Fertilising guide
How to fertilise Horse Mint (Mentha longifolia)— schedule & NPK
Also called Horse Mint, Horsemint, Bible Mint, Long-Leafed Mint.
More about horse mint
About Horse Mint
Mentha longifolia · also called Horse Mint, Horsemint · herb
Horse Mint is a tall, vigorous wild mint native from Europe to Central Asia, recognisable by its grey-green woolly leaves and branching spires of pale lilac to white flowers. More robust and wilder in character than culinary mints, it naturalises in moist, partly shaded spots and spreads freely by rhizome.
Growth habit: Vigorous, spreading herbaceous perennial with erect stems 60–120 cm tall, grey-green downy to woolly leaves, and stoloniferous rhizomes forming expanding clumps.
What fertiliser horse mint actually wants — and why
Horse Mint 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 horse mint: 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 horse mint, 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 horse mint:
Feed monthly during the growing season with a balanced liquid fertiliser at half recommended strength. As with most mints, avoid excessive nitrogen, which promotes leafy but less aromatic growth. A compost mulch applied in spring reduces the need for supplemental feeding. 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 horse mint 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 horse mint
Half strength is a sensible default for horse mint — 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 horse mint first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the horse mint watering schedule.
Signs you are over-feeding horse mint
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for horse mint:
- 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 horse mint
- 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 horse mint care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Pot-grown horse mint 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 horse mint
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 horse mint — frequently asked questions
What fertiliser does horse mint 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. Horse Mint 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 horse mint?
Feed monthly during the growing season with a balanced liquid fertiliser at half recommended strength. As with most mints, avoid excessive nitrogen, which promotes leafy but less aromatic growth. A compost mulch applied in spring reduces the need for supplemental feeding. Feed monthly during the growing season with a balanced liquid fertiliser at half recommended strength. As with most mints, avoid excessive nitrogen, which promotes leafy but less aromatic growth. A compost mulch applied in spring reduces the need for supplemental feeding. 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 horse mint?
Half strength is a sensible default for horse mint — enough to fuel regrowth after cutting, gentle enough that the leaves stay aromatic rather than watery.
What does over-feeding horse mint look like?
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. Over-feeding horse mint with strong nitrogen is the usual mistake — it grows fast and lush but the leaves turn bland and it bolts to flower sooner, ending the useful harvest early.
Should I flush the soil of horse mint?
Pot-grown horse mint 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.
Keep reading
- Horse Mint care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water horse mint — the watering schedule
- The houseplant fertiliser schedule — feeding through the year
- NPK ratio explained — what the three numbers on the bottle mean
- How to fertilise pelargonium 'ginger'
- How to fertilise pelargonium 'snowflake'
- How to fertilise pelargonium 'prince of orange'
- All 6887 fertilising guides in the Growli library