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):
Nitrogen deficiency is probably the most common deficiency reported by cannabis growers. It’s usually easy to identify and correct as soon as it becomes apparent, although it shares a basic resemblance to several other deficiencies in the early stages.
This guide will go over some of the most common problems that can be identified by simply inspecting the leaves. It also will explain how to proceed and get those plants on the road to recovery… and a better yield!
- 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
At the other end of the spectrum, nitrogen toxicity results in leaves taking on a very dark green hue that can look almost black in extreme cases.
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.
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.
Phosphorus deficiencies are rare, but should be cause for concern. If it affects plants in the vegetative stage, it can cause reduced growth rates, small leaves, weak roots, and plants that are generally lacking in vigour. In the early stages, phosphorus deficiency usually causes leaves to appear dark but washed-out, with very dark veins and somewhat lighter leaf tissue.
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.
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.
Your plant needs a lot of nitrogen in the vegetative stage, and it’s generally hard to give too much as long as you’re not going completely overboard with nutrients. Nitrogen is a big part of what makes leaves green, and is incredibly important to the process of photosynthesis (making energy from light).
Reduce the amount of nitrogen that is being fed to the plants. If you are feeding extra nutrients, cut down that amount. If you are in the flowering / budding stage, make sure you’re using a formula that’s specifically meant for flowering, or else it could have too much nitrogen.
Ok, you ruled out overwatering, now what?
Different strains react differently to nitrogen toxicity. Some plants get dark green leaves with no clawing. Some strains will get leaves that do the weird 90 degree bend at the tips, while other strains or individual plants start curling like claws and then turn yellow / brown and fall off like a deficiency. Yet these are all signs of too much nitrogen.
- If you use vegetative plant nutrients during the flowering stage, then they’ll deliver too much nitrogen. This is why you need to get special nutrients meant for the blooming / flowering stage. You’ll notice that flowering nutrients always contain a smaller percentage of nitrogen (the first number) compared to nutrients for the vegetative stage. Learn more about marijuana nutrients here.
- Many growers mistakenly keep raising nutrient levels or adding additional nitrogen when they see yellow leaves in the flowering stage, not realizing that it’s natural for plant leaves to start yellowing as harvest approaches. Adding too much nitrogen in the flowering stage can cause nitrogen toxicity even when you can see yellow lower leaves. Nitrogen toxicity in flowering results in smaller yields and airy cannabis buds, so make sure to watch out!
When it comes to nitrogen, this is what your plant needs:
Some examples of good one-part Flowering nutrient systems…
The Claw in the Flowering Stage
Effected leaves likely won’t recover, but you should see the problem halt with no new leaves being affected.
Does your plant have "the claw?" The talon-like leaves that are bent at the ends are a sign that your plant may have nitrogen toxicity. Learn how to fix it.