Optimist: The sun will be up soon.
Pessimist: It’s so dark I can’t see my hand in front of my face.
Optimist: I’m going to have free, primo weed for months when I harvest this crop!
Pessimist: I’m probably going to kill these plants before they even start to flower.
That last pessimist might be right if she doesn’t know the basics of growing cannabis — and if she doesn’t know that the leaves of her weed plants are telling her that they need immediate attention.
We’ll leave the discussion of best practices for tending marijuana grow for another article. But the warning signs that plants send when they need help are just as important to understand.
They usually send those signs through their leaves. Here are the most common problems that cannabis growers experience and their symptoms.
1. Too Much Water — or Not Enough
When the leaves of a cannabis plant start drooping, and the plant doesn’t seem to be growing very fast, the issue is likely to be that you’re not giving them enough water. Touch the soil to see if it’s moist or dry; chances are that it’s bone dry.
Good news: if you water the plant immediately, the leaves should bounce right back within an hour. Now, water more frequently.
On the other hand, leaves that seem stiff and curl down toward the stem are a clear sign that you’ve overwatered the plant. The leaves are stiff because they’re overloaded with water.
Don’t water for a few days, and the issue should resolve itself. Then cut back on your watering schedule, and always check the soil to see if your plant even needs water before you douse it again.
2. Temperature Issues
Weed plants only thrive in specific temperature ranges, generally between 65° and 80° depending on their growth stage. The “right” temperatures for a plant largely depend on its genetics, so if you’re growing outdoors, you should only choose strains that prefer the climate in your part of the country.
Once plants are in a grow room, though, it’s up to you to provide an environment in which they can thrive and watch their leaves for signs that they’re not happy.
Leaves that take on a deep purple color signal that the temperature is too cold; the plants may not grow properly or may freeze to death. If the environment is too warm, you’ll see the leaves start to fold up lengthwise (growers call it cupping). Growth will slow, and mold may start to develop.
Proper ventilation and insulation, using A/C or heating systems, and adjusting the types of grow lights may be expensive, but they can solve temperature issues permanently.
Gardeners call this problem nutrient burn (or eutrophication) because it’s caused either by using too much fertilizer or by using the wrong type and amounts of nutrients for your plants. Over-fertilizing is the more common issue because plants don’t need a lot of food until they’ve really started to grow.
Before panicking, first check the pH of your soil, fertilizer, and water. If it’s too high or too low for your plants (most prefer a pH of 5.5-6.5), “nutrient lockout” will prevent the plant from absorbing food. If that’s not the problem, read on.
The effects of nutrient burn usually start with the tips of the leaves, which start to turn brown. If it’s not caught early, the sides of the leaves will turn brown and/or gray and become brittle. Eventually, the leaves will take on a golden brown color and die if the problem hasn’t been solved.
Deal with this in the same way you deal with overwatering: stop feeding the plant for a few days until it recovers. If that doesn’t help, flush the soil with lots of pH-neutral water (five times the volume of the pot), let them recover, and start again.
4. Nutrient Imbalance
When weed plants receive the wrong balance of nutrients, bad things can happen.
The most common issues are not enough potassium, which causes leaves to turn bright yellow and then die, and too much nitrogen, which causes the leaves to turn dark green and curl downward (often called “clawing”).
Both are signs that you should check the balance of nutrients in the fertilizer you use. An N-P-K ratio (nitrogen-phosphorus-potassium) in the 3-1-1 range is desirable during the vegetative stage, and a ratio of about 1-3-3 is better for the flowering stage.
Cannabis plants also need micronutrients like boron, calcium, and manganese; deficiencies will show up as golden brown spots on the leaves (boron), fragile leaves (calcium), or yellow or brown spots (manganese). Be sure your nutrient mix includes all of the crucial micronutrients.
Indoor cannabis plants whose upper leaves turn yellow and look burnt are screaming for relief; the grow lights are too close. Raise the lights six inches higher if the symptoms are mild and by a foot if they’re severe.
Not all of the issues that cannabis plants suffer are completely the fault of the grower. High humidity and poor airflow can encourage powdery mildew to grow on the plants, but there’s no way to guarantee that mildew won’t become a problem in a grow.
The telltale sign is a white powder that appears on plants and begins to spread. Powdery mildew can be killed with a mildew spray available at all garden centers, but it can destroy your plants if it’s not caught early.
The two pests that most commonly attack weed plants are spider mites and aphids. They each attach themselves to the undersides of leaves, and they’re each too small to distinguish without a magnifying glass — at least until they proliferate and become a major threat to the plants.
Spider mites (specifically, two-spotted spider mites) love cannabis. Before you can tell that they’ve taken up residence, you’ll probably see small white or yellow spots on the leaves, giving them a mottled appearance. Over time, the leaves look bleached, fall off, and the plant eventually dies.
It’s a little easier to tell that aphids are the problem since mites only look like tiny dots, but aphids look like green dots with long legs. They feed on the plants’ nutrients and sap, so the leaves will turn yellow or curl up once aphids have started their feeding.
Introducing ladybugs into the grow (they may show up naturally outdoors if they sense there are insects available for dinner) will fight both pests. Predatory mites like P. persimilis will eat spider mites. You can find both at garden stores.
You can also try hitting the leaves with forceful water sprays for three or four days to wash off the pests, and contract sprays like insecticidal soap or horticultural oils (many say neem oil works well) may help. Continue the treatments for weeks after you think the problem has been solved; spider mites, in particular, can lay millions of eggs, which can hatch and start reproducing within a week.
Cannabis Leaf Problems and Symptoms: FAQ
Q: What does it mean if buds and small sugar leaves turn moldy and brown?
A: There’s a good chance it’s a bud rod caused by poor airflow and high humidity. Solve those problems, cut off the areas of the plant that have been affected (yes, including any flower that’s rotting), and you may be able to save the plant.
Q: My plant seems to be wilting during the day but looks better at night, and the leaves are turning light green. Is that because of something I did?
A: It’s probably another fungus called fusarium, which enters plants through the roots. So it probably isn’t anything you could have prevented, except by rotating crops or using new soil every year. There’s no way to “fix” the problem; first, isolate the affected plant so the fungus can’t spread, and then you’ll have to toss the plant and the soil it grew in. Sorry about that.