weed plants 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.

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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.

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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

Slow Cannabis Plant Growth And What You Can Do About It

When your cannabis plants grow slowly or stop growing altogether, there is always a reason. It could be a problem with nutrients, an environmental factor, or something else entirely. Let’s explore the reasons your cannabis plants or seedlings may experience slow or stunted growth.

“Why are my plants growing so slow?”. Sometimes, marijuana plant problems occur out of the blue. Your baby may not have shown any signs of an issue, but now you notice that development has halted and have no idea why. Here are some possible factors behind the slowed growth of your cannabis seedling or plant.



Old seeds don’t just take longer to germinate (if they germinate at all); plants grown from aged seeds can also sometimes grow at a reduced pace. Likewise, good genetics are essential for healthy and vigorous growth from seed to harvest. A random bagseed will not perform nearly as well as quality seeds obtained from a reputable seedbank.


Sometimes cuttings don’t root well, which hampers their growth. To prevent this from happening, apply a little bit of rooting hormone immediately after taking your cuttings.

Also, make sure your environment promotes root growth. The medium should be humid (but not too moist) with a pH level of about 6.0. Keep your cuttings at a temperature of around 22ºC. If they get too cold, they won’t root at all, and if it’s too hot, the roots will die.


When your plant’s roots can’t receive enough oxygen, metabolic functions slow down. In some cases, a lack of oxygen may stop their growth altogether. One common reason for this is overwatering or using substrates with poor drainage.

What to do about it? Create a light and airy growing medium with good drainage. You can improve poor-draining soil by adding some perlite.

The root zone for your cannabis plants should never get much hotter or colder than room temperature. Likewise, physical damage to the roots, mould, or bacteria can severely affect the growth of your plants. Always use non-transparent planters so light doesn’t reach the roots, as this is bad as well.


Stretching among seedlings can be particularly problematic. Multiple factors can induce this response, but the most likely culprit is a lack of light.

If your seedlings are spindly, increase light intensity or bring the lights closer. Prop them up with dowels as an aid during recovery. As a last resort, you can (carefully) replant them deeper into a new pot.


Although requirements can vary from strain to strain, light is nonetheless a critical factor for the development of all cannabis plants. A lack of “good” light can absolutely lead to slowed growth. If you grow indoors and suspect that your plants aren’t getting enough light, try to decrease the distance between your lamps and the tops of plants. If you grow outdoors in pots, move your plants to a sunnier spot.


Any type of stress on your cannabis plants, including many hours of exposure to direct sunlight without rest, can also halt or slow down growth. If you grow indoors and suspect light exposure to be the source of stress, decrease the intensity or move lamps further away from the canopy if possible. Know that seedlings are particularly sensitive to intense light! If you grow outdoors and you’re able to, move your plants into a spot where the light is diffused, such as around a shade tree.


How fast and how vigorously plants grow are influenced by the spectrum of light they receive. Make sure you use the correct type of light according to each stage of growth. For healthy vegetative growth, you want a cooler light with more blue in its spectrum, a so-called “vegging light”. Lights with a warmer, more reddish spectrum are used for the flowering phase.


Light is essential for all plants to grow. Any changes in light intensity or exposure will have an effect on growth. Flowering cannabis is especially susceptible to interruptions in the dark cycle. A light leak in your tent, stray light from a street lamp, and even a red light from a camera can disrupt flowering, and in a worst case scenario, can turn plants hermaphroditic. For that reason, it is very important to maintain complete darkness during the lights-off hours.

Exposing weed plants to irregular light hours can cause a hormone imbalance that confuses their internal clock. Your plants could flower prematurely, or they could revert back to the vegetative stage. If this happens, growth and yields will greatly suffer. For that reason, make sure to keep your light cycle consistent.

The above suggestions predominantly apply to photoperiod strains, as autoflowering cannabis flowers based on age rather than light exposure.


Overwatering is one of the most common mistakes made by new cannabis growers. It’s like suffocating your plants, and one of the main reasons behind slow growth, nutrient deficiencies, root rot, fungus, and many other problems. Don’t water too often and do not water on a fixed schedule. It is better water less frequently so that the soil can dry out between waterings. A good way to test whether you should water or not is to lift up the pot itself. If it feels quite light, it is time to water again.


Although not as common as overfeeding cannabis plants, an insufficient amount of nutrients for healthy growth can well be the reason for slow growth. Know that the nutrients found in most commercial potting mixes will only last for 3–4 weeks; afterwards, you will have to administer some more quality nutrients. Check the label of your nutrient products for the recommended dosage for healthy growth. Also know that your plant’s nutrient requirements are closely linked to the light intensity your plants are exposed to. Plants under intensive lights grow faster and will require more nutrients than plants under fluorescent lights, for example.


Calcium is among those vital elements that your plant needs for healthy development. A lack of calcium can manifest in the following symptoms:

  • Fresh growth is slow, twisted, and curled
  • Young shoots are discoloured and turn purple or yellow
  • Overall plant growth is slow and lacks vigour and vitality
  • You can avoid a calcium deficiency by adding dolomitic lime to your soil or growing medium

Address a calcium deficiency immediately with commercial CalMag products that contain liquid calcium. You can add these products to your nutrient solution or administer them as a foliar spray.

Be aware that some growing media, like coco, increase the risk for a calcium deficiency. If you grow in coco, you should use special coco nutrients and/or regularly add CalMag to your nutrient regimen.


Incorrect pH level of your nutrient solution is among the most common reasons for cannabis growing problems, including slow growth. The reason for this is that cannabis thrives only in a relatively small window of suitable pH values. If the pH is off, the plants are unable to take in nutrients, even if they are present.

Make sure to dial in the correct pH level depending on your growing method. If you grow in soil, make sure the pH level is from 6.5 to 7.0. If you grow in hydro, an optimal pH level is 5.6 to 5.8. For soilless grows, such as coco, a pH level of 6.0 to 6.3 is optimal.


Cannabis likes it warm to grow healthy, and does best at daytime temperatures between 25–30°C. Temperatures lower than that will slow down your plant’s metabolism, resulting in slower growth. But excessive temperatures are not optimal either. At very high temperatures, heat stress can also slow down or even halt plant growth altogether. If you grow indoors, adjust your temperature to a comfortable level. You can also provide some cooling with fans that blow a mild stream of air over your plants. This can also help prevent hot air pockets from forming inside your grow room.


Cannabis growers often start their seedlings in small cups. Later on, when the plants have reached an adequate size, they will “pot-up” to larger containers.

If you start your cannabis plants in containers that are too big, there is a high risk that you’ll overwater them. The issue is that seedlings cannot absorb all the moisture that is held in a large container, unlike mature cannabis, which can “drink” much more. Furthermore, a large pot will also take much longer to dry out.

To avoid the problems that come with too much soil and moisture, start seedlings in smaller containers until they’re growing vigorously. Once they have a set of 5–6 real leaves (not counting the cotyledons), then transfer them to a larger container, at least twice the current size.

If your seedling is already in a big container and you don’t want to or can’t move it into a smaller cup, water only a small area around the seedling.

• What Is The Right Size Pot For Your Cannabis Plant?

Use this rough guide to determine what size pot you should use for your cannabis plant:

  • Plant height 30cm: 7.5–11l container
  • Plant height 60cm: 11–19l container
  • Plant height 90cm: 18–26l container
  • Plant height 120cm: 22–37l container
  • Plant height 150cm: 30–37l+ container


Insects, pests, and disease can cause damage and compromise a plant’s immune system. In a best case scenario, your plant may survive, but you will have poor yields. In the worst case, your plants could die.

Insects may feed on the leaves, affecting a plant’s ability to retain water and transpire. Other pests may damage the roots or cause additional problems. Any time your plant is sick or infested with insects, it will spend most of its energy defending itself and recovering from damage, which will slow down growth.

If your plants are infested, you’ll want to treat them immediately with appropriate measures. Even better, you can use preventative methods (e.g. neem oil, slug barriers, etc.) to minimise the risk for pest infestations. During all stages of growth, ensure that you regularly check for symptoms of pest infestations, including under the leaves.


Physical damage, such as broken branches, can significantly slow your plant’s growth. Any damage will make the plant redirect valuable resources to repair wounds—resources that could be better spent on growing or flower production.

If you’re growing outdoors, situate your plants in an area sheltered from strong winds and heavy rains, and use chicken wire and stakes to maintain support.

Seedlings and young cannabis plants are especially vulnerable. Allow your seedlings to mature indoors for some weeks before setting them outside.


Tissue damage from high-stress plant training techniques always causes some delay in plant development. But when you’re pruning excessively or too frequently, your plant may ultimately spend more energy repairing itself than growing.

If you plan on pruning, don’t overdo it. Be aware that each pruning can delay the development of your plant for days, if not weeks.

If you’re using other plant training techniques such as topping, make sure you start as early as possible. If you’re growing autoflowers, don’t use any plant training techniques that involve tissue damage, such as pruning and cutting.


Older cannabis plants have different nutritional requirements than young plants. Their tissues become hard and woody, they’re less vigorous, and they’re unable to take in as many nutrients.

Because of this, you’ll want to adjust your feeding regimen accordingly. Otherwise you risk overfeeding, which in turn results in stunted growth, deficiencies, and disease. Keep this in mind if you’re keeping mother plants around for a long time.

Why is my cannabis plant growing slowly or not at all? Find out the answer to this question and see what you can do to solve it.