Fertilising guide

How to fertilise Rhubarb (Rheum rhabarbarum)— schedule & NPK

Also called garden rhubarb, pieplant.

About Rhubarb

Rheum rhabarbarum · also called garden rhubarb, pieplant · edible

Rhubarb is a hardy perennial grown for tart edible stalks used in pies and crumbles. Leaves are toxic — never eaten by people or pets. Plant crowns in rich soil; established clumps crop for 10-15 years. Toxic to pets.

Rhubarb, Rheum rhabarbarum, is a hardy perennial in the buckwheat family native to southern Siberia and long used medicinally in ancient China; grown for its tart leaf stalks.

A heavy feeder: apply balanced fertilizer or compost annually, favoring a low-phosphorus blend like 32-3-10 over 10-10-10 to protect water quality.

Growth habit: Clumping herbaceous perennial

Watch for — Thin pale stalks: Patch is hungry; mulch with compost.

Sources: extension.umn.edu, hort.extension.wisc.edu

What fertiliser rhubarb actually wants — and why

Rhubarb 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 rhubarb: 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 rhubarb, 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 rhubarb:

Compost top-dress and balanced feed in spring. 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 rhubarb 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 rhubarb

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

Signs you are over-feeding rhubarb

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

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 rhubarb

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 rhubarb 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 rhubarb 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 rhubarb

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 rhubarb — frequently asked questions

What fertiliser does rhubarb 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. Rhubarb 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 rhubarb?

Compost top-dress and balanced feed in spring. Compost top-dress and balanced feed in spring. 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 rhubarb?

What does over-feeding rhubarb 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 rhubarb 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 rhubarb?