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7 Reasons Why Cannabis Plants Grow Slow-Part I

Why are plants staying small? You plant your seeds, seedlings appear, but plants are growing soooooo slow. It’s a common problem with seedlings and vegetative plants, but often frustrating to figure out the fix. What’s the cause? How do you make plants grow faster?

This tutorial will help you overcome the 7 main causes of sluggish growth, so your cannabis plants grow blazingly fast! You’ll learn how to make plants grow fast and healthy.

Note: This article is about slow-growing seedlings and vegetative plants. 

7 Most Common Reasons Plant Growth is Slow

Here are the 7 most common reasons your seedlings or young plants are slow-growing:

1.) Grow Light

Plants tend to grow slow if they’re not getting enough light overall. If slow growth is combined with long, lanky seedlings, your plants need more light. In that case, either increase the power of the light, bring it closer, or get a bigger grow light. One recommendation: Foldable Strip Plant Grow Light Six/Eight/Ten Bars Full Spectrum

You can also give plants too much light. Powerful grow lights cause stress and can slow down plants if they’re kept too close. Too much light (when the grow light is too close) results in stress and unusual symptoms that can slow down plants.

2.) Extreme Environment

Cannabis plants enjoy a warm sunny day. Not too dry, not too humid. Similar to humans, actually. Certain strains are more resistant to an extreme environment, but plants grow best in moderate humidity and warm-but-not-hot air. A good temperature range is warmer than 70°F (21°C) at night and around 79°F (26°C) during the day. Humidity around 50% is perfect.


  • Cold daytime temperatures below 60°F (15°C) may slow growth down to a crawl, and freezing temperatures can kill.
  • Heat above 85°F (30°C) can also slow down growth or even slay plants if it stays too hot for too long


  • Dry air under 35% RH can limit growth, and under 25% RH you may even start seeing signs of deficiencies on the leaves and other odd symptoms
  • High humidity above 70% can make plants droopy and slow down growth by making it harder for the plant to move water efficiently through the plant

Seedlings have the most trouble dealing with a poor environment, while older plants tend to be more resistant to temperature and humidity.

This seedling is struggling to cope with a hot, dry environment. For the fastest growth, keep the temperature around 79°F (26°C) during the day and a little cooler at night. Optimal humidity is around 50%.

Cold temperatures can also stunt plants or make them grow at a snail’s pace. This plant experienced a cold night under 50°F/10°C and was droopy the next morning. It took several days for the plant to recover and start growing normally again. To make things worse, cold temperatures cause plants to drink less, so they’re more likely to suffer from symptoms of overwatering when the temperature falls.

Note: Although fans help equalize the temperature of your grow area, too strong a breeze can slow down seedling growth. Leaves should be gently rustling but you shouldn’t see stems waving around in the wind. If that happens, turn down the fan or point it in a different direction.

3.) Unhappy Roots

Unless you’re growing in a hydroponic setup, chances are your plant roots are hidden from view. Yet roots affect plant growth just as much as the environment aboveground. When the roots are unhealthy or unhappy, a plant languishes and stops growing. When roots are getting what they need, a plant can grow inches a day.

The most common cause of unhappy roots is overwatering, especially for seedlings and young plants. Overwatering is typically the result of giving too much water at a time, or watering too often.

But isn’t water good for the plant? The problem is that roots also need oxygen, which they typically get through air pockets in the soil. If the soil gets waterlogged from too much water without any pockets of air, the roots start “drowning” from lack of oxygen. The lack of oxygen is what causes the symptoms of drooping and slow growth.

In hydroponic growing, oxygen is provided to roots through bubbles. That’s why hydroponic plants don’t get droopy even though their roots live directly in water.

To be continued…

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