Fixing Upward or Downward Curling of Cannabis Leaves

Cannabis plants have all manner of ways to tell us that something is wrong and that they need our care. In many cases, this may be as simple as leaves curling downwards at their ends, or upwards like a canoe. While this may initially not look serious, it’s usually indicative that they need attention.

In this article, we look at the possible causes of curling cannabis leaves, and see what we can do about it!

What is leaf curling?

Leaf curling, or “clawing”, in cannabis is identified by a downward curl at the end of each leaf finger. In some cases, individual fingers may even curl up, causing them to take on the shape of a canoe or taco. When a whole leaf curls, it gives it an ominous claw shape, hence the name. When cannabis leaves claw, it’s a sure sign that something is amiss.

There is no single reason that cannabis leaves curl, so diagnosis usually involves identifying this symptom, and then others.

What are the common causes of cannabis leaves curling?

There are various causes of leaf curling in cannabis. Below, we’ll discuss each of them, and see which other symptoms could help you determine the issue at hand, so you can get to curing it!

Overfeeding cannabis plants

One of the main reasons that leaves curl is due to too much nitrogen. Most likely, this happens simply as a result of general overfeeding, which can first manifest as clawing, before going on to show signs of nutrient burn or lockout, such as leaf discolouration and a burnt appearance.

However, it may also occur because you’re using vegetative feed during the flowering stage. In the former stage, cannabis plants need more nitrogen than in the latter, so the feeds must be adapted accordingly depending on the plants' phase of life.

How to cure overfeeding cannabis plants

If you’ve identified that overfeeding is the problem, then fortunately the solution is fairly simple. First, you need to flush the soil or growing medium with pure water. Pour many litres through it, and if possible measure the runoff with an EC meter.

Once the medium has been flushed thoroughly, it should be devoid of nutrients. From this point on, feed carefully and make sure that you give it no more than it needs, and your plant will be able to return to good health.

Excessive grow room temperatures

If things get too hot in the grow room, then the leaves may begin to curl. Excessive temperatures can, if left unchecked, stunt plant growth and encourage the occurrence of pests, which can damage your plants further.

The perfect temperature for most cannabis strains is somewhere between 23 and 27ºC. This should be warm enough to keep them happy and healthy, but cool enough to let them grow properly.

How to fix it

The solution is simple: cool your plants down!

This may involve something as simple as moving the lights further away, or you might have to introduce an exhaust system to draw hot air out of the room and cool it down. Unventilated tents and grow rooms can be very hot, especially if you’re using HID lights. In some cases, an oscillating fan might even do the trick to keep temperatures optimal, while in others, a dedicated air conditioning system may be required.

Cold grow room conditions

On the other hand, a grow space that is too cold can also cause the leaves to claw. Indeed, temperatures that are too low can stunt growth and even kill plants if the conditions persist. Anything under 15°C is cause for alarm.

How to fix it

If a grow room is too cold, you’ll need to warm it up. The easiest way to do this is to bring in an electric heater. But make sure it has a thermometer attached so it doesn’t end up making the space too hot! Alternatively, you can use HID lights instead of LED lights, as these will help to keep a space warmer.

Overwatering cannabis plants

Overwatering is a trap that many novice growers fall into. It can seem that more is always better when it comes to watering, but that's definitely not the case. Too much water in the growing medium will deprive the roots of oxygen and cause them to rot. Above ground, one symptom of overwatering is, you guessed it, clawing leaves. This will start at the bottom of the plant and move upwards to newer growth. Leaves will also appear floppy and soggy, rather than dry and brittle.

How to fix overwatering cannabis plants

If roots have begun to rot, then you’ll need to remove your plants from their pots, trim off any rotting roots, and repot them in new, clean soil.

And next time, make sure that you only water when the top few centimetres of soil is dry. If it’s moist, then the plants don’t need water! If anything, it's better to underwater slightly than overwater.

Cannabis root issues

It’s not only overwatering that can cause roots to rot, but fungi, bacteria, and parasites as well. Likewise, overly dense soil may also damage roots. In any case, these problems will deprive roots of oxygen and eventually cause the plant to die.

The symptoms are the same as with overwatering, even though the soil might not be soaked. Rotting roots will smell, and will appear brown and slimy, rather than white and clean.

How to fix it

To fix these issues, follow the same steps as with overwatering. But you should also wash the roots, once pruned, with a bleach or hydrogen peroxide solution—and then wash this off with clean water. This step kills any pathogens that may be clinging onto your roots.

Also, you must always repot in clean soil, otherwise the problem will just return.

Clawing leaves in cannabis: A telltale sign that something is amiss

If your cannabis leaves are turning downwards, it’s a sure sign that something is wrong. Don’t wait around in the hope that they will suddenly recover, as the problems that give rise to this symptom usually require intervention.

Investigate immediately and treat as necessary. The quicker you act, the better the results will be!

Max Sargent

Max has been writing for over a decade, and has come into cannabis and psychedelic journalism in the last few years. Writing for companies such as Zamnesia, Royal Queen Seeds, Cannaconnection, Gorilla Seeds, MushMagic and more, he has experience in a broad spectrum of the industry.

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