Health Library

Nitrogen deficiency is the most common nutrient issue you'll run into. Because nitrogen is mobile, the plant pulls it from older lower leaves to feed new growth — so you'll always see symptoms starting at the bottom and working their way up. Affected leaves fade to a uniform pale yellow (tips first, spreading inward across the whole blade). Unlike magnesium deficiency, the yellowing is even across the leaf — veins and tissue all turn yellow together. It's most common during rapid veg growth when demand spikes, or when plants are root-bound or overwatered. Some yellowing of lower leaves during late flowering is completely normal as the plant redirects energy to buds — don't overcorrect it.

Potassium deficiency is one of the easier problems to identify — look for brown, crispy leaf edges on the lower leaves while the veins stay green. It's a mobile nutrient, so the plant robs older leaves first. Yellowing starts at the leaf margins and tips, then works inward toward the midrib with a blotchy, patchy pattern (not the smooth uniform fade you'd see with nitrogen). Stems and petioles may turn reddish. Plants often show increased stretching between nodes early on, then stunted growth as it worsens. K deficiency tanks disease resistance and makes plants vulnerable to drought. Common causes: pH above 8.0 in soil (7.5 in coco, 7.1 in hydro), excess calcium or nitrogen locking out K, overwatering, salt buildup, or cold root zones. Demand surges hard during flowering weeks 3–7.

Chlorine deficiency is arguably the least likely nutrient problem you'll ever encounter. Plants need only trace amounts of chloride and naturally accumulate 10–100x more than their minimum requirement from tap water, atmospheric deposition, soil reserves, and fertilizer residues alone. The only conceivable scenario would be using pure RO or distilled water with chlorine-free nutrients in a completely sealed system — something virtually no grower does. In reality, chlorine toxicity from heavily chlorinated municipal water is the far more common concern. Because chlorine is a mobile nutrient, deficiency would theoretically appear on older leaves first, though this has essentially never been documented in cannabis cultivation.

Sulfur deficiency looks almost identical to nitrogen deficiency — but on the opposite end of the plant. Because sulfur is only semi-mobile, initial chlorosis appears in the middle of the plant and on younger leaves rather than at the very bottom like nitrogen (Cockson 2019). That's the fastest way to tell them apart. The yellowing on individual leaves starts at the base near the petiole and spreads outward toward the tips — this base-to-tip progression is the key diagnostic pattern for sulfur. Initial yellowing is generally uniform (unlike iron, where veins stay distinctly green), though interveinal chlorosis can develop as symptoms progress on fan leaves. Flip affected leaves over — pink, red, or orange coloration on the undersides is the most reliable confirmation you're dealing with sulfur and not something else. Sulfur deficiency is uncommon because most fertilizers, tap water, and even atmospheric deposition provide enough. It's most likely in pure hydroponic systems, heavily leached sandy soils, or when pH drops below 5.5 (on the alkaline side sulfur holds on until 8.5 in soil, 8.0 in coco, 7.6 in hydro). Left uncorrected, it severely impacts terpene and aroma production — expect weak, bland, tasteless buds. Research shows yield reductions up to 34% in sulfur-starved plants.

Light burn happens when leaves sit too close to your grow light -- whether that's a high-powered LED or a big HPS -- and the intense PAR causes photo-oxidative damage to the tissue. The first thing you'll usually notice is the top leaves 'praying' upward or tacoing before any color change appears, and then the tissue closest to the light starts bleaching yellow-to-white while the rest of the leaf stays green. That's the key differentiator from heat stress: heat stress causes broad curling with browning edges across the canopy but no bleaching, whereas light burn is strictly proximity-based -- only the leaves physically closest to the bulb get hit. It's also different from iron deficiency, which targets new growth regardless of light distance. One easy diagnostic trick: light-burned leaves feel firm and resist plucking, while nitrogen-deficient yellow leaves wilt and practically fall off on their own.

Whiteflies are tiny, white, moth-like insects that cluster on leaf undersides — and the easiest way to confirm them is the shake test: gently shake the plant, and if a white cloud flies up, you've got whiteflies. They feed by piercing leaf tissue and sucking out sap, excreting sticky honeydew that quickly attracts black sooty mold. What makes whiteflies especially hard to eradicate is their life cycle — the adults you see flying around are only part of the problem. The immobile nymph stages are flat, translucent, and scale-like, glued to the undersides of leaves where they're easy to miss and resistant to most contact sprays. You'll often kill the adults only to have a fresh wave emerge from nymphs you didn't notice. Don't confuse them with aphids — whiteflies ALL fly when disturbed and are uniformly white, while aphids are mostly wingless and come in various colors. Whiteflies can also transmit plant viruses (like TYLCV and ToCV) that are incurable once established, so early detection matters. They thrive in warm, still conditions and are a common problem in greenhouses and indoor grows with poor air circulation.

Powdery mildew is one of the most common fungal diseases in cannabis — and one of the sneakiest. It's caused by obligate parasitic fungi that thrive in humid air (above 55% RH) with poor circulation, but here's what trips people up: unlike most fungi, PM doesn't need wet leaves to infect. It actually prefers dry leaf surfaces with moisture in the surrounding air. Spores spread easily through air currents, on clothing, or via contaminated tools, and a single colony can spread through an entire grow room within a couple of weeks if left unchecked. The easiest way to tell PM apart from trichomes is the wipe test — PM rubs off cleanly with a finger, while trichomes are sticky and don't wipe away. If you see white patches that smear when you touch them, you've got powdery mildew. It's especially dangerous during flower because infected buds are unsafe to consume.

Nutrient burn is one of the most common problems in cannabis growing — it's general salt toxicity from a nutrient solution that's too concentrated or from salts accumulating in the medium over time. The very tips of leaves yellow, then brown and crisp up, and unlike light burn (which hits the canopy tops) or single-element deficiencies (which target specific leaf zones), nute burn affects leaves throughout the entire plant simultaneously. It's especially common with synthetic fertilizers and in coco or hydro setups where salts build up fast. It often co-occurs with nitrogen toxicity — if you're seeing dark green clawing leaves AND burnt tips, you're likely overfeeding across the board. Seedlings in "hot" soil (fresh compost or amended super-soil) can show burn within days. A slight tip burn is actually considered acceptable by many experienced growers as a sign you're feeding near max, but any progression beyond the very tips means you need to back off. Damaged tissue won't recover — you're watching for healthy new growth as the sign things are improving.