Short answer. Copper is part of plastocyanin (photosynthetic electron transport) and enzymes that build lignin and strengthen cell walls [3]. Cannabis grown without Cu developed symptoms only after about 9 weeks: stunted, distorted new leaves with narrow, yellowish leaflet bases, fine interveinal chlorosis and a loss of turgor that looked like wilting; biomass fell 45% [1]. Plants need very little: about 0.03–0.1 mg/L Cu. Excess damages roots first.
What's happening
Cu²⁺ is strongly bound by organic matter and roots, and moves poorly to new growth, so new leaves show shortage [3]. Because Cu is needed for lignification, deficient plants have weak cell walls. Cockson et al. linked the odd wilting they saw to "gummi-hemp", a historical name for limp, lodging fibre hemp, and suggested hemp may need more Cu than many species [1]. Excess Cu accumulates in roots, damages membranes and induces Fe deficiency [3].
How to recognise it
- Deficiency: slight stunting; new and expanding leaves slightly distorted, especially at the leaflet base, which is narrower and slightly yellow; later more pronounced narrowing, marginal and then fine interveinal chlorosis, margins curling in and down, and reduced turgidity of new leaves [1]. Leaf Cu at onset was about 1.4 mg/kg against 4.7 in controls [1].
- Toxicity: stunted, thick, dark or brown roots with few fine laterals; then Fe-deficiency-like chlorosis on new leaves [3]. Cannabis-specific thresholds are not published.being reviewed
- Differential: B deficiency (also distorted tips and wilting from root damage), Ca deficiency, root rot (check roots), underwatering, heat.
Numbers that matter
| Parameter | Target | Conditions / notes |
|---|---|---|
| Solution Cu | 0.03–0.1 mg/L | Greenhouse standards [4]; cannabis trials used ≈0.19 mg/L without harm [1] |
| Leaf Cu, healthy (most recent mature leaf) | 5–11 mg/kg | NC State floral hemp [2] |
| Leaf Cu at first deficiency symptoms | ~1.4 mg/kg | [1] |
| Copper from water/plumbing | Test | New copper pipe and some ionisers release Cu.being reviewed |
What to check first
- Roots (both deficiency and toxicity show there; rot mimics both).
- Feed: a DIY salt mix with no Cu source, or RO water with base nutrients lacking micros.
- Copper sprays or ionisers: copper-based fungicides or Cu/Ag ionisers in recirculating systems.
- pH: Cu availability falls at high pH in soil and peat; very low pH increases it.
How to fix it
- Deficiency: copper sulfate pentahydrate (≈25.5% Cu; 0.3 mg/L product ≈ 0.08 mg/L Cu) or Cu-EDTA (≈14% Cu). Weigh carefully; this is a tiny dose. Mixing a 1,000× micronutrient stock makes accurate dosing possible.
- Toxicity: remove the source, change the solution, keep pH in range. Root recovery takes 1–2 weeks.
- Safety: copper sulfate is harmful if swallowed and irritates eyes; gloves and eye protection.
How to confirm it worked
New leaves expand normally and hold turgor within 1–2 weeks. Tissue Cu 5–11 mg/kg [2].
Common mistakes
- Treating limp new growth as underwatering.
- Spraying copper fungicide repeatedly in veg and then wondering about root issues in hydro.
- Dosing copper sulfate by "pinch".
