Fertilising guide

How to fertilise Zucchini / courgette (Cucurbita pepo)— schedule & NPK

Also called courgette, summer squash, marrow (mature fruit).

About Zucchini / courgette

Cucurbita pepo · also called courgette, summer squash · edible

Zucchini (US) or courgette (UK) is a fast-growing summer squash that crops heavily through summer. One or two plants feed a household. Needs sun, rich soil, and steady water. Pet-safe; fruit and foliage are non-toxic.

Zucchini is a summer-squash form of Cucurbita pepo, a species domesticated in Mesoamerica thousands of years ago; the modern cylindrical zucchini was selected in 19th-century Milan, Italy.

Apply phosphorus and potassium to a soil test and side-dress with nitrogen when the vines begin to spread.

Growth habit: Bushy or short-vining annual

Watch for — Yellow leaves: Nitrogen depletion in heavy producers; side-dress with compost.

Sources: extension.umn.edu, plants.ces.ncsu.edu

What fertiliser zucchini / courgette actually wants — and why

Zucchini / courgette feeds in two distinct phases — balanced to build the plant, then high-potassium the moment flowering starts to set and fill a heavy crop.

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 zucchini / courgette: 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 zucchini / courgette, 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 zucchini / courgette:

A balanced feed at planting; switch to a high-potash tomato feed once flowering starts. So: a balanced feed or compost at planting, then a high-potash liquid every 1-2 weeks from first flower through harvest across the main season (spring through early autumn).

The dormant-season rule matters more than the exact interval: skip feeding entirely when zucchini / courgette 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 zucchini / courgette

Follow the crop-feed label rate for zucchini / courgette — these are calibrated for hungry vegetables. Consistency through fruiting matters more than strength; erratic feeding causes problems like blossom-end rot.

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

Signs you are over-feeding zucchini / courgette

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

Vigorous dark-green leafy growth but few flowers or fruit (excess nitrogen).
Lush foliage hiding the crop; soft growth prone to pests and disease.
Salt crust on the soil and scorched leaf edges in containers.

Signs you are under-feeding zucchini / courgette

Pale, yellowing lower leaves and stunted growth.
Small fruit, poor set, and a quickly exhausted plant.
Blossom-end rot and weak cropping from erratic or insufficient feeding.

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

Flushing and leaching the salts

In containers, fertiliser salts build up fast — water zucchini / courgette thoroughly so excess drains from the base each time, and flush pots with plain water every few weeks to prevent a damaging salt build-up.

Organic vs synthetic feeds for zucchini / courgette

Organic options

Garden compost or well-rotted manure dug in before planting, plus a liquid comfrey or seaweed feed once fruiting starts. UK: comfrey feed or organic Tomorite; US: Espoma Tomato-tone or Neptune's Harvest. Builds soil and feeds in one.

Synthetic / liquid feeds

A balanced feed at planting then a high-potash tomato feed in fruiting — UK: Growmore at planting then Tomorite (Levington) or Phostrogen; US: a balanced 10-10-10 then Miracle-Gro Tomato or a bloom booster.

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 zucchini / courgette — frequently asked questions

What fertiliser does zucchini / courgette need?

Balanced (even N-P-K) at planting for roots and frame, then switch to a high-potassium ("high-potash") tomato-style feed once the first flowers open — potassium is what sizes and ripens fruit, not nitrogen. Zucchini / courgette feeds in two distinct phases — balanced to build the plant, then high-potassium the moment flowering starts to set and fill a heavy crop.

How often should I feed zucchini / courgette?

A balanced feed at planting; switch to a high-potash tomato feed once flowering starts. A balanced feed at planting; switch to a high-potash tomato feed once flowering starts. So: a balanced feed or compost at planting, then a high-potash liquid every 1-2 weeks from first flower through harvest across the main season (spring through early autumn).

What strength of feed for zucchini / courgette?

What does over-feeding zucchini / courgette look like?

Vigorous dark-green leafy growth but few flowers or fruit (excess nitrogen). Lush foliage hiding the crop; soft growth prone to pests and disease. Salt crust on the soil and scorched leaf edges in containers. Staying on a high-nitrogen feed once zucchini / courgette starts flowering is the classic error — you get a huge leafy plant and a disappointing crop. Switch to high-potash the moment flowers appear.

Should I flush the soil of zucchini / courgette?