What is foliar feeding and when does it actually work?

Foliar feeding occupies an odd position: it is a real, well-established horticultural technique that works exactly as described, and it is routinely sold as something it is not. The distinction is quantity. A leaf can absorb enough iron to green up a chlorotic plant within a week. It cannot absorb anything like enough nitrogen to grow a crop.

Safety

What you'll need

• A pressure sprayer or a hand mister
• A soluble feed suitable for foliar use
• Eye protection
• A wetting agent, or a drop of washing-up liquid (optional)

Step by step

1. Understand what a leaf can absorb.Nutrients enter through the cuticle and through stomata in small quantities. Trace elements — iron, manganese, zinc, boron, copper — are needed in such small amounts that a spray can supply a plant's whole requirement. Magnesium works well too. Nitrogen is absorbed in modest amounts. Phosphorus and potassium are absorbed poorly and are needed in bulk, so foliar application is close to pointless for them.
2. Use it where it genuinely earns its place.Correcting a visible trace element deficiency quickly. Supporting a plant whose roots cannot take up nutrients — waterlogged, damaged, recently transplanted, or growing in soil at a pH that locks the nutrient out. Getting a fast response late in a season when there is no time for soil feeding to work.
3. Use chelated forms for trace elements.Chelated iron and manganese stay soluble and available where the sulphate forms precipitate out. This matters both in the spray tank and on the leaf, and it is the main reason a proper foliar product outperforms a general feed diluted more.
4. Spray in the right conditions.Early morning or evening. Not in bright sun, which scorches wet leaves and evaporates the spray before absorption. Not when rain is due within a few hours. Cool, humid, still conditions keep the droplets on the leaf longer, and absorption is proportional to how long the leaf stays wet.
5. Cover the undersides of the leaves.Absorption is substantially better through the underside, where the stomata are concentrated and the cuticle is thinner. A spray that only wets the top surface achieves a fraction of what it could.
6. Use a much weaker dilution than for root feeding.Foliar rates are typically a quarter to a half of root-drench rates, and often less. Leaf tissue burns at concentrations roots tolerate easily. If a product does not state a foliar rate, it is not intended for foliar use.
7. Add a wetting agent if droplets bead off.Waxy leaves — brassicas, alliums, many evergreens — shed water. A commercial wetting agent, or the smallest drop of washing-up liquid in the tank, breaks the surface tension so the spray spreads into a film.
8. Keep feeding the roots.This is the point people miss. Foliar feeding is a correction and a supplement. The plant's nitrogen, phosphorus and potassium still have to come from the soil or the compost, in quantities no spray can deliver.
9. Fix the underlying cause.Recurring iron chlorosis means the pH is wrong. Recurring magnesium deficiency in a pot means the compost is exhausted or you are over-feeding potassium, which competes with magnesium for uptake. A spray buys time to address the real problem.

Tips

• Seaweed extract is the most commonly used foliar product in gardens, and its value is as a biostimulant — improving stress tolerance and rooting — rather than as a nutrient source.
• Epsom salts as a foliar spray is the classic magnesium correction and gives a visible response within a week on tomatoes and roses in pots.
• Chelated iron sprayed on a chlorotic acid-loving plant greens it up in seven to ten days. It lasts weeks, not months, and needs repeating unless the pH problem is fixed.
• Never spray a plant that is wilting from drought. Water it first — a stressed plant closes its stomata and absorbs very little, and the spray is wasted.
• Greenhouse crops respond better to foliar feeding than outdoor ones, simply because rain is not washing it off an hour later.

Questions people ask