Fertilising guide

How to fertilise Perforate St John's-wort (Hypericum perforatum)— schedule & NPK

Also called Perforate St John's-wort, Common St John's Wort, St John's Wort, Klamath Weed.

More about perforate st john's-wort

About Perforate St John's-wort

Hypericum perforatum · also called Perforate St John's-wort, Common St John's Wort · herb

Hypericum perforatum is the medicinal St John's Wort — a upright, freely branching perennial native to grasslands, roadsides, and scrubby habitats across Europe and the UK, bearing bright golden-yellow flowers with distinctive black-dotted margins from June to September. The leaves have translucent oil glands visible when held to the light, giving it the species name 'perforatum'. It thrives in full sun and poor to moderately fertile, well-drained soils and self-seeds prolifically once established. It is confirmed toxic to cats, dogs, and horses via hypericin-driven photosensitization.

Growth habit: Erect, freely branching herbaceous perennial with two distinct ridges running down the stems and opposite, oval, translucent-dotted leaves.

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

Perforate 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 perforate 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 perforate 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 perforate st john's-wort:

Rarely required; a light balanced feed in spring on very poor soils is sufficient. Avoid nitrogen-rich fertilisers. 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 perforate 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 perforate st john's-wort

Half strength is a sensible default for perforate 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 perforate 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 perforate st john's-wort watering schedule.

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

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for perforate 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 perforate 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 perforate st john's-wort care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

Pot-grown perforate 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 perforate 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 perforate st john's-wort — frequently asked questions

What fertiliser does perforate 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. Perforate 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 perforate st john's-wort?

Rarely required; a light balanced feed in spring on very poor soils is sufficient. Avoid nitrogen-rich fertilisers. Rarely required; a light balanced feed in spring on very poor soils is sufficient. Avoid nitrogen-rich fertilisers. 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 perforate st john's-wort?