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.
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.
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.
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.
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.
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.
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.
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.
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.
Only one of my 3 plants is having this issue.
Having good air movement, like a small fan in your grow tent blowing over the tops of your plants, can help prevent hot spots from forming directly under your grow lights.
A friend suggestted that the issue may be low humidity.
When the the serrated edges of marijuana leaves curls or tipped up like that, it’s often a sign of temperature stress, overwatering/root problems or extreme humidity levels. This plant was overwatered and living in high heat, which is what caused these symptoms.
If plants are also droopy, it’s likely you’re dealing with watering or root problems.
You mentioned that this is happening to only some of your plants. If this is happening to the plants which are closest to your grow light, that’s a hint that the problem may be temperature related.
When growing with hot HID lights, good ventillation and an exhaust fan will go a long way towards preventing all these problems. If the whole grow area is too hot with stagnant air, you will want to consider venting out all that extra heat.
This looks like the result of excessive heat, root problems (possibly from overwatering) and/or suffering from extreme humidity levels (such as very high or low humidity).
If your light is simply too close, you can move the bulb further away from these plants and this issue may resolve itself. However, these leaves are showing symptoms of heat stress, without light stress (burning / spotting), so it’s more likely that the light is a good distance away and the heat just isn’t being controlled properly.
[Answer inside!] Only one of my 3 plants is having this issue. A friend suggestted that the issue may be low humidity…