Diagnose a Cannabis Plant ProblemMay 27, 2019
For new growers, and even experienced ones, to know how to diagnose a cannabis plant problem is not always easy.
Sometimes it can be difficult for you to pinpoint exactly what the problem is.
By looking for certain symptoms on the plant leaves, you can diagnose a cannabis plant problem easily. And with the correct diagnosis, you can get the problem fixed quicker.
This list is going to help you find the problem, fix it, and ensure it doesn’t happen again. If at any point you feel as if you need some extra help, please sign up to our cannabis growers forum and ask for help!
There is no shame in having problems with your cannabis plants, it is all part of the learning curve, all growers have been there, and we will be there again. Ask for help if you need it.
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Yellow or Pale Green Leaves
Usually, when the leaves on a cannabis plant start to turn yellow, it is because of a lack of certain nutrient.
Looking where the yellowing is occuring, can give you a good idea of what the issues is.
Leaves turning yellow at the top of a cannabis plant, can be from light bleaching, whilst yellow leaves at the bottom of the plant can be a nitrogen deficiency.
Leaves turning yellow at the top of a cannabis plant, can be from light bleaching, whilst yellow leaves at the bottom of the plant can be a sign of nitrogen deficiency.
Yellow Leaves on the Top of a Cannabis Plant
If the leaves at the top of your cannabis plant are turning yellow, it is most likely down to 2 things:
- 1: Your light it too close to the top of the plant, or
- 2: The plant could be suffering from a nutrient deficiency such as Iron or Sulphur.
Solution: Whether you’re using LED lighting or HID lighting, your light should be no closer than 18 inches ( unless you’re using a cooling tube or hood). Adjust your light if you think it is too close to the canopy, and see if it makes a difference.
If you think the cause is more likely to be a nutrient deficiency, the cause is probably due to a PH imbalance. Ensure the PH of your medium is set to the correct levels.
If the ph is right, then you should increase the amount of food you’re giving the plants. Just a litttle, small changes are the right way to go, you do not want to over feed your plants.
New Growth Growing Yellow
When new growth start to turn yellow, is is a sure sign that the plant is not eating right. This could be down to either the pH level being off balance, or the nutrient content of the medium being too low.
A deficiency in Iron, and Sulphur can cause new growth to grow yellow on your cannabis plants.
Solution: Adjust the pH of the medium, and feed your plant a slightly higher dosage than what you have been.
If you are growing in soil and haven’t started feeding your plant yet, then this would be a good time to transplant or introduce nutrients to the medium.
Yellowing on Lower Leaves
If the leaves are turning yellow on the bottom of your cannabis plant it will most likely be caused by 2 things:
- 1: A lack of light to the bottom of the plants
- 2: Not enough Nitrogen available to the plants
If the lower leaves are not receiving enough light to be worth being on the plant, the plant will use the stored nutrients in the leaf and let it drop off.
If you have a big canopy, and the lower leaves of the plant can not get enough light, it will start to turn yellow and fall off.
The same will happen with a Nitrogen deficiency. Nitrogen is a mobile nutrient, so it will be moved around the plant to where is is needed most.
As the bottom part of the plant will get less light, your plant will take nitrogen from the lower leaves and move it to where it is needed most.
Solution: You should defoliate and “lollypop” the lower parts of your plants if they get too bushy or too tall. This will ensure good light penetration, so the plant can get a good, even, amount of light.
If you think the cause is from nitrogen deficiency, simply increase the amount of nitrogen in the medium but adding more grow nuits in your next feed.
Leaf Veins Turning Yellow
If the veins of your cannabis plant leaves are turning yellow, but the rest of the leaf stays green, this could be a sign of calcium and magnesium deficiency.
Calcium and magnesium work together in a cannabis plant, if there is a problem with one, there will be a problem with the other.
Most water will contain some trace of calcium, and you only really need to supplement it if you’re using very soft water, or an RO system.
If your plants are suffering from cal mag deficiency, they will not be able to process their nutrients properly and will soon start to suffer.
Solution: First off, you can supplement cal/mag with many different nutrient brands. But as mentioned, your water may already have calcium in it. This means you’re most likely to be deficient of magnesium, rather than calcium
By foliar feeding your plant with 1 tablespoon per gallon of epsom salts, you can give your plants the magnesium it need, right where it needs it.
If your plant is flowering, than foliar feeding is not recommended. In this case, you should use a cal mag supplement, as your plants will be using a lot of calcium at this stage, and can handle the excess.
Purple Stems and Branches
This can be a normal genetic trait displayed by a cannabis plant and is not an immediate concern to a grower.
Purple stems can happen for a number of reason other than genetics:
1, Temperature: If a plant gets too cold, it will release hormones to protect itself. These hormones will make the plant display a purple pigment, and it is also how purple buds are grown.
2, LED Lighting: Many “Blurple” LED lights, will bleach your plants purple. If you’re growing with led, and your plants have purple stems, check the shaded areas of the plant.
If the shaded areas are green, but the parts of the plant that receive direct light are purple, the cause is probably down to light bleaching.
3, Nutrient Deficiency: Deficiencies in Magnesium and phosphorus can force your plant to display purple stems and branches. This is usually down to either the ph is incorrect in the medium, or there is not enough nutrients available to the plant.
Solution: Always ensure your lights are at the correct distance from the top of your plants. Around 40 cm is average.
Temperatures must not fall below 18c, even when the lights are off. Cold temperatures will encourage the plant to express the purple genetics.
Check the Ph and EC of the medium and make sure your plant is getting the right nutrients.
Spots on Cannabis Plant Leaves
If you have found spots on your cannabis leaves, it can be from a few things, bugs, nutrient imbalances, or lights burning the plant after foliar feeding them.
To diagnose a cannabis plant problem by the spots, you should first make sure there are no bugs on your plants.
Look under the leaves where the spots are. This is where bugs will hide out, and if they are the cause, you will find them there.
Orange or Rusty Spots on Cannabis Leaves
Orange spots can be caused by a deficiency in phosphorus, or calcium. You can tell the difference because calcium deficiency will show smaller spots than phosphorus, that will display patches, rather than spots.
In some cases, bugs such as thrips, and spider mites, may cause orange or rusty patches on your leaves. Check on the underside of the leaves an look out for eggs that may be laid there.
Solution: Like most cannabis plant problems, pH or Ec is probably the problem Make sure the Ph is set at the right level for your medium, and your plant is getting the right amount of nutrients.
If you suspect bugs, there will be other symptoms display along with the orange spots. See our pest diagnosis guide if you find bugs.
White Dots on Leaves
Small white dots on the leaves are usually a sign of mites. Mites are small insect that will bite into the leaves and suck juices from them. This leaves behind tiny white or yellow dots.
If you have white dots on your cannabis plant, you most likely have bugs, and you need to check the grow room throurally.
Solution: Find the bugs, diagnose which one it is, and treat accordingly. We have guides on how to kill each bug that can potentially harm your cannabis plants. You can get help finding out which bugs is in your grow room here
Dark Purple/ Black Patches on Cannabis Plant Leaves
There is really only one cause for purple or black blotches on the lower and middle leaves on your cannabis plant, phosphorus deficiency.
This is most likely to occur during the flowering phase of your cannabis grow. Your plants will use a lot more Phosphorus during flower, to build big buds.
If you see purple blotches on your leaves during flower, your yield is being affected. The plant needs more phosphorus, so it can build bigger flowers, so it is taking it from the lower section of the plant.
Solution: When you try to diagnose a cannabis plant problem, always make sure the pH of the medium in the correct range. This can increase the amount of Phosphorus available to the plant.
During the flowering stage of your grow, you should use a PK booster, to give your plant the energy it, and minerals it need to grow bigger buds.
When you first start to see flowers form after veg, then introduce the PK booster for best results.
Silvery “Spit like” Patches
Patches likes this are caused by thrips. The females pierce the leaves of your plant, and lay eggs into them. The patches will look like dry spit, or “snail Trial”.
Solution: Treat for thrips. A neem oil pesticide used every three days, for three treatments will get rid of most infestations.
If the problem continue, use diatomaceous earth on top of your medium. This will stop the larvae emerging from it.
White Powder on Cannabis Leaves
A white powder on your cannabis leaves can be cause by 2 things, nutrient splash back, or powdery mildew.
If you foliar feed, or splash some nutrients onto your leaves, the salts will be left behind when the water evaporates. This will leave a fine white powder on your leaves, that can simply be rinsed off.
Powdery Mildew on the other hand is a type of mould that can grow on your cannabis plant. If the condition are right the mould spores will germinate and soon infect your whole grow room.
If you diagnose a cannabis plant problem to be powdery mildew, the crop might need to be destroyed.
PM can be treated on vegetating plants, but if it is found on flowering plants, the buds should not be used.
Solution: If it is simply salt build up, then wash the leaves a little with some water to remove it.
For a guide on treatment of powdery mildew, and decide whether to scrap the crop or treat it, you should see our guide in plant problems
Diagnose a Cannabis Plant Problem by Misshapen Leaves
Misshapen leaves on a cannabis plant can be caused by a number of things. Environment, damage, genetics, and pest.
Your leaves may not be fully damaged, but show sign of curling, or drooping. All of these symptoms are signs of different problems:
Edges of leaves Curling Upwards
This is called “Canoeing”. Is is a common sign to show that the plants are suffering from heat stress. Either the temperatures are too high in the grow room, or the light is too close to the plants.
Solution: Increase extraction, and bring in cooler air to the grow room. Also try raising your lights, and add another fan to the grow room, this can help with reducing temperatures.
Edges of Leaves Curling Downwards
If the edges of your leaves are curling inwards, it is a classic sign of “The Claw”. This is when too much nitrogen is in the plant. The leaves will be a dark shade of green and claw under themselves.
Solution: To fix a nitrogen Excess, you should flush your medium and re feed with a lower EC, or a lower dose of nitrogen. Plants in the flowering stage need less nitrogen than plants in veg. Always use specific bloom food when your plants are flowering.
Droopy Leaves on a Cannabis Plant
Before you can diagnose a cannabis plant problem, that is causing droopy leaves, you have to think about your watering schedule.
If the leaves on your cannabis plant are drooping they can be suffering from a number of issues. The main 2 being, over, or underwatered.
When a cannabis plant receives too much water, the leaves will be full of it, and because of that they will be heavier.
On the other hand, if you do not water enough, the plant will not be able to move nutrients around itself, and will soon lack the energy it needs to hold the leaves up.
It may also be down to a root bound plant. The roots may be constricting themselves because of lack of space, causing them to starve of oxygen.
Solution: Your plants need to be in a suitable sized pot for their root ball. If the root ball becomes restricted, then the plant can not eat, drink and breath properly.
Make sure you’re using the correct water method for the medium you are growing in!
- If you’re growing in coco, watering daily is fine, just make sure you remove all run off.
- For hydroponics systems, make sure your air stones are working, and not blocked by any roots.
- For soil growers, water your plants correctly, once every 2 or 3 days is enough. You do not want the medium to fully dry out, but cannabis plants in soil prefers dry spells. This helps them get the air the root zone needs.
Leaves Pointing Upright
For leaves to be point upwards at a slight angle is a good sign. But, if they start to grow vertically upwards, then they could be suffering from heat stress, or too much light.
Solution: Leaves point up, is an environmental factor, so you need to adjust temperatures and humidity to suit the stage of plant growth.
Raise the lights and increase the amount of air being drawn out of the grow room.
Final Notes on How to Diagnose a Cannabis Plant Problem
This list can go on, and be broken down into smaller sections. These are the most common problems you will find on a cannabis plant.
It is a rare a cannabis plant problem can not be fixed. In most cases, a plant can be fixed and back to health within a week. You juts have to treat the problem when you see it, don’t wait.
Anytime you feel like you need some extra help to diagnose a cannabis plant problem correctly, visit out cannabis growers forum.
Thanks for reading! You can find more information on anything related to growing in our cannabis grow guides. All of our guides are written by experienced growers, and members of our cannabis growers forum. If you have any questions, sign up. It is free, and we are always happy to help!