Short answer. If the top is yellow and the bottom is green, the plant is short of something it cannot move from old leaves: iron, sulfur, manganese, zinc, copper, boron or calcium. In most grow rooms the fertiliser contains all of these; what has failed is uptake, usually because root-zone pH is too high or the roots are wet, cold or damaged. Before you buy micronutrients, rule out light bleaching (tops nearest the light), broad or russet mites (twisted, shiny, glassy new growth) and viroid infection (stunted, pale, brittle plants).
What's happening
Immobile elements must arrive fresh through the roots to every new leaf. Any interruption shows first at the growing tips. In cannabis trials, Fe deficiency produced interveinal chlorosis on new and expanding leaves while lower foliage stayed dark green [1]; Mn deficiency produced bright yellow netted interveinal chlorosis on upper and central foliage in sand culture [1] but no visible symptoms in DWC [2]; Zn deficiency produced marginal yellowing of new leaves turning to tan necrosis [1]; S deficiency gave general paling with the base of leaflets most yellow, intensifying on new leaves [1], and in DWC flowering plants it yellowed upper sugar leaves first [2]; Ca and B deficiency distorted new growth, narrow leaflet bases and dying tips [1].
Iron and manganese availability falls as pH rises above ~6.5 in soilless media [3], which is why "iron deficiency" in coco or hydro is so often a pH problem.
How to recognise it
| Pattern on new growth | Most likely | Also consider |
|---|---|---|
| Interveinal yellow, veins sharply green, top only | Iron (often pH-induced) | Mn; cold, wet roots |
| Netted yellow between veins, small tan specks | Manganese | Fe; pH high |
| Pale overall, leaflet base yellowest, top half | Sulfur | N if spreading from bottom too |
| Yellow margins on new leaves, then tan edges | Zinc | Heat/light stress |
| Distorted, narrow, crinkled leaflets; dying tips | Calcium or boron | Broad/russet mites, herbicide drift, HLVd |
| White/bleached tops directly under light, lower canopy fine | Light/heat bleaching | Not a nutrient at all |
| Twisted, shiny, glassy, bronzed new growth; leaves cupping | Broad or russet mites [5] | Ca/B deficiency |
| Whole plant stunted, small pale leaves, short internodes, brittle stems | Hop latent viroid [6] | Root rot, root aphids |
Numbers that matter
| Parameter | Target | Notes |
|---|---|---|
| pH, hydro | 5.5–6.0 | [3] |
| pH, coco/rockwool | 5.5–6.5 | Above 6.5, Fe and Mn availability falls [3] |
| Leaf Fe, healthy | ~100–150 mg/kg | Survey range in [1]; deficient plants 60 mg/kg |
| Leaf Mn, healthy | ~41–93 mg/kg | [1] |
| Leaf Zn, healthy | ~24–52 mg/kg | [1] |
| Leaf S, healthy | ~0.17–0.41% | Survey 0.17–0.26%; controls 0.41% [1] |
| Canopy PPFD | flower ~900–1500 µmol/m²/s | Yield rose linearly to 1800 in one trial [4], but bleaching occurs on some cultivars at high PPFDbeing reviewed |
| Canopy leaf temperature | < ~30 °C (86 °F) | Grower practice |
What to check first
- Light. Are the yellow/white tops only the ones closest to the fixture? Measure PPFD or raise the light 10–15 cm. Bleaching starts as pale, then white, leaves and bracts at the very top.
- pH at the root zone. Runoff or reservoir, calibrated meter. A pH of 6.8 in coco is a likely Fe/Mn lockout.
- Root zone. Cold (< 18 °C / 64 °F), waterlogged, or rotting roots. New growth suffers first when roots are compromised.
- Hand lens on new growth. 20× or better. Broad mites are nearly invisible to the eye [5]; look for shine, bronzing and twisted tips.
- Whole-plant pattern. One plant stunted and pale in a room of healthy ones (same feed) suggests a pathogen or viroid; test if it is a mother or a clone source.
- Feed check. Is it a complete fertiliser with micronutrients? RO water with an incomplete base nutrient can genuinely lack S or micros.
How to fix it
- Light: raise or dim to reduce top-of-canopy PPFD and leaf temperature. Bleached tissue will not recover.
- pH: bring the root zone back into range. In coco, irrigate to 20–30% runoff with correctly pH'd solution; in hydro, change the reservoir. Fe and Mn uptake typically recovers in days.
- Root zone: warm it, aerate it, water less often. See drooping and root browning in hydro.
- True micronutrient shortage (rare with complete feeds): use a chelated micronutrient product at label rate. A chelated iron foliar spray can green new growth in veg within days; do not spray flowers.
- Sulfur: most complete feeds supply it via sulfate salts (e.g. magnesium sulfate). Check your base nutrient label.
- Mites: see broad and russet mites. Do not delay; they move fast.
- Suspected viroid: isolate, test (RT-PCR on roots and leaves), and remove positives from propagation. See HLVd.
How to confirm it worked
- The next pair of leaves at each tip emerges green, within 5–10 days.
- The yellowing does not spread to further new growth.
- With mites, new growth stops twisting within 1–2 weeks of control.
Common mistakes
- Adding iron to a pH 7 coco pot. It precipitates and the plant still can't use it.
- Mistaking light bleaching for nitrogen or iron shortage.
- Spraying micronutrients on a plant with broad mites while the mites keep feeding.
- Overdosing boron or zinc "just in case"; boron toxicity in cannabis caused marginal yellowing and necrosis of lower leaves [1].
Evidence
- Upper-leaf symptoms of Fe, Mn, S, Zn, Ca and B deficiency in cannabis are Established in sand culture [1], with partial confirmation in DWC [2]. The Mn result differs between studies: Contested how often Mn deficiency is visible.
- pH-induced Fe/Mn unavailability is Established in horticulture and Supported for cannabis [3].
- Light bleaching thresholds are Grower practice; yield responses to high PPFD are Supported [4].
- Broad mite and HLVd symptoms are Supported by extension sources [5][6].
