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.
Overwatering is watering so often that the medium stays saturated, starving roots of oxygen, which causes drooping, slow growth and root disease.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.The programmed ageing and breakdown of leaves and other tissues, in which the plant recycles nutrients (especially nitrogen) from old leaves to developing flowers and seeds.Nitrogen (N) is the macronutrient plants use most, built into proteins, chlorophyll and nucleic acids; it drives leaf and stem growth and is fully mobile.DWC (deep water culture) suspends a plant's roots in an aerated nutrient solution, typically in a bucket with a net pot lid and an air stone.Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.Yellowing of the leaf tissue between the veins while the veins themselves stay green; on old leaves it points to magnesium, on new leaves to iron or manganese.Decay of roots by pathogens such as Pythium, Fusarium, Rhizoctonia or Phytophthora, usually triggered by low oxygen, heat and waterlogging around the roots.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Aphids that colonise roots and the crown below the medium surface, on cannabis chiefly the rice root aphid (Rhopalosiphum rufiabdominale), causing slow, unexplained decline.Salt build-up is the accumulation of unused fertiliser and water salts in the growing medium, raising root-zone EC and causing osmotic stress and lockout-type symptoms.Feeding only plain water for days or weeks before harvest to "clean" the plant; GenetixBay rates it a Myth that harms yield, unlike a root-zone salt flush.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.
