Problem: Dark green leaves, weak stems, and overall slow growth. Marijuana leaves that are nitrogen toxic often get “The Claw” or talon-like leaves that are bent at the ends. The also do a weird curving / cupping that is often mistaken for overwatering, but is unique to nitrogen toxicity. You can see a picture of this leaf “clawing” below.
Leaves that turn into claws often start turning yellow and dying if the nitrogen toxicity is not treated, much like a nitrogen deficiency, only the leaves will continue to get more and more clawed. Leaves eventually turn yellow or brown and fall off. You can tell if yellowing is caused by too much nitrogen because the rest of the plant will be dark green, and the yellowing leaves will turn into claws first.
Different strains react differently to nitrogen toxicity. Some plants get dark green leaves with no clawing. Some strains will get leaves that do the weird 90 degree bend at the tips, while other strains or individual plants start curling like claws and then turn yellow / brown and fall off like a deficiency. Yet these are all signs of too much nitrogen.
Solution: Reduce the Nitrogen
Reduce the amount of nitrogen that is being fed to the plants. If you are feeding extra nutrients, cut down. If you are in the flowering / budding stage, make sure you’re using a formula that’s specifically meant for flowering, or else it will have too much nitrogen.
If you are not feeding extra nutrients, you may have “hot” soil that has been supplementing with extra nutrients. In that case, flush your plants with filtered, pH’ed water to help leech out the extra nitrogen.
Effected leaves likely won’t recover, but you should stop seeing the problem spread to more parts of the plant.