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.
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.
Only one of my 3 plants is having this issue.
More grow details: 250W HID, 2x3x5 grow tent with 4″ inline fan. Northern Lights strain. Seeds germinated in rockwool and transplanted into 4 gallon smart pots w/ mix of peat moss and perlite. Fox Farms Nutrient Trio for nutes – following the dosing schedule but started with 1/2 doses for the first week. pH of water is 6.5-6.6 and runoff from pots tested at 6.4-6.6.
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.
When the problem is low humidity, the leaf will also often fold in the middle, like a taco.
If plants are also droopy, it’s likely you’re dealing with watering or root problems.
I live in a dry area so the humidity in my house is normally below 20%. However, when I checked the humidty in my grow tent, it was 36% so I’m not actually sure it’s actually too low.
[Answer inside!] Only one of my 3 plants is having this issue. A friend suggestted that the issue may be low humidity…
As it progresses, purplish discolourations begin to appear on the main part of the leaves and the leaf edges begin to turn brown and curl downwards. The leaf petioles (the small stalk attaching the leaf to the main stem) may also begin to turn a purplish colour. In the final stages, large patches of the leaves become purplish-brown and dead, while the remaining sections turn mottled yellowish-green.
While this definitely isn’t a conclusive list, here are some common growing problems and how they may be identifiable on cannabis leaves (read the rest of this guide for more in-depth help):
One of the biggest mistakes new growers make is overreacting, though. If an iffy situation is spotted, growers should simply acknowledge that and take the time to learn what the problem is and how to properly fix it.
Again, phosphorus deficiencies usually affect the outer, lower leaves first. Unlike nitrogen deficiency though, phosphorus deficiency at any stage is cause for concern, as the plants continue to require high levels of phosphorus throughout the flowering period.
In severe cases of nitrogen deficiency, the leaves appear lighter and lighter in colour, and eventually begin to dry out and die. The larger leaves on the lower and outer parts of the plant are often the first to be affected and the first to die.
In the early stages of nitrogen deficiency, leaves appear to be too light in colour—almost yellowish-green—compared to the desired deep green of healthy leaves. Nitrogen-deficient leaves also typically have slightly darker veins compared to the very light colour of the leaf tissue itself—although the contrast is not as marked as in some other deficiencies.
However, a nitrogen deficiency in vegetative growth or early flower can severely impact overall yield and quality, as it affects the plant’s ability to photosynthesize energy. Nitrogen deficiency can usually be corrected by simply increasing the strength of your normal NPK-based feed, but if fine-tuning is required then nitrogen-only additives are not hard to find.
- Yellow leaves: Could be a sign of all deficiencies mentioned below, or light burn
- Leaves that curl upward: Could be a sign of potassium deficiency, or overwatering
- Brown leaves: Could be a sign of phosphorus, potassium, magnesium, sulphur, manganese deficiency. Could also be nutrient burn (more solid brown colouring) or heat stress (brown on the edges)
- Leaves droop: Could be a sign of overwatering
- Leaves curl downwards: Could be a sign of potassium or phosphorus deficiency, or overwatering
A cannabis plant is hardy, but its health and vigour will be negatively impacted if it is deprived of essential nutrients. If this is not corrected, it can cause plants to stop growing or even die. Deficiencies in cannabis can be hard to identify, so we have compiled a basic list of what to look out for if things go wrong, and how to fix them.
Tutorial: Cannabis plant nutrient deficiencies can be hard to identify. Use this guide to help you figure out what’s wrong and how you can fix it.