Short answer. Brown, crispy tips on leaves that are otherwise dark green is classic salt (nutrient) burn: the root zone is too concentrated, so water is pulled out of the leaf margins first. Measure runoff or reservoir EC and compare it with what you are feeding. If EC is fine, look at watering rhythm (pots drying out concentrates salts), heat and very low humidity, and only then at potassium or boron disorders.
What's happening
Leaf tips and margins are the end of the water path. Anything that limits water supply or concentrates salts there kills those cells first. High root-zone EC lowers the osmotic potential around roots; in drying media the salts concentrate further. Heat and low RH raise transpiration demand and deliver more salts to the tip. True K deficiency starts on lower leaves with yellowing of the serrated margin before browning [1]; in DWC flowering trials K deficiency caused scorching of fan-leaf tips and margins and dark lesions on sugar leaves [2]. Boron toxicity caused marginal yellowing then browning on lower leaves [1].
How to recognise it
| Pattern | Most likely | Also consider |
|---|---|---|
| Tips only, 1–3 mm brown, leaves dark green, newest mature leaves | Mild salt/nutrient burn (EC high) | Feed too strong for seedlings |
| Tips plus dark, glossy leaves curling down at tips ("claw") | Excess N plus high EC | See clawing |
| Brown margins with yellow band, lower leaves first | Potassium deficiency [1] | Salt build-up, Mn/B toxicity |
| Brown edges plus upward cupping at top of canopy | Heat or light stress | Low RH, see taco |
| Brown tips across many plants after a dry spell | Drought plus salt concentration | Irrigation failure |
| Brown spots in the leaf body, not just tips | Not tip burn: see rust spots | Ca, P, leaf septoria |
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| Seedling feed EC | ~0.4–0.8 mS/cm (200–400 ppm500) | Grower practice |
| Veg feed EC | ~1.0–1.8 mS/cm (500–900 ppm500; 700–1260 ppm700) | Grower practice; DWC trial used 1.8 dS/m [2] |
| Flower feed EC | ~1.6–2.4 mS/cm (800–1200 ppm500) | Grower practicebeing reviewed |
| Runoff EC vs input | within ~0.5 mS/cm | Grower practice; > 1.0 above input = build-up |
| Runoff volume (coco/rockwool, drain-to-waste) | 10–30% of irrigation | Grower practice |
| RH, veg | ~55–70% | Grower practice; very low RH worsens tip burn |
| Air temperature | photosynthesis declined above ~30 °C (86 °F) in one cultivar | [5] |
What to check first
- Runoff/reservoir EC versus input EC. The single most useful reading.
- Feed strength vs stage. Seedlings and fresh clones burn at EC that a flowering plant would find light.
- Watering rhythm. Coco or rockwool allowed to dry hard between irrigations concentrates salts; small sips with no runoff build salts.
- Climate. Leaf temperature and RH at the canopy. Hot, dry rooms produce tip burn at moderate EC.
- pH. Rarely the direct cause of tip burn, but out-of-range pH changes which salts accumulate; targets are 5.5–6.0 in hydro and 5.5–6.5 in soilless media [4].
- Pattern by leaf age to separate K deficiency (old leaves, yellow band first) from burn (newest mature leaves, tips first).
How to fix it
- Hydro: dilute or replace the reservoir to a lower EC (drop 0.2–0.4 mS/cm). Top up evaporation losses with plain pH-adjusted water, not full-strength feed.
- Coco/rockwool: do a root-zone flush: run about 2–3× the container volume of a light solution (EC ~0.4–0.8 mS/cm, with Ca and Mg, pH ~5.8–6.0) through in several passes until runoff EC is within ~0.5 mS/cm of the flush input. Then go back to normal feed with consistent runoff. Plain water strips Ca and Mg from coco.
- Soil: root-zone flush with 2–3× the pot volume of pH-adjusted plain water (pH ~6.3–6.8), slowly and in passes, then check runoff EC and resume normal feeding once the top few cm dry. Do not drown a pot that is already wet.
This root-zone flush treats the medium, not the plant, and is fine at any stage (see the root-zone flush). It is not a pre-harvest flush, which is a Myth that costs yield and resin. - Climate: raise RH towards the stage target, lower leaf temperature, reduce light intensity if leaves are hot. - True K deficiency (confirmed by pattern and normal EC): increase K in the feed modestly. Bevan et al. found flowering yield in soilless cannabis was relatively insensitive across moderate K rates, so big K additions are rarely justified [3].being reviewed
How to confirm it worked
- New leaves develop without brown tips (5–10 days).
- Runoff EC trends toward input EC over 2–3 irrigations.
- Existing burnt tips stay burnt; they do not spread inward if fixed.
Common mistakes
- Reading the brown tip as hunger and feeding harder.
- Topping up a hydro reservoir with full-strength nutrient; EC creeps up as water evaporates.
- Ignoring runoff in coco. No runoff = salts with nowhere to go.
- Cutting off every crispy tip; harmless, but pointless.
