Fertilising guide

How to fertilise Purple Choy Sum (Brassica rapa var. parachinensis 'Purple')— schedule & NPK

Also called purple choy sum, purple Chinese flowering cabbage.

More about purple choy sum

About Purple Choy Sum

Brassica rapa var. parachinensis 'Purple' · also called purple choy sum, purple Chinese flowering cabbage · edible

Purple Choy Sum is an ornamental-yet-edible flowering cabbage with striking purple stems and leaf veins, topped by yellow buds. Grown for sweet, tender stems and shoots, it matures in about 40-55 days, holds colour best in cool weather, and crops cut-and-come-again, adding vivid colour to beds and stir-fries alike.

Growth habit: Upright, branching plant with purple-flushed stems and veins, green-to-purple leaves, and yellow buds; cutting the main stem prompts colourful side shoots.

What fertiliser purple choy sum actually wants — and why

Purple Choy Sum 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 purple choy sum: 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 purple choy sum, 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 purple choy sum:

Feed as a leafy/stem crop: incorporate compost or balanced fertiliser at planting, then apply nitrogen-rich liquid feed every 2 weeks; cool-season feeding combined with bright light supports the strongest purple colour. 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 purple choy sum 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 purple choy sum

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

Signs you are over-feeding purple choy sum

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

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 purple choy sum

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 purple choy sum 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 purple choy sum 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 purple choy sum

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 purple choy sum — frequently asked questions

What fertiliser does purple choy sum 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. Purple Choy Sum 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 purple choy sum?

Feed as a leafy/stem crop: incorporate compost or balanced fertiliser at planting, then apply nitrogen-rich liquid feed every 2 weeks; cool-season feeding combined with bright light supports the strongest purple colour. Feed as a leafy/stem crop: incorporate compost or balanced fertiliser at planting, then apply nitrogen-rich liquid feed every 2 weeks; cool-season feeding combined with bright light supports the strongest purple colour. 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 purple choy sum?

What does over-feeding purple choy sum 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 purple choy sum 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 purple choy sum?