The Causes and Cures for Deformed Leaves in Cannabis Plants
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Evaluating Cannabis Leaves for Health Problems
When raising cannabis, it’s helpful to think of your plants as close friends. Get to know them, especially their leaves, and you’ll know when they need some extra TLC.
Your plants may not have the words to tell you how they feel, but if you pay attention to their leaves, you’ll find out they’re pretty good communicators.
Leaf Structure and Genetics
When growing cannabis, it’s helpful to become familiar with the types and structure of the leaves you will commonly see. There are three main types of cannabis–sativa, indica, and ruderalis.
Generally, sativas provide an uplifting, energizing effect, giving the user a greater “high” and spurring increased creativity. These plants grow tall with leaves that tend to be long and thin and of a lighter green color.
The hands of these leaves can have as many as 13 “fingers.”
Indicas, on the other hand, are known for their strong relaxing effects. Indica plants are considerably shorter with leaves that are wide and broad and deeper in color than sativas.
The hands of these leaves have fewer and fatter fingers, usually in the range of about seven to nine. Ruderalis, like sativas, have slender fingers, though only about three to five on each leaf hand.
Ruderalis is different than its two cousins because of its special autoflower qualities.
Most plants cultivated in the U.S. today are hybrids, strains that attempt to combine into one the best qualities of sativas and indicas. The types of leaves your plants produce can give you an idea if your plants will lean more toward a sativa type or an indica.
All three types of cannabis have both sugar and fan leaves, both of which help with plant growth and development. Located within and reaching out from the buds, sugar leaves give the buds structure.
Due to their harsh taste, however, growers tend to trim them away. Fan leaves play a huge role in photosynthesis, creating energy for plants to grow during the vegetative phase and for storing nutrients.
Because of their largesse, fan leaves may be trimmed throughout the flowering phase to allow sunlight to reach lower leaves on a stem.
Leaf Condition as a Sign of Overall Health
Leaves are the lifeblood of your plants, so no matter the strain you grow, their condition will give you a good indication of whether or not you are giving them the proper care they need. Keeping an eye on any changing conditions of your leaves will alert you to changes in their health.
If you do find a problem developing, catching it early can make a huge difference in their health and their subsequent yield.
What Causes Deformed Leaves in Cannabis Plants?
There are many reasons why your cannabis plants’ leaves can become deformed. Deformed leaves are simply a plant’s way of calling out for help.
When formerly healthy leaves start to curl upwards or downwards, if they start to droop or develop burnt edges, you’re being alerted that something is wrong. In most cases, this is an indication that the nutrient or soil pH range is incorrect.
And it’s the first thing you should check and seek to correct.
If, however, the pH for your nutrients as well as that for your soil are in the correct range, then you’ll have to dig a little deeper. Often deformities are caused by serious problems and unless you want a substantially reduced yield, you’ll need to attack those problems head-on.
Nitrogen is good for your plants, but sometimes too much of a good thing isn’t good and causes harm to your plants. Your plants will tell you they are getting too much nitrogen at any one stage of their growth by curling downwards and taking on a clawlike appearance.
Nitrogen toxicity will show as very dark green leaves on your plants. It is commonly caused by growers who use a nitrogen-heavy fertilizer during the vegetative stage and who forget to back it off during flowering.
This can be corrected by switching out your fertilizer to one with less nitrogen.
If your plants become droopy and wilt, if they are experiencing some curling and cupping, or if they begin to turn yellow, it is their way of telling – even screaming to you – that you are overwatering them. This is not an uncommon problem for growers, especially those new to cultivating cannabis.
Overwatering can be dangerous as too much water will suffocate the roots so they can’t get the oxygen they need. It can also lead to the development of mold and fungus and may invite pests.
Like most solutions, early intervention is key. If you wait until the roots are too rotten, your plants may not be able to come back.
If you discover your plants becoming sickly from overwatering, stop watering them all together for a while. This will allow your plants to use up what water is in the soil and for some of it also to evaporate.
Distressed plants have difficulty uptaking nutrients, so you should also stop using nutrients until your plant has recovered.
Drill some holes in the bottoms of your containers so water won’t build up unnecessarily in the soil. You may want to combine this with the use of a stronger fan as such a fan will remove more water through the leaves so the plant will uptake more water from the soil when it is needed.
If your plants are wilting, this means they have reached an advanced stage. You’ll want to move them into the shade and remove and discard the dying leaves.
Don’t reintroduce water until the soil is dry, and when you do, introduce it slowly.
As for outdoor plants, water them during the day when the evaporation rate is higher. This way, your plants won’t be overly exposed to wet earth.
Underwatering can also be a problem that will usually show itself in downward-curling leaves that barely hold to their stems. The solution for this is easy enough. Water them and check the soil regularly.
Keep to a healthy schedule, and they should bounce back fairly quickly.
Light burn is exactly what the name suggests. Your plants become burned because the light source is too close to their tops. Typically, light burn will cause your leaves to yellow and curl.
Make sure you are using the correct lights and use your dimmer to reduce the level of light intensity for your grow. Place them at a healthy distance from your plants.
As noted earlier, overwatering can cause root rot, and like everything with your plants, it will show itself in their leaves. Leaves will become frail and curl downwards, and brown splotches may appear on their face.
Good grow soils and adhering to a healthy watering schedule will help avoid root rot.
Healthy plants need a good supply of nutrients, particularly the NPK big three of nitrogen, phosphorous, and potassium. Cannabis also requires several other nutrients.
A shortage of any type can stunt plant growth or even kill it.
Once again, a deficiency of any type will again show itself in the leaves. Determining what nutrients your plants may be in short supply of, however, can be challenging.
For example, leaves that curl upward can indicate a potassium deficiency, but it can also be caused by overwatering. Likewise, if your leaves curl downwards, it could indicate potassium or phosphorous deficiencies–or, once again, overwatering.
Plants need plenty of phosphorous while flowering and a shortage of this important nutrient will also cause your plants to turn a purplish color with dark patches.
Potassium is known to be highly soluble, meaning it can leach from soils. It also arises due to the presence of too much calcium and nitrogen.
This can make it hard to fix when you are using a combined-nutrient fertilizer. The best way to fix a potassium deficiency is to use a single-nutrient potassium fertilizer.
Fortunately, phosphorous deficiencies are less common, but if you do encounter such a problem, it can be treated by adding phosphorous-rich additives to your soil, including fish meal, worm castings, crab shell, or soft rock phosphates. You should also keep your grow room temperatures at or above 15 degrees Celsius so your plants can better absorb the phosphorous.
Don’t worry if the impacted leaves don’t improve in color and vitality. This is to be expected. Instead, watch to see that any new growth is healthy in appearance.
Bugs and Pests
These are the bane of every grower’s existence and may include various types of fungus, gnats, or mites. Typically, they will cause leaves to turn yellow.
To prevent an infestation, you’ll need to pay special attention to your leaves by regularly inspecting them with a loupe or microscope. If an infestation develops, you may need to sterilize your soil.
Products like neem oil and slug traps will also help to reduce the risks of infestations.
Fungus gnats often appear as a result of high humidity or overwatering (or both!). You can fix such a problem by limiting watering so the soil dries out, then covering the soil with sand or perlite to prevent such gnats from returning.
You can also use foliar sprays or insecticidal soaps to rid your plants of common pests while inviting in beneficial insects like ladybugs to keep the unwelcome types away.
A good friend listens to his friend when he has a problem, and if you want to be a good friend to your plants, you’ll need to “listen” to the messages they send through their leaves. Paying attention and catching and addressing any problems early on is key.
With proper attention and care, if your plants experience distress, you should be able to nurse them back to good health.