Fertilising guide

How to fertilise St. John's Wort (Hypericum perforatum)— schedule & NPK

Also called St. John's wort, common St. John's wort, perforate St. John's wort.

More about st. john's wort

About St. John's Wort

Hypericum perforatum · also called St. John's wort, common St. John's wort · herb

Common St. John's wort is a hardy, sun-loving perennial herb bearing bright yellow five-petalled flowers in midsummer over wiry stems and small, gland-dotted leaves. Vigorous and drought-tolerant, it self-seeds and spreads freely. Long used medicinally, it is a regulated weed in some regions and is toxic to pets and grazing animals.

Growth habit: Upright, clump-forming herbaceous perennial with branching, somewhat woody-based stems, opposite gland-dotted leaves, and terminal clusters of starry yellow flowers; spreads by rhizomes and seed.

What fertiliser st. john's wort actually wants — and why

St. John's Wort is a soft, fast leafy herb that you harvest hard — a modest balanced feed keeps tender growth coming without tipping it into bland or bolting.

A balanced general feed (even N-P-K) at modest strength — enough nitrogen to keep replacing the leaves you pick, but not so much that flavour thins or it bolts to seed.

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 st. john's wort: 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 st. john's wort, 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 st. john's wort:

Minimal. Performs well in poor soil and rarely needs feeding; a light spring compost mulch is ample. Excess fertiliser produces lax, disease-prone growth and worsens its weedy spread. In practice: a balanced liquid feed every few weeks through the main growing and harvesting season (spring through early autumn), more often the harder you are picking it.

The dormant-season rule matters more than the exact interval: skip feeding entirely when st. john's wort 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 st. john's wort

Half strength is a sensible default for st. john's wort — enough to fuel regrowth after cutting, gentle enough that the leaves stay aromatic rather than watery.

Feeding always goes onto already-damp soil, never dry roots — water st. john's wort first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the st. john's wort watering schedule.

Signs you are over-feeding st. john's wort

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for st. john's wort:

Fast, soft, pale growth with diluted, less aromatic flavour.
Early bolting (running to flower) and a bitter edge.
Salt crust and scorched tips on container plants.

Signs you are under-feeding st. john's wort

Pale, slow regrowth after cutting and small leaves.
A tired, stalled plant that cannot keep up with harvesting.
Yellowing older leaves in a long-spent pot.

If the symptoms point at watering, light or roots rather than nutrition, the full st. john's wort care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

Pot-grown st. john's wort builds up feed salts quickly — water until it drains each time and flush the pot with plain water every few weeks, especially on a sunny windowsill.

Organic vs synthetic feeds for st. john's wort

Organic options

A diluted seaweed feed or worm-casting tea keeps soft growth coming without overdoing it. UK: dilute seaweed or Westland; US: Espoma Garden-tone or Neptune's Harvest. Gentle, hard to overdo, flavour-friendly.

Synthetic / liquid feeds

A balanced liquid feed at half strength through harvesting — UK: Phostrogen, Baby Bio or Westland; US: Miracle-Gro all-purpose at half strength. Fast regrowth; just do not overdo the nitrogen.

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 st. john's wort — frequently asked questions

What fertiliser does st. john's wort need?

A balanced general feed (even N-P-K) at modest strength — enough nitrogen to keep replacing the leaves you pick, but not so much that flavour thins or it bolts to seed. St. John's Wort is a soft, fast leafy herb that you harvest hard — a modest balanced feed keeps tender growth coming without tipping it into bland or bolting.

How often should I feed st. john's wort?

Minimal. Performs well in poor soil and rarely needs feeding; a light spring compost mulch is ample. Excess fertiliser produces lax, disease-prone growth and worsens its weedy spread. Minimal. Performs well in poor soil and rarely needs feeding; a light spring compost mulch is ample. Excess fertiliser produces lax, disease-prone growth and worsens its weedy spread. In practice: a balanced liquid feed every few weeks through the main growing and harvesting season (spring through early autumn), more often the harder you are picking it.

What strength of feed for st. john's wort?