Fertilising guide

How to fertilise Skirret (Sium sisarum)— schedule & NPK

Also called skirret, crummock, sweet water-parsnip.

More about skirret

About Skirret

Sium sisarum · also called skirret, crummock · edible

Skirret (Sium sisarum) is a hardy perennial root vegetable in the carrot family, once a staple before the potato. Each crown produces a cluster of slender, sweet white roots with a flavour between parsnip and sweet potato, best lifted after autumn frosts. It bears umbels of small white flowers and is grown for its tasty, fiddly-to-clean fingered roots.

Growth habit: Clump-forming herbaceous perennial with ferny pinnate foliage and tall white flower umbels; forms a crown of multiple slender finger-like roots.

What fertiliser skirret actually wants — and why

Skirret 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 skirret: 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 skirret, 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 skirret:

Moderate feeder. Incorporate compost or well-rotted manure before planting and side-dress with a balanced fertiliser in midsummer. Avoid heavy fresh nitrogen, which favours top growth over the roots. 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 skirret 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 skirret

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

Signs you are over-feeding skirret

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

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 skirret

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 skirret 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 skirret 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 skirret

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 skirret — frequently asked questions

What fertiliser does skirret 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. Skirret 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 skirret?

Moderate feeder. Incorporate compost or well-rotted manure before planting and side-dress with a balanced fertiliser in midsummer. Avoid heavy fresh nitrogen, which favours top growth over the roots. Moderate feeder. Incorporate compost or well-rotted manure before planting and side-dress with a balanced fertiliser in midsummer. Avoid heavy fresh nitrogen, which favours top growth over the roots. 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 skirret?

What does over-feeding skirret 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 skirret 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 skirret?