weed plant growing slow

Why Are My Weed Plants Growing Slowly? Top Reasons Behind The Common Problem

It’s one of the most frustrating issues a grower faces – stunted growth. Don’t get down on yourself if you’re wracking your brain wondering “why are my plants growing so slow?” Stunted growth can be symptomatic of numerous problems. To speed up cannabis growth, you’ll need to address the correct issue plaguing your garden. That’s why today, we’re going to help you identify the root of the problem and figure out how to fix stunted growth in plants.

Main Reasons Behind Stunted Cannabis Plants

Several factors can cause stunted growth in your cannabis plants. Environmental factors like lighting and temperature could be the culprits. Grower errors like overwatering, underfeeding, pH issues and insufficient nutrients can also cause stunted growth in marijuana plants. Finally, there may be some other reasons, like seed quality or pot size causing slow growth in your cannabis.

Small Plants

Poor Quality Seeds

If you’re noticing stunted or slow growth in your plants, consider starting your search for answers at the beginning of their lifecycle. If you purchased low-quality seeds, you’re going to grow low-quality plants. Even if you bought great genetics, the seeds you ended up with may just randomly be duds. In addition, the older a seed gets without germinating, the less robust it will be once it does germinate. If you’ve noticed stunted young plants and are wondering “why are my seedlings growing so slow?”, bad seeds may be the reason.

Lack Of Light

This is usually a problem that affects indoor grows more than outdoor ones. Without enough light, cannabis plants won’t be able to perform photosynthesis and therefore can’t grow. Indoor growers can try lowering their lamps and placing them closer to their plants. If your plants are packed close together, defoliating will also help light penetrate through your canopy and get to those hard-to-reach spots.

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Excess Light

In contrast, your weed plants may be growing slowly because they’re getting too much light. If you’re seeing the leaves closest to your light source yellowing and curling, consider raising your lamps higher to increase the distance between them and your plants. This is another problem that mostly affects indoor growers.

Wrong Spectrum Of Light

Different types of bulbs emit different spectrums of light. For example, metal halide (MH) lights give off more blue light than high-pressure sodium (HPS) bulbs, which radiate a yellow and red-colored glow. In general, vegetative plants prefer blue light (in the 400-500 nanometer spectrum), while flowering plants like red lights (in the 620-780 nanometer spectrum). Different strains all like different wavelengths too. If you notice stunted growth on some of your marijuana plants while others are thriving, you may need to get lamps that emit a different light spectrum.

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Often, if you notice a weed plant growing slowly, overwatering is the problem. This is especially true with newer growers who are growing in soil. Overwatering can lead to a multitude of problems like nutrient deficiencies, fungal infections and more. One good way to check and see if you’re overwatering is by picking up your pots. If they’re heavy, let them dry out. If they feel light, go for it!


This isn’t as common as over-feeding is, but it’s still a viable issue that may cause stunted growth in your cannabis plants. This problem usually arises in hydroponic grows, although it can occur in soil grows as well. Most potting soil mixes only have enough nutrients to last three or four weeks. After that, you’ll need to introduce your own nutrients. If your plants seem to hit a wall after a month of growth, they may need more nutrients.

Insufficient Calcium

If you’re noticing stunted plant development combined with yellow or brown spots appearing on new growth, you may need to add calcium to your nutrient mix. Calcium is a vital mineral that strengthens plant cell walls and encourages stem and root health. You can introduce a calcium-magnesium (CalMag) supplement or dolomitic lime to boost the available calcium in your plants’ media. As with under-feeding, hydroponic gardens are especially susceptible to calcium deficiencies.

pH Issues

If your soil isn’t at the right pH, your plants won’t be able to absorb the crucial nutrients they need. As a result, pH issues can often manifest as nutrient deficiencies. If you’re pumping nutrients into your garden but notice your weed plants aren’t growing, you need to fix your pH. You should keep the pH of the solution you feed your plants between 6.5 and 7.0 for soil grows and between 5.6 and 5.8 for hydroponic gardens.

Temperature Problem

Weed plants can’t survive outside a certain temperature range. If you notice your cannabis experiencing slow growth, check the ambient temperature in your garden. You need to keep it between 77 and 86 degrees Fahrenheit (25 and 30 degrees Celsius) for your plants’ metabolism to run at an optimal rate. If the ambient temperature in the air is outside of this range, correct it to speed up cannabis growth in your garden.

Wrong Size Pots

Using a bigger pot is often the answer to growers wondering, “how do I make my weed plant grow faster?” If a plant’s root system doesn’t have enough room to grow, it will affect the plant’s size above ground. Plant height depends on the size of its container. For example, plants that are 30 centimeters (about a foot) tall need a container between 7.5 and 11 liters (between two and three gallons) in size.

The Verdict

If you’re noticing stunted growth and are wondering how to make your cannabis plants grow faster, check to see if any of these issues are affecting your plants’ health. These are some of the most common problems you’ll face, and once you fix them, your plants will be off to the races once again.

If you’re noticing stunted growth and are wondering how to make your cannabis plants grow faster, check out our article. Here are some of the most common problems you’ll face, and once you fix them, your plants will be off to the races once again

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.

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.

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.

Stage Humidity Level
Seedling 60%
Vegetative 50%
Flowering 40%

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.


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.

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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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