Short answer. Yellowing that starts on the oldest, lowest leaves means a mobile nutrient is being pulled out of them to feed new growth: most often nitrogen, sometimes magnesium or potassium. But the commonest reason a plant runs short is not an empty feed bottle; it is roots that cannot absorb (overwatered, cold, rotting, or pH out of range). Check water and pH first. In the last 2–3 weeks of flower, a slow fade from the bottom is normal.
What's happening
N, Mg and K are mobile in the phloem. When supply at the roots falls short, the plant cannibalises old leaves. In controlled cannabis trials, N deficiency began as pale yellowing of the lower foliage that climbed upward, then leaves turned fully yellow, necrotic and dropped [1]; in DWC it began at lower fan-leaf tips around week 3 of flower, with red-brown petioles on the worst leaves [2]. Mg deficiency showed as interveinal yellowing of lower leaves with veins staying green [1][2]. K deficiency started as yellowing on the leaf margins and serrations of lower leaves, then tan scorch [1].
How to recognise it
| Pattern on lower leaves | Most likely | Also consider |
|---|---|---|
| Even, pale-to-lemon yellow, whole leaf, climbing up | Nitrogen shortage | Overwatering, root rot, cold root zone, root aphids |
| Yellow between veins, veins green | Magnesium | pH too low (soilless/hydro), excess K or Ca antagonism |
| Yellow margins and serration tips, then brown scorch | Potassium | Salt build-up, Mn or B toxicity (marginal yellowing of lower leaves in trials [1]) |
| Brown/olive spots, sunken, on lower leaves | Phosphorus (see rust spots) | Calcium spots, leaf septoria |
| One or two leaves, shaded, deep in the canopy | Shade senescence | Nothing; normal |
| Whole bottom third, late flower, slow | Normal ripening fade | N cut too early if before week 5–6 |
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| pH, hydro | 5.5–6.0 | [3] |
| pH, coco/rockwool | 5.5–6.5 | Mg uptake falls at low pHbeing reviewed |
| pH, soil | 6.0–7.0 | Grower practice |
| N supply, mineral feed, flower | ~194 mg/L N predicted optimum in one DWC flowering trial | Bevan et al. [4]; organic feeds needed more (~260 mg/L in flower) |
| Leaf N, healthy (most recent mature leaf) | ~3.3–4.8% dry weight | Survey range cited by Cockson et al. [1]; deficient plants had 1.6% |
| Leaf Mg, healthy | ~0.40–0.81% | [1] |
| Leaf K, healthy | ~1.8–2.9% | Survey 1.83–2.35%; controls 2.85% [1] |
| Solution temperature (hydro) | 18–22 °C (65–72 °F) | Warmer = less oxygen, more root disease |
What to check first
- Stage. Late flower (last 2–3 weeks) and slow bottom-up fade: probably normal. Stop here, and keep feeding at stage strength; don't force the fade with a pre-harvest flush, which is a Myth that costs yield and resin (see the flush debate).
- Watering. Is the pot heavy and wet for more than 2–3 days? Is the medium sour-smelling? Overwatered roots absorb poorly (see drooping).
- Roots. Hydro: are they white, or tan/brown and slimy? Brown roots with yellowing and stunting are classic Pythium/Fusarium root rot in hydro cannabis [5]. Soil/coco: tip a plant out; look for white roots at the edges, and for root aphids (small, pear-shaped insects on roots, often with white waxy residue) [6].
- pH of input and runoff. Calibrated meter.
- EC of input and runoff. Very low input EC = genuine hunger. Very high runoff EC = salts, often with tip burn too.
- Feed record. Did you switch to a "bloom" formula with low N in early flower, or plain water for weeks (a "flush")?
- Only then decide between N, Mg and K from the leaf pattern.
How to fix it
- Root zone first. Let overwatered pots dry back until the pot is noticeably lighter; improve aeration; warm a cold root zone to ~20 °C (68 °F); in hydro, change the reservoir and check the air supply.
- Correct pH to the system range, measured at the root zone (runoff or reservoir), not just the input.
- If N is genuinely low: raise the N in the feed modestly, about 10–20% of the existing rate, rather than doubling. In one hydroponic flowering trial, mineral N around 194 mg/L was the predicted yield optimum [4]. Organic growers: top-dress or liquid organic N; it is slower to act.
- If Mg: check pH first; then a magnesium sulfate supplement at label rate for your system. A foliar Mg spray can green up new leaves faster in veg; avoid foliar sprays on buds.
- If K: confirm EC is not excessive (salt stress mimics K burn), then adjust to a formula with more K in flower.
- Organic/living soil: do not chase with bottled salts. Check moisture and pH, top-dress, and give the soil biology time (1–2 weeks).
How to confirm it worked
- The yellowing stops climbing within 5–7 days. Already-yellow leaves stay yellow.
- New leaves emerge mid-green, not pale.
- For Mg, the next leaves above the damage have no interveinal yellowing.
- Runoff pH and EC stay in range at the next two checks.
Common mistakes
- Adding nitrogen to an overwatered or root-rotted plant. It won't be absorbed and the EC rises.
- Calling late-flower fade a deficiency and pushing N into ripening buds.
- Using Cal-Mag as a cure-all; excess Ca can compete with Mg and K.
- Defoliating every yellow leaf before diagnosing; you lose the evidence and the plant's reserves.
Evidence
- Bottom-up yellowing as the signature of N, Mg and K shortage in cannabis is Established by two controlled studies in different systems [1][2].
- Root rot in hydro producing yellowing and stunting is Established [5].
- N rate optima are Supported (single cultivar trials; organic and mineral differ) [4].
- Soil pH 6.0–7.0 for cannabis is Grower practice.
