Fertilising guide

How to fertilise Kale (Brassica oleracea var. sabellica)— schedule & NPK

Also called curly kale, Tuscan kale, cavolo nero, Lacinato kale.

About Kale

Brassica oleracea var. sabellica · also called curly kale, Tuscan kale · edible

Kale is a cold-hardy leafy brassica that crops from late summer through deep winter and into the following spring. Frost sweetens the leaves. Pair with brassica-friendly companions and protect from cabbage white butterflies. Toxic to pets in large amounts.

Kale is the non-heading (Acephala Group) form of Brassica oleracea, the same species as cabbage and broccoli, derived from wild cabbage of the eastern Mediterranean and Asia Minor and grown as a leafy crop since Greek and Roman times.

A leafy-growth crop that responds to steady nitrogen; side-dressing keeps new leaves coming through a long harvest window.

Growth habit: Upright leafy biennial grown as an annual

Watch for — Yellow lower leaves: Nitrogen depletion in mature plants; side-dress with compost.

Sources: gardens.duke.edu, plants.ces.ncsu.edu, pmc.ncbi.nlm.nih.gov

What fertiliser kale actually wants — and why

Kale is grown entirely for its leaves, so nitrogen is the priority — steady, nitrogen-leaning feeding keeps it growing fast, tender and unbolted.

For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for kale: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.

How often to feed kale, and which months

Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For kale:

A balanced feed at planting; top-dress with nitrogen every 6 weeks during heavy harvesting. In practice: a balanced or compost-rich start, then a nitrogen side-dress or liquid feed every 3-4 weeks through the cropping period in the main season (spring through early autumn).

The dormant-season rule matters more than the exact interval: skip feeding entirely when kale is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.

What strength to mix for kale

Use the vegetable-feed label rate for kale. Steady availability matters more than a strong dose — a check in growth makes leaves tough and can trigger bolting.

Feeding always goes onto already-damp soil, never dry roots — water kale first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the kale watering schedule.

Signs you are over-feeding kale

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for kale:

Very soft, floppy, dark-green growth that attracts aphids.
Excess leafy growth at the expense of hearts/heads in cabbage and the like.
Salt crust and scorched leaf edges in containers; nitrate-heavy leaves.

Signs you are under-feeding kale

Pale, yellow-green leaves, oldest first, and slow growth.
Small, tough, bitter leaves and premature bolting.
Weak, stunted heads in cabbage, broccoli and cauliflower.

If the symptoms point at watering, light or roots rather than nutrition, the full kale care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

For container-grown kale, water until it drains freely each time and flush pots monthly with plain water to stop nitrogen salts accumulating; in the ground, good compost levels naturally buffer this.

Organic vs synthetic feeds for kale

Organic options

Well-rotted manure or compost dug in, plus nitrogen-rich liquid feeds like diluted chicken-manure pellets or nettle feed. UK: pelleted chicken manure or Westland; US: Espoma Garden-tone or blood meal. Steady and soil-building.

Synthetic / liquid feeds

A balanced feed at planting then a high-nitrogen liquid or granular side-dress — UK: Growmore then a nitrogen feed or Phostrogen; US: a 10-10-10 then a high-N (e.g. 21-0-0) side-dress or Miracle-Gro.

Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.

Fertilising kale — frequently asked questions

What fertiliser does kale need?

A nitrogen-leaning feed (higher first number) or compost-rich soil — nitrogen drives the fast, tender leafy growth this crop is grown for. Phosphorus and potassium matter far less here than for fruiting crops. Kale is grown entirely for its leaves, so nitrogen is the priority — steady, nitrogen-leaning feeding keeps it growing fast, tender and unbolted.

How often should I feed kale?

A balanced feed at planting; top-dress with nitrogen every 6 weeks during heavy harvesting. A balanced feed at planting; top-dress with nitrogen every 6 weeks during heavy harvesting. In practice: a balanced or compost-rich start, then a nitrogen side-dress or liquid feed every 3-4 weeks through the cropping period in the main season (spring through early autumn).

What strength of feed for kale?