Fertilising guide
How to fertilise Moth Mullein (Verbascum blattaria)— schedule & NPK
Also called Moth Mullein, Moth Verbascum.
More about moth mullein
About Moth Mullein
Verbascum blattaria · also called Moth Mullein, Moth Verbascum · flowering
Moth Mullein is a slender, graceful biennial or short-lived perennial producing tall, elegant spikes of yellow or white flowers with distinctive purple-hairy stamens that resemble antennae — giving the plant its common name. Native to Europe and western Asia, it naturalises readily in dry, sunny conditions and is a valuable pollinator plant with a long summer blooming season.
Growth habit: Biennial or short-lived perennial; slender upright unbranched to lightly branched spike; more delicate habit than most Verbascum species; self-seeds freely and naturalises
Watch for — Mullein moth (Cucullia verbasci): Caterpillars feed on flowers and leaves in late spring; the slender spikes of moth mullein are particularly vulnerable to complete defoliation — inspect regularly and remove caterpillars by hand or apply Bt biological spray.
What fertiliser moth mullein actually wants — and why
Moth Mullein 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 moth mullein: 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 moth mullein, 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 moth mullein:
No fertiliser required in typical garden soils. Moth mullein flowers best in low-fertility conditions. In very poor sandy soils, a light application of balanced granular fertiliser in early spring encourages a stronger flowering spike without causing the lax, overly lush growth seen in over-fed plants. 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 moth mullein 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 moth mullein
Half strength is the safe default for moth mullein — 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 moth mullein first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the moth mullein watering schedule.
Signs you are over-feeding moth mullein
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for moth mullein:
- 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.
Signs you are under-feeding moth mullein
- Uniformly pale or yellow-green leaves, oldest first.
- Noticeably small new leaves and stalled growth in good light and season.
- A generally tired, lacklustre look despite correct watering and light.
If the symptoms point at watering, light or roots rather than nutrition, the full moth mullein care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Flush the pot of moth mullein 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 moth mullein
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 moth mullein — frequently asked questions
What fertiliser does moth mullein 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. Moth Mullein 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 moth mullein?
No fertiliser required in typical garden soils. Moth mullein flowers best in low-fertility conditions. In very poor sandy soils, a light application of balanced granular fertiliser in early spring encourages a stronger flowering spike without causing the lax, overly lush growth seen in over-fed plants. No fertiliser required in typical garden soils. Moth mullein flowers best in low-fertility conditions. In very poor sandy soils, a light application of balanced granular fertiliser in early spring encourages a stronger flowering spike without causing the lax, overly lush growth seen in over-fed plants. 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 moth mullein?
Half strength is the safe default for moth mullein — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
What does over-feeding moth mullein 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 moth mullein 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 moth mullein?
Flush the pot of moth mullein 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.
Keep reading
- Moth Mullein care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water moth mullein — 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 tiger orchid
- How to fertilise nun's orchid
- How to fertilise cockleshell orchid
- All 8452 fertilising guides in the Growli library