Foliar feeding is applying nutrients in dilute solution directly to leaves. Ions enter through the cuticle, stomata and small pores, and are used locally or moved on through the phloem. Uptake works best for elements needed in small amounts, such as iron, manganese, zinc, boron and magnesium, and for urea.
In practice
Growers use foliar sprays as a quick, temporary fix for a micronutrient shortage while correcting the root-zone cause, usually pH. Macronutrients cannot be supplied in meaningful amounts this way: leaves cannot absorb enough without burning. Spray at low strength, with lights off or low, covering leaf undersides, and stop once flowers form. Wet buds invite bud rot, and residues can end up in the harvested flower.
Don't confuse with
- Root feeding (fertigation): the main route for all nutrients.
- Foliar pest sprays: pesticides, a separate topic with their own rules.
- Misting for humidity: plain water, no nutrients.
Myth check
Foliar feeding in flower is not a shortcut to quality, and it adds disease risk. See botrytis bud rot.
Microscopic pores, mostly on the underside of leaves, each opened and closed by two guard cells to control gas exchange and water loss.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.Urea is a 46% nitrogen organic compound that soil enzymes convert to ammonium; it is cheap in field fertilisers but a poor choice for hydroponic cannabis.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.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.A macronutrient is an essential element plants need in large amounts, measured in tens to hundreds of mg/L: nitrogen, phosphorus and potassium, plus calcium, magnesium and sulfur.Botrytis cinerea, the fungus behind bud rot or grey mould, which rots dense colas from the inside in cool, humid conditions late in flower.Fertigation is delivering fertiliser dissolved in irrigation water, so plants are fed every time they are watered rather than through solid amendments.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.An iron chelate is iron bound to an organic molecule (EDTA, DTPA or EDDHA) that keeps it dissolved and available; each type holds up to a different pH.An amino acid chelate is a metal bound to amino acids such as glycine; it is a gentler, biodegradable alternative to synthetic chelates, often sold for foliar sprays.
