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Fertilising guide

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

Also called Hairy St John's-wort, Hairy St John's Wort.

More about hairy st john's-wort

About Hairy St John's-wort

Hypericum hirsutum · also called Hairy St John's-wort, Hairy St John's Wort · flowering

Hypericum hirsutum is a softly hairy, erect perennial native to calcareous woodland edges, scrub, and hedgebanks across the UK and Europe, reaching 40–80 cm with terminal clusters of pale yellow five-petalled flowers from July to August. It grows in partial to full shade, tolerating conditions too shady for most Hypericum species, and prefers moist, well-drained soils. The most important care fact is that, like all members of the genus, it contains hypericin, which is toxic to cats, dogs, and horses. Keep pets away from all parts of the plant.

Growth habit: Erect, softly hairy herbaceous perennial with opposite, oval, translucent-dotted leaves.

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

Hairy St John's-wort is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.

A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula.

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 hairy 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 hairy 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 hairy st john's-wort:

A light application of a balanced fertiliser (e.g. 5-10-5) in spring supports flowering; avoid high-nitrogen feeds that promote soft, fungal-susceptible growth. Treat that as sparingly through the growing season between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.

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

Half strength is the safe default for hairy st john's-wort — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.

Feeding always goes onto already-damp soil, never dry roots — water hairy 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 hairy st john's-wort watering schedule.

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

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

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

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

Flushing and leaching the salts

Flush the pot of hairy st john's-wort with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.

Organic vs synthetic feeds for hairy st john's-wort

Organic options

A diluted seaweed or worm-casting feed, or fish emulsion if you can tolerate the smell indoors. UK: Westland or Baby Bio Organic, dilute seaweed; US: Espoma Indoor! or Neptune's Harvest fish & seaweed. Slow, gentle and hard to overdo.

Synthetic / liquid feeds

A general-purpose houseplant liquid at half strength — UK: Baby Bio, Westland Houseplant Feed or Phostrogen; US: Miracle-Gro Indoor Plant Food or Schultz. Convenient and fast-acting; the only risk is overdoing it.

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 hairy st john's-wort — frequently asked questions

What fertiliser does hairy st john's-wort need?

A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula. Hairy St John's-wort is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.

How often should I feed hairy st john's-wort?

A light application of a balanced fertiliser (e.g. 5-10-5) in spring supports flowering; avoid high-nitrogen feeds that promote soft, fungal-susceptible growth. A light application of a balanced fertiliser (e.g. 5-10-5) in spring supports flowering; avoid high-nitrogen feeds that promote soft, fungal-susceptible growth. Treat that as sparingly through the growing season between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.

What strength of feed for hairy st john's-wort?

Half strength is the safe default for hairy st john's-wort — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.

What does over-feeding hairy st john's-wort look like?

Brown, crispy leaf tips and edges with no sign of underwatering. A white, crusty salt deposit on the soil surface or pot rim. Weak, pale, stretched new growth that flops. Lower leaves yellow and drop while the soil is correctly watered. Feeding hairy st john's-wort year-round on a fixed schedule, including dark winter months, is the most common mistake — it cannot use the nutrients in low light and the surplus simply burns the roots and crusts the soil.

Should I flush the soil of hairy st john's-wort?

Flush the pot of hairy st john's-wort with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.

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