Marijuana plants “talk to you” in many ways, even when you’re not hallucinating. One of the most important methods they use is the condition of their leaves.
Successful cannabis growers know that daily leaf examination is a way to detect nutrients problems, pests, diseases, and other issues. The issues we’re dealing with today involve drooping and wilting marijuana leaves…
Drooping Versus Wilting Marijuana Leaves
Drooping marijuana leaves aren’t caused by the same problems that cause wilting leaves. In fact, the causes are often oppositional.
How do you know the difference between wilting and drooping?
When marijuana leaves are wilted, they are deflated, empty-looking, wrinkled, hanging down limply.
When marijuana leaves are drooping, they retain their turgidity, the leaf stems stay relatively parallel to the root plane, leaves may look puffy rather than empty, and don’t hang down as much or as limply as wilted leaves.
The article photo shows you a young plant with drooping leaves, but we don’t have a photo of wilted leaves, because wilted leaves are a sign of extreme mistakes, or a fusarium disease plague, and fortunately so far we haven’t had have those mishaps in our marijuana grow rooms.
But if you ever see your marijuana leaves hanging downward instead of horizontal to the root plane, or even reaching towards the sky, all you have to do is look at our drooping leaves photo and compare it to your marijuana leaves. The difference between wilted and drooping leaves will be obvious to you.
What Causes Wilting Marijuana Leaves?
There are two main causes for wilting marijuana leaves: lack of moisture in the root zone, or a pathogenic or mechanical attack on cannabis roots.
The plant disease Fusarium Wilt causes devastating, fast, catastrophic wilting marijuana leaves. Fusarium is a root zone pathogen most often found in poor-quality soils and soilless mixes, but it can infiltrate outdoor marijuana root zones and indoor grow ops through other vectors too.
Other root zone diseases such as root rot destroy roots to the point that they can’t supply sufficient water, nutrients and oxygen to your marijuana plants, and this can result in wilting leaves.
Underwatering causes wilting and is easy to correct—you just need to water properly, perhaps with a little horticultural Vitamin B and iron to help the plants recover. Unless your plants’ roots have been dry too long, your plants will recover within a few hours of watering.
In contrast, an attack of root rot, fusarium or other pathogenic attackers is almost impossible to stop once it has taken hold in your root zone.
Prevention of root diseases is the best tactic, and includes installing beneficial bacteria and fungi in your root zone, using only high quality soil, soilless mix or inert media (such as rockwool or coco), and avoiding habits such as overwatering, or adding sugars to your root zone.
Root water intake is impeded when your marijuana plants are in pots too small for them, which creates root girdling and a “rootbound” condition.
Prevent that by using adequate-sized pots to grow in, and making sure they’re cloth pots, which naturally root prune to prevent girdling and rootboundness.
Dropping Marijuana Leaves: Overwatering & Poor Porosity
When your root zone is too wet and/or lacking in enough pore space to hold adequate amounts of oxygen, your marijuana roots drown. They can’t intake moisture or meet their full obligations for plant metabolism, and leaves droop.
The leaves don’t wilt, because they are full of water. Instead, they hang down from the weight of excess water and failures in the root zone, leading to subdued transpiration (leaf evaporation) and metabolism.
These conditions slow plant growth to an almost full stop, stunt plants, and set up your root zone for pathogenic infections.
The obvious fix is not to overwater, and to add aeration spacers such as coarse, washed perlite into your root zone.
And again, cloth pots are recommended. Unlike plastic pots, they are porous, so oxygenation of the root zone is increased.
Drooping Marijuana Leaves: Grow Light Intensity
Marijuana leaves sometimes droop in a futile effort to get away from intense light dosages. Measuring and analyzing photosynthetic photon flux density (PPFD) at the leaf surface is crucial for marijuana growers, but most growers don’t know how to do it or they have defective light meters. Read here to discover the world’s best cannabis light meters.
As with overfeeding nutrients, if you feed too much light to your plants, it hurts them—and their leaves show it.
It isn’t only that your grow lights are sending too much light to your plants because they’re too close or otherwise too intense, it can also be that your light cycles are too long.
Plants can only process a finite amount of photons each lights-on cycle. This is called the “daily light integral.” After that amount has been delivered, excess light often drives the leaves downward. This is why running your grow lights in grow phase for more than 18 hours per day is probably a waste of electricity, and may be damaging your plants.
Grow lights too close to the canopy, too intense, or too hot (especially HID grow lights), can cause “bleaching” and burning of your cannabis leaves, which also may show signs of drooping.
Grow room temperatures that are too hot, coupled with underwatering, vapor pressure deficit issues, or other problems, can cause drooping or wilting.
Drooping Marijuana Leaves Caused by Vapor Pressure Deficit
All growers should read our article about vapor pressure deficit right here. It explains that the percentage of water in your grow room air relative to the grow room’s air temperature determines how efficiently your marijuana plants’ leaves can transpire oxygen and moisture.
If vapor pressure deficit is out of range, your marijuana plants might not be able to process enough root zone moisture or conduct other metabolic functions that maintain healthy, firm leaves.
Out-of-range vapor pressure deficit is a commonly overlooked and misunderstood marijuana growing problem that slows plant growth and creates many other negatives. Outdoors, the grower can do nothing about it. Indoors, the use of exhaust fans, temperature control, and dehumidifiers can keep VPD within range.
Drooping Leaves Caused by Circadian Rhythm
Your plants are smart. They know when the sun is supposed to come up and set, and they know when grow room lights are supposed to come on and go off. This feature, caused by the plants’ internal circadian rhythm, is more pronounced in some strains compared to others, and at some stages of plant growth more than others.
Seedlings and clones are often seen to be drooping an hour or two before the end of light cycle, especially during grow phase, and older plants may do that too.
You often see marijuana plants drooping during dark cycle or for a little while after lights come on. This is totally natural, and no cause for concern, but if you see persistent drooping near the end of the lights-on cycle, experiment with incrementally and slightly shortening the light cycle to see if the droop goes away.
This often works when the drooping is keyed not just to circadian rhythm, but when plants are being pushed too hard by 19 or more hours of light per day as is sometimes done with autoflowering cannabis.
Now here’s a short cheat sheet for drooping or wilting marijuana leaves:
- If leaves droop for a short time at the end and/or the beginning of the light cycle, this usually isn’t cause for concern.
- If leaves droop for substantial periods during lights-on, look for overwatering, root trapping, poor soil, root zone diseases, vapor pressure deficit or temperature issues.
- If leaves wilt, give your plants water right away. Plants that wilt for more than a couple of hours are permanently damaged and may never fully recover.
- Learn to tell the difference between wilting and dropping leaves.
- Examine your plants’ leaves closely every day you grow.
- For more information, watch the videos embedded in this article.