What to do About Slow Growing Marijuana Plants

The waiting is the hardest part. That’s not just a fantastic lyric from the late Tom Petty, singing about sexual frustration. Marijuana growers have similar frustration when it comes to how fast their plants grow.

We know that under ideal conditions, photoperiod marijuana plants need only 3-5 weeks in grow phase before they’re ready for bloom phase. Autoflowering marijuana plants spend 2-3 weeks after germination in grow phase before automatically beginning bloom phase.

A healthy photoperiod marijuana plant grown from seed should be at least a couple of feet high four weeks after germination, gaining at least an inch or more height per day. Plants grown from cuttings should be slightly taller than that.

If the plant is growing slower than what I just described, immediately figure out why, because slow growth easily leads to stunting, and your plants may end up as pathetic little dwarfs.

Even if slow-growing plants eventually get big enough to deliver a worthwhile bloom phase and harvest, the slow growth already cost you lost time and money.

Here are some tactics to help you solve the slow growth problem…

Every cannabis seed has genetics that partially determine how fast the strain grows. Seeds that are old, poorly-bred, or otherwise defective may germinate, but plants lack vigor and may never grow properly or yield big, potent buds.

If you’re growing clones, slow growth and lack of vigor often derive from a worn-out or poorly-maintained motherplant. In general, the older a motherplant and the more times it’s had cuttings taken from it, the less vigorous the clones will be.

Incorrect grow room environment is a major cause of slow-growing plants. Temperatures below 74°F, and especially a cold root zone, slow plant growth.

Out-of-range vapor pressure deficit (VPD) slows plant growth. Read here about VPD so you fully understand important facts about grow room temperature and relative humidity.

Grow rooms that are too hot also slow plant growth. In rooms without added carbon dioxide (C02), temperatures shouldn’t exceed 77°F. In rooms with added C02, temperatures shouldn’t exceed 84°F.

Speaking of C02, grow rooms without grower-added C02 or enough fresh air intake may lack sufficient ambient C02, which slows plant growth.

Root zone conditions have major impact on growth rate. Growers often use dense, poorly aerated, poorly-made root zone media that lacks air space, sufficient porosity, beneficial microbes, and correct pH buffering.

Overwatering causes several root zone problems. Roots drown, unable to get enough oxygen or absorb nutrients. Wet conditions welcome pathogenic microbes and root pests such as fungus gnats.

Inferior hydroponic nutrients pollute the marijuana root zone and don’t contain the right amounts, ratios and types of nutritional elements your plants need. They may also skew root zone pH out of the narrow 5.7-6.3 range it should be in, making it hard if not impossible for your marijuana plants to absorb nutrition.

Realize that most nutrients brands are NOT precisely engineered after testing on cannabis plants, with incorrect feed charts and potentially harmful ingredients.

Growers relying on pH meters and pH adjustments may have defective pH meters, leading them to improperly adjust nutrients water. Read here for the most reliable pH and parts per million meters.

Overfeeding nutrients, which is very easy to do, also slows growth. Root zones that aren’t flushed periodically or are too dense retain nutrients overload and become toxic for roots.

When you see leaf tips turning brown and/or yellow, that’s a sign of overfeeding and other nutrients problems.

To make things more complicated, underfeeding nutrients is also a problem, and harder to detect. Slow growth, pale leaves, and slow or incomplete leaf development are signs of underfeeding. Calcium and magnesium deficiencies are particularly hard to recognize and remedy.

Pathogenic root diseases such as fusarium and root rot, along with root-eating pests such as fungus gnats and root aphids, also cause slow growth.

Pests and diseases aboveground, including mites, thrips, aphids, powdery mildew and gray mold, can also slow growth while crippling plants or rendering buds unusable.

Most growers only think of water and nutrients when they plan marijuana feed programs, but light is food for your plants too.

The amount, duration and type of electromagnetic radiation you deliver to your marijuana plants has a massive impact on growth rate, morphology, resin production, and harvest weight.

As with nutrients, most grow light brands are inferior. Their units aren’t delivering wavelengths based on actual scientific marijuana research. Grow light manufacturers’ instructions about grow light distance from plants may also be faulty.

Read this article about grow light guru Dr. Bruce Bugbee to understand how to feed photons to plants. And know that as of right now, no grow light manufacturer is making a perfect grow light for cannabis, whether it be HID or LED.

The main thing to realize is that not enough light and too much light can both slow down or stop marijuana growth.

If your plants aren’t getting enough light, you’ll see them stretching and growing slowly. If they’re getting too much light, you’ll see them growing horizontally, drooping their leaves, and growing slowly.

If your grow lights have the wrong spectral output, your plants may grow slow or otherwise not develop to their full potential.

Make sure your grow light timers are 100% accurate, consistent, and coordinated for predictable lights-on cycles, and that there’s absolutely no light at all in your grow room during dark cycle.

Most marijuana growers known that topping and/or trimming your plants, especially just before bloom phase starts, slows growth.

The process of “lollipopping,” which removes a plant’s lower branches, stresses the plant and slows growth.

Constant trimming, leaf removal, topping, low-stress training and other plant manipulation similarly slows growth.

These harsh techniques when overused or done incorrectly can kill your marijuana plants, or at the very least result in slower growth, low harvest weight and lower bud potency.

We’re working on an article that tells you how to push your marijuana plants to their maximum growth rate and yield.

But all you need to do right now is look at what the things we just talked about that cause slow growth, and make sure these situations don’t exist in your grow room. 

When you give your plants an ideal grow room environment, along with quality reverse osmosis water, nutrients and grow light spectrum, they grow as fast as their genetics allow. And you’ll be singing this song a little bit less often…