It’s never a good sign when the serrated edges of cannabis plants take on a “canoe” shape and start curling upwards. In the immortal words of Jim Carrey: “It’s becoming the claw!” So what causes cannabis leaves to curl? Well, a lot of things.

While cannabis is a hardy and resilient plant, it’s still susceptible to a fair share of pests and problems. Unfortunately, it’s just part of the deal. With more growing experience, you’ll be able to identify the main issues that contribute to clawing earlier.

Clawing is a common vernacular for the curling of a cannabis plant’s leaves. When a cannabis plant’s leaves curl, it’s often a distress signal. Many variable factors can contribute to clawing, and it’s important to address them. Otherwise, you risk stunting growth or even losing the plants. Pause your freak out. If you’ve got some curly leaves, it’s not game over yet. Below, we’ll go over five of the most common culprits behind curly cannabis leaves, a.k.a the dreaded “claw,” and how to tackle them.

5 reasons your cannabis leaves are curling

Overwatering your cannabis harvest

Making sure you’re not overwatering is step one in diagnosing the cause of clawing. Overwatering puts the plant at risk of drowning and bud rot while creating an ideal environment for the parasite that causes root rot, Pythium, to propagate. Both waterlogging or root rot will cause cannabis leaves to curl. Overwatering is one of the most common problems encountered by growers just starting out, and luckily, it’s easy to fix.

Before you grab the watering can check your plants! Take a look at the leaves and the soil. If the leaves are drooping and seem to be growing slowly and the soil is dry, an inch deep or more, and you’ve got the green light for watering. Otherwise, it’s best to hold off. Overwatering, in general, will do more damage to your plants than slightly underwatering. If the leaves perk up within about 30 minutes after watering, then underwatering was your culprit.

If you’ve got an overwatered plant on your hands, let it dry out for a few days and adjust your watering schedule accordingly. Another more high-tech option for checking plant moisture is using a moisture meter. Like a thermometer, a moisture meter measures the water saturation of your plants from the root up.

Overfertilizing the cannabis plants

Overfertilizing can introduce a host of issues. Nitrogen is essential for your plants to thrive. But as with water, there’s too much of a good thing. Excess nitrogen, in particular, will cause nitrogen toxicity, and subsequently, your cannabis leaves to curl. This issue is called nutrient burn, and it’s another fairly common problem among amateur growers. Curling leaves due to nutrient burn are usually accompanied by a deep green hue or even brown “burnt” edges on the leaves. Thankfully, this is also an easy fix.

If you’ve isolated nutrient burn as the culprit behind your curlies, you’ll want to start by flushing with slightly acidic water, around 6.0 pH. A flush should help get rid of excess nutrients near the roots, and your plants will level out within a few days. Remember, you should be maintaining a stable pH. For hydro, the pH should be between 5.6 and 6.5. In soil, pH should be between 6 and 7. Testing your pH is easy with a test kit.

Heat Stress

Cannabis plants should be kept at 23 C (or 75 F), whether indoors or outdoors. When exposed to temperatures higher than 27 or 28 C, the plant will start losing water faster than it can replace it in the root system and thus, experiences heat stress.

The first sign of heat stress is slow growth, browning leaves and curling or “canoeing” fan leaves to preserve moisture. A robust root system can partially mitigate heat stress, but even a short episode of high heat (summer 2021, much?) is enough to cause permanent damage to fan leaves.

The solution is obviously to lower the temperature in the grow room. Easier said than done for indoor growing setups. You’ll know heat stress was the reason for your curling leaves if you spot new growth within a couple of weeks.

cannabis leaves curling

Wind Burn

It’s important to ventilate your grow room, but too much wind (seeing a bit of a pattern here?) directly on your cannabis plants could be one of the reasons your cannabis leaves are curling.

If using fans in an indoor grow room, turn them down a little or make sure they are angled away from the plants and not hitting them directly. If growing outdoors, you may need to bring the plants inside if they are potted.

Light Burn

Heat stress and light burn can sometimes go hand in hand, but you can still encounter light burn even with ideal temperatures. Light burn is more common in indoor setups, usually when the plants are too close to the grow lights. Like heat stress, the curling will typically happen towards the top of the plant or whichever part is closer to the light source. With light burn, the leaves will begin to turn yellow and eventually curl. If you’re dealing with light burn, move your plants further away from your light source, and they should recover.

What other growing tips would you like us to cover? Let us know in the comments! For the latest cannabis coverage, news and more, be sure to follow CLN.

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