Fertilising guide

How to fertilise Courgette (Cucurbita pepo 'Defender')— schedule & NPK

Also called courgette, zucchini, summer squash.

More about courgette

About Courgette

Cucurbita pepo 'Defender' · also called courgette, zucchini · edible

Courgette, or zucchini, is a fast, prolific bush summer squash cropping heavily from midsummer to first frost. It is a hungry, thirsty, tender annual needing full sun, very rich moist soil and warmth. Keep picking fruits young to sustain production; a couple of healthy plants can overwhelm a household with fruit through the season.

Growth habit: Vigorous, frost-tender annual; most modern cultivars are open bush types with large lobed leaves and short internodes, producing fruit continuously at the plant's centre rather than running like a trailing squash.

Watch for — Cucumber mosaic virus: Yellow-mottled, distorted leaves and stunted, deformed fruit signal this aphid-spread virus. There is no cure; control aphids, remove and destroy infected plants, and grow resistant varieties.

What fertiliser courgette actually wants — and why

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 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 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 courgette:

A heavy feeder. Plant into compost-rich soil, then once fruiting begins feed every 1-2 weeks with a high-potassium tomato-type liquid feed to sustain flowering and fruit set. Too much nitrogen gives lush leaves but few fruits. 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 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 courgette

Follow the crop-feed label rate for 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 courgette first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the courgette watering schedule.

Signs you are over-feeding courgette

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for 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 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 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 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 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 courgette — frequently asked questions

What fertiliser does 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. 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 courgette?

A heavy feeder. Plant into compost-rich soil, then once fruiting begins feed every 1-2 weeks with a high-potassium tomato-type liquid feed to sustain flowering and fruit set. Too much nitrogen gives lush leaves but few fruits. A heavy feeder. Plant into compost-rich soil, then once fruiting begins feed every 1-2 weeks with a high-potassium tomato-type liquid feed to sustain flowering and fruit set. Too much nitrogen gives lush leaves but few fruits. 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 courgette?

What does over-feeding 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 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 courgette?