Two of the most common problems that puzzle cannabis growers are light burn and nutrient burn. Both cause leaves to yellow, brown, and look scorched, and at a quick glance they can seem like the same issue. The trouble is that they have completely different causes and require opposite solutions, so misdiagnosing one for the other can make the problem worse. The good news is that once you know what to look for, the two are usually easy to tell apart based on where the damage appears on the plant and the conditions that led to it. This guide explains how to distinguish light burn from nutrient burn so you can fix the right problem.
Understanding Nutrient Burn
Nutrient burn happens when a plant receives more nutrients than it can use, and the excess damages the plant tissue. The classic sign is burnt, brown, crispy tips on the leaves, often starting at the very ends of the leaf points and progressing inward along the edges if the overfeeding continues. In its early stages the leaf tips may look slightly yellow or take on a darker, almost glossy green before turning crispy and brown. Because nutrient burn comes from the nutrients the plant takes up through its roots, it tends to show first on the parts of the plant that are growing most vigorously and consuming the most, and it is not tied to proximity to the light. Overly strong nutrient solutions, feeding too frequently, or a buildup of salts in the medium are the usual culprits. A plant suffering from nutrient burn is essentially being overfed, and the damaged leaf tips are its way of showing that it has had too much.
Understanding Light Burn
Light burn occurs when the grow light is too intense or positioned too close to the canopy, and the leaves nearest the light are damaged by the excessive light and heat. The telltale sign is that the damage appears on the uppermost leaves, the ones closest to the fixture, while the rest of the plant looks healthy. Light-burned leaves often turn yellow while their veins may stay greener, and the tops of the plant can take on a bleached, faded, or yellowed appearance, sometimes with the buds nearest the light losing color or developing a washed-out look in severe cases. Because light burn is about proximity and intensity, it is concentrated at the top of the canopy directly under the light, which is the opposite of where you might expect a nutrient problem to concentrate. Plants under powerful modern LEDs are particularly susceptible if the light is run too close, since growers sometimes assume cooler-running LEDs can be placed nearer than they actually should be.
Telling Them Apart
The single most useful clue for distinguishing the two is location. Nutrient burn shows up as crispy brown tips and edges that can appear throughout the plant and is driven by what the roots are taking up, whereas light burn is concentrated on the top leaves closest to the light while lower foliage stays healthy. If your damage is clustered at the very top under the fixture and the leaves are bleaching or yellowing with greener veins, light burn is the likely cause. If instead you see burnt brown tips on leaves regardless of their position relative to the light, and especially if you have been feeding heavily, nutrient burn is more probable. Context fills in the rest of the picture, so consider whether you recently increased your nutrient strength, which points toward nutrient burn, or whether your light is positioned very close to the canopy or was recently moved nearer, which points toward light burn. Checking the distance from your light to the canopy and reviewing your feeding routine usually settles the question quickly.
Fixing Each Problem
The fixes are as different as the causes, which is why correct diagnosis matters so much. For nutrient burn, the solution is to reduce the nutrient load, which means cutting back the strength of your feeding, flushing the medium with plain pH-adjusted water to wash out excess salts, and then resuming feeding at a gentler concentration once the plant recovers. The damaged leaf tips will not heal, but new growth should come in clean if the overfeeding stops. For light burn, the answer is to reduce the light intensity reaching the plant, most simply by raising the fixture farther from the canopy or dimming it if your light allows, until the affected leaves are no longer being scorched. In both cases, the already-damaged leaves remain damaged, so success is measured by whether new growth comes in healthy. Watching the plant over the following days confirms whether your correction worked, and resisting the urge to make multiple changes at once keeps the diagnosis clean.
Frequently Asked Questions
Where does light burn appear on the plant?
Light burn appears on the uppermost leaves closest to the light, often bleaching or yellowing them while veins stay greener, while the lower plant looks healthy. Its concentration at the top is the key clue.
How do I fix nutrient burn?
Reduce your feeding strength and flush the medium with plain pH-adjusted water to remove excess salts, then resume feeding more gently. Damaged tips will not heal, but new growth should come in clean.
Can LED lights cause light burn?
Yes. Powerful modern LEDs can burn the canopy if run too close, partly because growers assume cooler-running LEDs can be placed nearer than they should. Raising or dimming the light corrects it.