# 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.

- Canonical: https://genetixbay.com/knowledge/answers/does-foliar-feeding-work.html
- Topic: Nutrients · Type: How-to
- Evidence: Supported · Status: Draft: awaiting expert review
- Full article: [Building a cannabis feed by stage: ppm targets per element and how to hit them](https://genetixbay.com/knowledge/nutrients/feed-by-stage.md)
- Also asked as: Do foliar nutrient sprays work? | Can I spray leaves with Cal-Mag? | Is it OK to foliar feed in flower? | How much Epsom salt for a foliar spray? | Can foliar feeding replace root feeding?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

As a quick top-up, yes; as a feed, no. A [foliar spray](https://genetixbay.com/knowledge/glossary/foliar-feeding.md) can green new cannabis leaves within days for magnesium, iron or [zinc](https://genetixbay.com/knowledge/glossary/zinc.md) 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](https://genetixbay.com/knowledge/glossary/micronutrient.md) 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](https://genetixbay.com/knowledge/glossary/magnesium-sulfate.md) 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](https://genetixbay.com/knowledge/glossary/manganese.md) and zinc [deficiencies](https://genetixbay.com/knowledge/glossary/nutrient-deficiency.md) 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](https://genetixbay.com/knowledge/glossary/iron-chelate.md) ([DTPA](https://genetixbay.com/knowledge/glossary/dtpa.md) or [EDTA](https://genetixbay.com/knowledge/glossary/edta.md)) | ~50–100 mg/L Fe | Iron [chlorosis](https://genetixbay.com/knowledge/glossary/chlorosis.md), veg |
| [Calcium nitrate](https://genetixbay.com/knowledge/glossary/calcium-nitrate.md) or [chloride](https://genetixbay.com/knowledge/glossary/chloride-nutrient.md) | ~0.5–1 g/L | Sprayed new leaves only |
| [Zinc sulfate](https://genetixbay.com/knowledge/glossary/zinc-sulfate.md) | 0.1–0.5 g/L of product | Zinc, veg |
| Any spray on buds | None | Never |

## What to do

1. Diagnose first: check root-zone [pH](https://genetixbay.com/knowledge/glossary/ph.md) and [runoff](https://genetixbay.com/knowledge/glossary/runoff.md) [EC](https://genetixbay.com/knowledge/glossary/ec.md).
2. Fix the root-zone cause; the spray only buys time.
3. Spray at lights-off or low light, with a wetting agent.
4. Test one branch and wait 24 h for burn.
5. 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.

## Related

- Full article: [Building a cannabis feed by stage](https://genetixbay.com/knowledge/nutrients/feed-by-stage.md)
- Answers: [deficiency in late flower](https://genetixbay.com/knowledge/answers/fix-deficiency-in-late-flower.md) · [macro vs micronutrients](https://genetixbay.com/knowledge/answers/macro-vs-micronutrients.md)
- Articles: [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md) · [bud rot](https://genetixbay.com/knowledge/diseases/botrytis-bud-rot.md)
- Terms: [foliar feeding](https://genetixbay.com/knowledge/glossary/foliar-feeding.md) · [immobile nutrient](https://genetixbay.com/knowledge/glossary/immobile-nutrient.md)

## 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

1. Fernández V, Brown PH, 2013. From plant surface to plant metabolism: the uncertain fate of foliar-applied nutrients. Frontiers in Plant Science 4:289. https://doi.org/10.3389/fpls.2013.00289
2. Marschner P (ed.), 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. https://doi.org/10.1016/C2009-0-63043-9
3. Cockson P, Landis H, Smith T, Hicks K, Whipker BE, 2019. Characterization of nutrient disorders of Cannabis sativa. Applied Sciences 9(20):4432. https://doi.org/10.3390/app9204432