    The perfect grow—my first grow ————-God loves me–

  15. nODDy July 27, 2014 at 7:11 am -

    I’ve literally just started growing my first batch only days ago. I know nothing about growing, other than you need to begin with red spectrum light. And finish with the blue spectrum. first phase gives a bit of height. And the second does leaves and bud. How soon do I top the plant. And how can I tell how potent she’ll be. As far as I am aware. I have a common strain. Nothing particularly special. Any help would be great. Cheers.