Nitrogen Toxicity

Nitrogen toxicity is too much nitrogen for too long. The plant keeps taking it up because it cannot refuse, and the excess distorts the leaves and delays flowering.

What you are seeing#

Color first. The leaves are a dark, almost blue-green instead of a healthy medium green, and they have a glossy or waxy sheen that catches the light. Healthy cannabis leaves are matte.

Then shape. The leaf tips bend downward into a hook, and in a bad case the whole leaf curls under like an eagle's claw. This shows up on the bigger, more mature fan leaves in the middle and upper canopy first, because those are the leaves doing the most uptake.

Late stage adds burnt, brown tips and yellowing at the very ends of the leaf blades. In flower you get loose, leafy, slow-ripening buds and stems that stay soft and green.

Why it happens#

Excess nitrogen drives leafy vegetative growth at the expense of everything else. It also raises salt levels in the root zone and interferes with calcium and potassium uptake, which is where the tip burn comes from.

Beginner causes:

• Following the bottle's full-strength schedule. Most nutrient labels are written at roughly double what a beginner plant in soil actually needs.
• Using a grow formula into flowering instead of switching to a bloom formula around week two or three.
• Feeding nutrients on top of fresh pre-amended soil that already has four weeks of food in it.
• Adding bat guano, blood meal or a "boost" on top of a complete feed.

How to fix it#

1. Stop feeding nitrogen today. Next watering is plain pH-corrected water, nothing else.
2. In coco or hydro, flush the root zone with three times the pot volume of pH-corrected water until runoff EC drops to roughly 0.8 to 1.2, or 400 to 600 ppm on the 500 scale.
3. In soil, one heavy watering to runoff is enough. Repeated soil flushes waterlog the root ball and cause a second problem.
4. Resume feeding after two waterings at half your previous strength. If you were running EC 1.8, come back at 0.9.
5. In flower, switch to a bloom formula with a lower first number. Aim for a nitrogen level around a third of what you ran in veg.
6. Over the next week, watch canopy temperature and keep the fan moving. Dark, clawed leaves transpire poorly and sit wetter than they look.

How to tell it apart#

Versus nutrient burn. Related but not the same. Nutrient burn is a general salt overload and its signature is crisp brown tips on the newest, fastest growing leaves, with normal leaf color. Nitrogen toxicity changes the color and shape of the whole leaf before any tip burns. If the leaves are dark and clawed, it is nitrogen. If they are normal green with burnt tips, it is overall salt.

Versus overwatering. Both produce drooping, downward-curled leaves. The discriminator is stiffness and color. Overwatered leaves are limp, soft and normal green, and the whole plant sags from the stem. Nitrogen-clawed leaves are firm, dark and glossy, and the stems are still upright. Also lift the pot. Overwatering means the pot is heavy days after watering.

Versus pH lockout. Lockout produces deficiency symptoms, not excess ones. If your leaves are too dark rather than too pale, lockout is not your answer.

How long until it looks better#

Slower than most problems. Clawed leaves rarely uncurl, and the dark color takes ten to fourteen days to fade back toward normal as the plant grows out of it.

Judge the fix by new growth again. Leaves that open after the flush should come in a lighter, matte green and lie flat. That takes about seven days in coco or hydro and up to two weeks in soil, because soil releases stored nitrogen slowly and there is no fast way to hurry it. If you were deep into flower when it started, expect the harvest to ripen a few days late.