Does foliar feeding work on cannabis?

Q: Does foliar feeding work on cannabis?

Foliar feeding works on cannabis as a quick, small top-up, not as a feed. Leaves absorb little and unevenly, so a spray can green new growth within days for magnesium, iron or zinc in veg, but it cannot supply nitrogen, potassium or calcium in useful amounts. Use it in veg only, at low light or lights-off, and never spray flowers. Fix the root zone at the same time.

Short answer

As a quick top-up, yes; as a feed, no. A foliar spray can green new cannabis leaves within days for magnesium, iron or zinc in veg, but leaves absorb too little to supply nitrogen, potassium or calcium. Spray in veg only, at low light or lights-off, and never on flowers. Supported for micronutrient fixes; the specific rates are Grower practice.

Why

Leaves are built to keep water in, not let solutes through. The cuticle limits uptake, and how much enters depends on the nutrient, the formulation, humidity and the leaf itself, which makes results uncertain ^[1].

Small doses work for nutrients needed in small amounts. A few mg/L of iron or a few grams of Epsom salt per litre can correct a micronutrient or magnesium shortfall on the sprayed leaves.

Large doses can't. A flowering plant takes up hundreds of mg of nitrogen and potassium per litre of water it drinks; no spray competes with that.

Mobility decides reach. Calcium and boron barely move in the phloem, so sprayed calcium stays where it lands ^[2]. Iron, manganese and zinc deficiencies show on new growth first, which is where a spray should be aimed ^[3].

The numbers

Spray	Rate (grower practice)	Use for
Epsom salt	2–5 g/L + wetting agent	Magnesium, veg
Iron chelate (DTPA or EDTA)	~50–100 mg/L Fe	Iron chlorosis, veg
Calcium nitrate or chloride	~0.5–1 g/L	Sprayed new leaves only
Zinc sulfate	0.1–0.5 g/L of product	Zinc, veg
Any spray on buds	None	Never

What to do

Diagnose first: check root-zone pH and runoff EC.
Fix the root-zone cause; the spray only buys time.
Spray at lights-off or low light, with a wetting agent.
Test one branch and wait 24 h for burn.
Stop once buds form.

Common mistakes

Using foliar feeds instead of fixing pH.
Spraying under full light and scorching leaves.
Spraying flowers.
Expecting foliar calcium to protect new growth.

Full article: Building a cannabis feed by stage
Answers: deficiency in late flower · macro vs micronutrients
Articles: Cal-Mag · bud rot
Terms: foliar feeding · immobile nutrient

Foliar feeding is spraying a dilute nutrient solution onto leaves for quick uptake through the leaf surface; it suits micronutrient fixes but cannot replace root feeding.Zinc (Zn) is a micronutrient needed for auxin production and many enzymes; deficiency gives small, twisted new leaves and shortened internodes.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.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.Manganese (Mn) is a micronutrient that splits water in photosynthesis and activates many enzymes; low pH can raise it to toxic levels.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.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.DTPA (diethylenetriaminepentaacetic acid) is a synthetic chelating agent that keeps iron available up to about pH 7.0-7.5, a common choice for coco and hydro.EDTA (ethylenediaminetetraacetic acid) is the most common synthetic chelating agent in fertilisers, effective for iron up to about pH 6.0-6.5.Yellowing of normally green plant tissue caused by too little chlorophyll, from nutrient shortage, poor root uptake, pH lockout, light damage, pests or normal ageing.Calcium nitrate is the standard hydroponic calcium source, supplying about 19% calcium and 15.5% nitrogen, mostly as nitrate.As a plant nutrient, chlorine is used as the chloride ion (Cl-) in tiny amounts for photosynthesis and osmotic balance; excess from water or salts is the usual problem.Zinc sulfate (ZnSO4) is the standard soluble zinc salt for micronutrient mixes, about 22.7% Zn as the heptahydrate and 35.5% as the monohydrate.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.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.

Follow-up questions

How much Epsom salt for a foliar spray?

Grower practice for a cannabis magnesium foliar spray is about 2-5 g/L of Epsom salt (magnesium sulfate heptahydrate) with a wetting agent, applied in veg at lights-off or low light. Test on one branch first and wait a day for burn. New leaves green within days, but fix the root-zone cause, usually pH, potassium excess or a low-magnesium feed.

Can I foliar feed in flower?

No, do not foliar feed cannabis in flower once buds are forming. Water and residues sit in the flowers, which invites bud rot, and anything sprayed may end up in what is smoked. Correct flower-stage deficiencies through the root zone instead: check pH and runoff EC first, then adjust the feed itself.

Can I foliar feed Cal-Mag for calcium deficiency?

Foliar calcium helps only the leaves it lands on. Calcium barely moves in the phloem, so a spray of calcium nitrate or calcium chloride at about 0.5-1 g/L can help sprayed new leaves in veg but cannot reach new growth that forms later. Most calcium problems are delivery problems: fix VPD, roots and root-zone pH.

Sources

Fernández V, Brown PH, 2013. From plant surface to plant metabolism: the uncertain fate of foliar-applied nutrients. Frontiers in Plant Science 4:289. doi.org/10.3389/fpls.2013.00289

Marschner P (ed.), 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. doi.org/10.1016/C2009-0-63043-9

Cockson P, Landis H, Smith T, Hicks K, Whipker BE, 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. doi.org/10.3390/app9204432