How To Deal With Slow Cannabis Growth

If your cannabis plants are growing too slow it’s most likely there’s something wrong with them.
Author Enzo Schillaci
29 June 2020
9 min read
How To Deal With Slow Cannabis Growth


If you feel like your plants are not growing as fast as they should then it’s most likely there’s something wrong, slow growth can be a result of a number of things. If your plants are suffering from slow root growth, or you're asking yourself why your plants are growing so slow, here's a couple of tips to help you fix your problem, remember these problems can affect cannabis in all stages of plant growth.

When growing cannabis there are a lot of variables that can affect the growth and final result of your harvest. From genetics, possible nutrient deficiencies, root system and pests problems to the environment, you need to make sure everything is okay if you want to have a smooth growth cycle. Providing your plants with a warm slightly humid climate will not only help you to deal with their slow growth but can result in a bigger and better harvest.

1. Lights

Not enough light is one of the main problems associated with slow growth. Not providing enough light will cause your plants to take way longer to develop because they don’t have a good amount of light to photosynthesize.

When your plants are still young you can see this, for example, when your seedling is stretching too much, this is a sign that your plant is not getting enough light and is trying to get closer to it.

Now, have in mind that you can also give your plants too much light (if the environment is not appropriate for the amount of light you’re providing).

This happens because most high-intensity lights have to be used in combination with higher CO2 levels so your plant can absorb the light properly, if the CO2 level is too low your plant can get stressed and show heat stress signs, resulting in slow growth.

How to deal with it

If you’re experiencing light-related problems (like heat stress symptoms), you should try to adjust the intensity (with a dimmer) or the height your light fixture is at.

A good way to test this is to keep your hands under the light for around 30 seconds, if it’s too hot for you, it definitely is too hot for your plants.


How To Deal With Slow Cannabis Growth: Light distance

Approximate light distance for LEDs and light bulbs.

As a general guide, LEDs should be kept in between 60-100 cm from your plants (depending on the model) while light bulb fixtures should be around 30cm from your canopy.

2. Environment

Even though some plants can withstand harsh environments, most cannabis plants enjoy a warm slightly humid climate, it’s essential you have a thermo hygrometer to measure this and adjust when necessary.

If the temperature in your grow room is below 15°C or above 30°C for too long, you will start to see slower growth and if left like that for too long, you’ll see signs of unhappy plants like droopy leaves or the leaves starting to yellow, crisp and die.


The ideal humidity levels are around 60% for seedlings, 50% in the vegetative stage and 40% in the flowering stage.

This also applies to humidity. Humidity levels below 35% can stunt growth and if they’re even lower than that (around 25%), you’ll start to see signs of deficiency on your plant. 

A humidity level higher than 70% can also have a toll on your cannabis plant, slowing your plant’s growth, making them droopy because it makes it harder for the plant to move water internally.

How to deal with it

If your seedlings are not growing or you're wondering why your seedlings are growing so slow, this could be the problem.

To avoid environment-related problems you have to ensure your growing environment is optimal for your plants.

For your cannabis plants to thrive you need to provide a temperature of around 22-25 celsius and a humidity level of 60% for seedling, 50% in the vegetative stage, and 40% in the flowering stage.


StageHumidity Level


These numbers are just a guideline, you should always look for the signs your plants give you and adjust the environment accordingly.

3. Root problems

Unless you’re growing in a hydroponic (or aeroponic) setup you won’t be able to see your plant’s roots.

Keeping a good environment for your plants also includes maintaining the medium oxygenated and with the right amount of water. Root problems are the main cause of slow growth with new growers, adjusting and maintaining a good growing medium for the roots is the best way to fix stunted growth in plants.

The lack of appropriate care with the roots can cause the following problems:


Overwatering is a common occurrence with new growers, excessive watering inhibits oxygen from reaching the roots and can result in the drowning of your plant.

Overwatered plants will start to droop, show slower growth, start yellowing, and show signs of deficiencies because they can’t absorb nutrients properly.


Underwatering isn’t as common as overwatering but can also affect your plant in a bad way.

If there’s a lack of water in the medium, your plant will show signs similar to the signs of overwatering but instead of the leaves looking “fat” because of the excess of water, they will look thin and fragile because there’s no water (or a tiny amount) in them.


Oxygen is essential to the roots, by providing the appropriate amount of oxygen in the mediu, you allow the roots to develop healthy, resulting in a healthier plant.


Rootbound happens when you plant your seeds in a small pot or container and the roots don’t have any more room to grow.

When this happens, your plant will start to show different symptoms than can confuse you, when you see your plant getting much wider than the pot you should start thinking about transplanting it, failing to give the roots the space they need to grow will result in droopy leaves and other signs associated with overwatering of nutrient deficiencies.

How to deal with it

To avoid root problems you have to make sure you’re giving the roots enough oxygen, room to grow, and water only when needed.

You should water only when at least 60% of the medium is dry and transplant your cannabis when she's growing out the pot she’s in.

If you’re having problems with this we recommend you try different mixes of soil, perlite, and coco fiber, this will help you provide the right amount of oxygen and water to the roots.

4. Nutrient deficiencies

Plants need nutrients to grow, depending on the medium you’re growing in, you’ll need to not only provide all the macronutrients but also a good amount of micronutrients.

You also have to remember that nutrient absorption is directly related to the pH level.

If you don’t provide the amount of nutrients your plant needs or if you fail to adjust the pH level your plant won’t be able to grow properly and will result in nutrient deficiencies, stunting growth and damaging the leaves.

How to deal with it

To fix this you need to adjust the amount of nutrients you’re giving your plant and check the pH level every day.

 How To Deal With Slow Cannabis Growth: Ph nutrient absorption

PH levels for optimal nutrient absorption.

If you’re giving the right amount of nutrients then the problem will most likely be the pH level, remember the pH level changes according to the medium you’re growing in and you should always check it and adjust it if you want your plants to grow healthy.

It is crucial you keep your plant well fed and with the correct pH levels, if you see even the slightest yellowing on the leaves then your plant is most likely not growing properly.

5. Bugs or pests

Bugs and pests feed on your plant, they can feed on the sugars or on the plant matter, either way, they will damage your plant and slow its growth.

Some bugs like Spider mites can also rot the buds, making them unhealthy to smoke so it’s crucial you keep your plant healthy and check on them every day to spot bugs early.

How to deal with it

There are different ways to deal with bugs but the best way is to prevent them.

By checking on your plants every day you avoid bugs, you can also spread yellow sticky traps so you can spot them early.

You can also spray your plants with a mix of water and a small dose of organic insecticide to prevent them although this is not recommended, insecticides should only be used when you already have bugs and are used to eliminate and not to prevent them.

6. Genetics

Slow plant growth can also be the result of bad genetics, even though you cannot change this once you’ve already started to grow your plant, you should always start with good genetics. This will save you time and money and can result in a bigger and better harvest.

How to deal with it

If you’re dealing with slow plant growth and can’t seem to get to the root of it, the cause of your problems can be bad genetics.

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7. In conclusion

Slow plant growth can be the result of a not-optimal growing environment or even a mix of some of the elements said above.

If you’re having problems with your plants, we recommend checking every one of the topics we talked about and if you cannot solve your problems try to grow good quality genetics in your growing cycle. This can not only solve your problems but surprise you with the results you’ll have.

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