Overfertilized Cannabis: How to Spot Nutrient Burn and Fix It

To prevent overfertilizing your cannabis, feed a little less than the label says, check your pH, and watch for the warning signs: leaves clawing downward, burnt tips and an unnaturally dark green color. Fertilizer is a great way to boost a weed grow and get a bigger, better harvest, but contrary to what a lot of rookie growers think, more food doesn't mean bigger plants. Overdo it and you get the opposite: stalled growth and fewer, lower-quality buds.

When a plant is overfertilized, you get the reverse of what you wanted: growth stalls and the plant puts out fewer buds, and worse ones. That's because excess fertilizer leaves a toxic buildup of salts in the growing medium, and the root system can no longer absorb and use the nutrients properly.

How do you recognize overfertilizing?

Broadly speaking, the classic signs of overfertilizing are leaves curling down like claws, burnt tips and very dark green coloring. But the exact symptoms depend on which element is in excess, because the plant reacts differently to each one. Below are some pointers to help you figure out which nutrient is causing trouble.

How do you avoid overfeeding your cannabis plants?

Measure the pH:

This is huge for keeping overfeeding in check, because pH is directly tied to how soluble nutrients are and how well roots absorb them: if it's too high (or out of range in either direction), plants can't take up and use nutrients properly.

Measure your fertilizer dose exactly:

Always measure the cannabis fertilizers you're about to apply, because you should start with a little less than the manufacturer recommends (if the plants respond well, you can bump the dose up).

Overfertilizing usually shows up as leaves clawing downward, burnt tips and very dark green tones; depending on which element is in excess, the plant will react differently.

Nutrient excess in the vegetative stage

In general, overfeeding during veg means too much nitrogen (the nutrient that dominates grow formulas). Here's more on what that looks like.

Signs of overfertilizing in cannabis: nitrogen

Too much nitrogen can leave leaves unnaturally dark green and glossy. Fungus and pest attacks get more common with excess nitrogen; stems turn brittle and gradually droop; and with severe overfeeding, leaves turn a coppery brown, dry out and drop. If this happens in veg, the plant grows a lot less; in flower, buds will be sparse and smaller.

How do you recognize overfertilizing in flower?

Overfeeding during flowering usually means an excess of one specific nutrient: phosphorus (P) or potassium (K). Here's how to tell.

Phosphorus:

Phosphorus excess in flower is hard to spot because cannabis tolerates a lot of it, but when it happens you may notice brown spots on the leaves. Too much phosphorus translates into a shortage of calcium, magnesium, copper, iron and zinc.

Potassium:

Looks a lot like a magnesium, manganese, zinc or iron deficiency. If those nutrients are running short, a potassium overdose is probably right around the corner.

Either way, if you want specifics, here are the telltale symptoms of excess for each nutrient:

Calcium:

the plant wilts and develops shortages of potassium, magnesium, manganese and iron.

Magnesium:

magnesium excess is very rare; when it happens, it blocks the uptake of other nutrients, especially calcium.

Sulfur:

if you're not using sulfur-based products, sulfur will never be a problem. With too much sulfur, leaves grow small and very green and vegetative growth stalls (severe excess causes burn).

Iron:

iron excess isn't common, but it can cause a phosphorus shortage.

Boron:

yellowing foliage. Leaf edges dry out first and the rest of the leaf gradually follows; boron excess comes from overusing crop-protection products rich in boric acid. It causes necrosis (dead tissue) and leaf drop.

Zinc:

excess zinc shuts down iron uptake and is highly toxic, so in severe cases it can kill the plant fast.

Manganese:

manganese plays a role in how the plant handles heat stress, but with too much of it, growth slows down.

Chlorine:

leaf tips and edges get burnt.

Copper: ​​

excess copper causes an iron shortage and chlorosis (yellowing from lost chlorophyll), stalls the plant's development and clogs up the root system, usually from overusing copper-based crop-protection products.

Molybdenum:

too much molybdenum means a shortage of copper and zinc.

Cobalt:

it could interfere with nitrogen uptake; cannabis doesn't need cobalt to develop.

All these different symptoms show how tough it is to pin down a specific nutrient problem, and as mentioned above, you don't actually need to nail the exact one. In most cases, the problem is simply too much fertilizer, and the fix is a lower dose.

How do you fix overfertilized cannabis?

It's worth pointing out that excesses rarely come from individual nutrients, since few growers apply nutrients one by one. They usually come from overdoing one particular fertilizer product, so cutting back that dose is enough to clear things up.

For mild overfeeding, just water with plain, nutrient-free water for one to two weeks. That washes accumulated salts and excess nutrients out of the growing medium. For severe overfeeding, if you think the plant is in danger, the only real fix is a full flush.

You can avoid overfertilizing. At first it can feel like information overload that leaves you more confused about what's going on with your plants. So slow down, observe and start writing down the symptoms you see. Bit by bit, you'll figure out what's happening.

And if you're still a little lost, ask other growers to take a look at your plants and help you sort it out.

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