Fertilising guide
How to fertilise Common Mullein (Verbascum thapsus)— schedule & NPK
Also called Common Mullein, Great Mullein, Wooly Mullein, Aaron's Rod, Flannel Leaf.
More about common mullein
About Common Mullein
Verbascum thapsus · also called Common Mullein, Great Mullein · herb
Common Mullein is a stately biennial forming a basal rosette of large, densely woolly grey-green leaves in year one, then sending up a towering flower spike to 2 m covered in yellow flowers in year two. Historically used in herbal medicine for respiratory complaints, it thrives in poor, dry, disturbed soils and full sun. Excellent for wildlife gardens.
Growth habit: Biennial; year one = flat ground-hugging rosette; year two = erect single flower spike; self-seeds prolifically and forms colonies over time
What fertiliser common mullein actually wants — and why
Common Mullein 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 common 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 common 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 common mullein:
No fertilising needed or recommended. Adding fertiliser encourages lush, soft growth more prone to disease and reduces the plant's architectural quality. In extremely impoverished soils, a single application of balanced slow-release granules at planting supports establishment of the year-one rosette. 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 common 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 common mullein
Half strength is a sensible default for common mullein — 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 common mullein first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the common mullein watering schedule.
Signs you are over-feeding common mullein
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for common mullein:
- 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 common mullein
- 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 common mullein care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Pot-grown common mullein 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 common mullein
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 common mullein — frequently asked questions
What fertiliser does common mullein 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. Common Mullein 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 common mullein?
No fertilising needed or recommended. Adding fertiliser encourages lush, soft growth more prone to disease and reduces the plant's architectural quality. In extremely impoverished soils, a single application of balanced slow-release granules at planting supports establishment of the year-one rosette. No fertilising needed or recommended. Adding fertiliser encourages lush, soft growth more prone to disease and reduces the plant's architectural quality. In extremely impoverished soils, a single application of balanced slow-release granules at planting supports establishment of the year-one rosette. 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 common mullein?
Half strength is a sensible default for common mullein — enough to fuel regrowth after cutting, gentle enough that the leaves stay aromatic rather than watery.
What does over-feeding common mullein 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 common mullein 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 common mullein?
Pot-grown common mullein 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
- Common Mullein care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water common 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 pelargonium 'clorinda'
- How to fertilise pelargonium 'ginger'
- How to fertilise pelargonium 'snowflake'
- All 8452 fertilising guides in the Growli library