Fertilising guide

How to fertilise Cucumber (Cucumis sativus)— schedule & NPK

Also called garden cucumber.

About Cucumber

Cucumis sativus · also called garden cucumber · edible

Cucumber is a thirsty warm-season vining fruit grown for fresh eating and pickling. It demands consistent moisture, heavy feeding, and warm soil — stress causes bitter fruit. Pet-safe by ASPCA standards.

Cucumis sativus originated on the Indian subcontinent and reached China about 2,000 years ago; it is a subtropical vine that demands long warm days, full sun and steady moisture.

Heavy feeder that benefits from soil well supplied with organic matter and nutrients; black plastic mulch also warms soil to push earlier, stronger growth.

Growth habit: Sprawling or climbing annual vine

Watch for — Yellow leaves: Powdery mildew, downy mildew, or nutrient depletion.

Sources: extension.umn.edu, extension.psu.edu, extension.uga.edu

What fertiliser cucumber actually wants — and why

Cucumber 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 cucumber: 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 cucumber, 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 cucumber:

Balanced feed at planting; side-dress with compost or a balanced liquid feed every 3 weeks once vines run. 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 cucumber 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 cucumber

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

Signs you are over-feeding cucumber

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

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 cucumber

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 cucumber 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 cucumber 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 cucumber

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 cucumber — frequently asked questions

What fertiliser does cucumber 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. Cucumber 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 cucumber?

Balanced feed at planting; side-dress with compost or a balanced liquid feed every 3 weeks once vines run. Balanced feed at planting; side-dress with compost or a balanced liquid feed every 3 weeks once vines run. 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 cucumber?

What does over-feeding cucumber 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 cucumber 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 cucumber?