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

Last updated: 15 August 2020

how to fix stunted growth in plants

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

why are my plants growing so slow

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.

Excess Light

why are my seedlings growing so slow

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!


weed plants growing slow

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

marijuana stunted growth

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 plant growing slow

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

cannabis stunted growth

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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