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What Causes Curly Cannabis Leaves And How To Cure Them

There are numerous reasons why you might find curly cannabis leaves in your grow-op. This guide will explain why this phenomenon occurs, and what you can do to prevent it from ruining your final product.

Contents:

Cannabis plants can’t vocalise a call for help—but they can send signals to tell you all is not well. If you see leaves either clawing or curling, there is trouble with the trees. Don’t ignore their pleas. This blog will help you identify the causes and cures for curly cannabis leaves.

CAUSE: OVERWATERING

Overwatering will literally drown your plant’s roots. Excess water will not only rinse most of the beneficial microbes from the medium, a sodden substrate can also become colonised by algae and nasty fungi. Persistent overwatering invariably invites the parasitic Pythium, better known as root rot. Cannabis plants with droopy, claw-like leaves could be trying to tell you they are waterlogged.

“Water mould” microorganisms are just like vampires; you have to invite them in before they can do any harm. Keep them out of the garden by making sure they are not welcome. Maintaining an effective wet-dry cycle is all it takes. If you can pick up your pots, do it. Then you can tell by their weight when it’s time to water.

If you can’t lift the containers, then consider a moisture meter and make sure to carefully monitor post-watering plant behaviour. Try reducing the volume of water. Alternatively, take longer intervals between waterings. Unfortunately, Pythium is virtually incurable and will turn your plant’s roots into brown sludge. If you see droopy, curly cannabis leaves, especially with young plants, look to the roots for answers.

CAUSE: OVERFERTILISATION

A heavy-handed approach to nutrients is ill-advised. Excessive doses of nitrogen-rich vegetative growth base nutes can cause clawing in leaves. Sometimes, they will even canoe. Similarly, overdoing it with the phosphorus and potassium during flowering will cause curly cannabis leaves and scorch the tips. Chlorosis is a common symptom in both cases.

Dial in feeding. Easier said than done right? Wrong! Almost every brand of well-known cannabis fertiliser offers a feeding chart free to download from their respective websites. Granted, not all cannabis varieties will respond in the same way to fertilisers.

It’s better to start low and go slow. You can incrementally increase doses without seeing leaves curling or clawing. But if you dive right in at maximum strength, you can expect plenty of curly cannabis leaves that will probably die and eventually drop-off.
It should go without saying, but we’ll say it again for good measure; make sure the nutrient solution is the right pH. That’s about 6.0pH for soil and a more precise 5.8pH for coco/hydroponics.

CAUSE: TEMPERATURES ARE TOO HOT

Heat stress can occur indoors or outdoors. If you see curling and nasty-looking brown fringing, your cannabis leaves are sending you a distress signal. Cannabis plants can photosynthesise efficiently at moderate temperatures up to 28°C. Anything above 30°C and your plants are in the danger zone. Combine this with low RH and you’ve got real problems. New leaves will grow gnarly and old leaves will curl yellow and maybe even burn to a rusty, brown crisp.

Indoor growers must constantly maintain the optimal environmental conditions. This starts with optimal light distance. The only way to keep the plant canopy in the sweet spot is to measure and adjust until mature plants peak in height during mid-late bloom, depending on the strain. Moreover, indoor growers can utilise air-con and fans to keep the grow-op cool.

Outdoor growers confronted with heat waves and drought conditions have less control than the indoor grower. Constructing a simple screen shade will keep plants slightly cooler and may prevent leaves from fraying and curling further. You can’t really revive scorched leaves and will have to remove older foliage beyond saving. Also, planting in white pots instead of black pots will keep the root zone cooler.

CAUSE: TEMPERATURES ARE TOO COLD

Cold temperatures can cause curly cannabis leaves too. Eventually, all kinds of leaf discolouration will develop. Sure, cooler nighttime temps late in bloom can add a dash of purple charm to buds, but prolonged exposure to temperatures below 10°C will kill your plants. Flowers will be loose and leafy if plants even make it to harvest. Coupled with high RH, buds will be moist and become vulnerable to Botrytis, AKA bud rot.

Indoors, if temps are too low, you can always add more grow lights and turn a negative into a positive. Outdoor growers might consider an early harvest, or if possible, moving plants indoors at night. Cannabis is a hardy plant species, but outside of the optimal 20–28°C temperature range, leaves will curl or claw.

A FINAL WORD ON CURLY CANNABIS LEAVES: GENETICS

Genetics are the cause of all kinds of cannabis leaf deformities and mutations. Some strains occasionally have a tendency towards curly leaves or other odd traits. Most growers will thin out these plants. All the shrewd cultivator can do is write it off as bad luck.

Sativa strains and many autoflowering varieties are sensitive to high doses of fertilisers. This is a trait that can cause problems for beginner growers. The solution is to research your reefer. Always find out as much as you can about the genetics of a strain before you decide to grow it. Curly cannabis leaves can be completely avoided if you know how to keep your plants healthy.

Cannabis leaves curling or clawing are signs your plants are suffering. Something is going wrong in the garden. This blog will help you save the stash.

Why are Leaves Curling or Clawing? (“The Claw”)

The following symptoms are for when your cannabis leaves are “clawing” or curling up or curling down. Sometimes known as “The Claw”. I’ll give a short explanation with pictures of each problem, plus links to the solutions! Fix this common (but hard to diagnose) marijuana problem today!

Nitrogen Toxicity

A Nitrogen toxicity is the result of the plant getting too much Nitrogen (usually from too high levels of nutrients overall, or by using a Vegetative nutrient in the flowering stage). It causes dark green leaves and curled tips (“the claw”).

One of the main symptoms of a Nitrogen toxicity is curled tips (“the claw”)

A plant with a Nitrogen toxicity tends to be dark green all over

Wind Burn

Caused by too much wind. You’ll notice that the leaves further from the fan don’t have symptoms.

Example of too much wind on your leaves

Bad Soil / Overwatering / Underwatering

You can help prevent over and under-watering your cannabis plants by always starting with good soil or coco coir.

Bad Soil

Bad soil is usually thick and muddy. Plants in poor soil will droop (often with unhappy curly leaves) no matter your watering practices.

Avoid thick soil that stays wet for a long time and doesn’t drain well

Overwatering

Overwatering makes leaves fat with water, and they tend to curl down and droop

Overwatering (especially when combined with heat) can also cause leaves to curl up

This plant was grown in muddy soil, and the curling, unhealthy leaves kept getting worse and worse over time!

Underwatering

Underwatering causes symptoms that often look like overwatering, but you’ll know it’s underwatering if the plants perk up each time after you water them.

Root Problems

Although often caused by overwatering, once the roots are sick you’ll see symptoms for a little while even after you start watering your plants properly.

Unhealthy roots can cause all sorts of problems including curling and clawing!

Unhealthy Roots in Soil/Coco

This plant’s roots were damaged from being overwatered and too hot for several days. As a result, the leaves took on a strange, blistery appearance.

This plant suffered from heat combined with overwatering for several days. This damaged the roots and gave it this odd leaf curling.

Root Rot

Root rot is something marijuana hydroponic growers can suffer from if pathogens attack the roots. It is often triggered by heat and/or lack of bubbles near the roots.

Root rot can cause curling leaves and brown patches as well as sometimes other nutrient deficiencies

Rootbound

If a plant stays in the same container for too long, the roots will eventually start wrapping around the edges of the pot. This is known as being “rootbound” and causes symptoms similar to other root problems.

A rootbound plant has been in the same container for too long. Roots wrap around the edges and “choke” the plant.

Rootbound plants often droop, appear yellow, get nutrient deficiencies, and stay small. Even if you’re caring for them perfectly!

If you see tons of white roots when transplanted, that means the plant was in that container too long

When this happens, the main solution is to transplant the plant into a bigger container. Another solution is to grow in fabric pots or air pots. These types of pots let air in from the sides, killing the circling roots (“air-pruning” them) and prevents the plant from getting rootbound for months.

To help a rootbound plant, transplant to a bigger container with fresh potting mix

Or start with air pots or fabric pots in order to prevent plants from getting rootbound at all

Heat Stress

If plants are experiencing a lot of heat, it can make leaves droop and/or curl. Some strains can handle a lot of heat, while other strains tend to droop when it gets warm.

Heat can cause leaves to curl up

Read more about heat and growing weed:

Light Burn

Plants can get light burn (sort of like a sunburn) even if the temperature is completely under control. The symptoms are usually concentrated close to the grow lights. Sometimes this can cause leaves to claw and curl downwards.

Light burn can cause the leaves closest to the light to turn yellow

Bugs & Pests

Often a bug infestation caused general plant unhappiness, but these are some of the most likely to cause curling or clawing leaves.

Broad Mites

Usually, you can’t see broad mites because they live inside the plant. The main symptom of an infestation is strange leaf curling that is specific to this pest, as well as “wet” looking leaves.

Hemp Russet Mites

Hemp russet mites can also cause drooping and other strange symptoms, but the bugs are so small many growers don’t realize what they’re dealing with.

Hemp russet mites cause drooping and yellow mold-like growth on the tops of plants

A closeup of the hemp russet mites

Fungus Gnats

Fungus gnats look like tiny flies buzzing around your topsoil. Although a few fungus gnats won’t really hurt your plants, a big infestation can damage the roots, causing symptoms similar to other types of root problems.

Learn about the various things that cause cannabis leaf curling or clawing, and get the solutions!