Fertilising guide
How to fertilise Olympic Mullein (Verbascum olympicum)— schedule & NPK
Also called Olympic Mullein, Greek Mullein, Branching Mullein.
More about olympic mullein
About Olympic Mullein
Verbascum olympicum · also called Olympic Mullein, Greek Mullein · flowering
Olympic Mullein is a dramatic, architectural biennial or short-lived perennial from Greece and Turkey, producing a massive basal rosette of silver-white woolly leaves followed by a candelabra-branched flower spike reaching 1.8–2.5 m and studded with golden-yellow blooms. Spectacular as a focal point in dry, sunny borders, gravel gardens, and Mediterranean-style planting schemes.
Growth habit: Biennial or monocarpic perennial; large flat rosette in year one; tall, candelabra-branched flower spike in year two or three; dies after flowering but self-seeds
Watch for — Mullein moth (Cucullia verbasci): Striking yellow-and-black-spotted caterpillars feed on foliage and flowers from late spring; inspect plants regularly and remove by hand or apply Bt-based biological spray.
What fertiliser olympic mullein actually wants — and why
Olympic 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 olympic 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 olympic 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 olympic mullein:
No feeding required in poor soils; this is the preferred condition. In moderately fertile garden soil, a light dressing of slow-release balanced fertiliser in spring of year two supports flower spike development. Over-feeding produces fast, weak growth and reduces the striking silver colouring of the leaf rosette. 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 olympic 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 olympic mullein
Half strength is the safe default for olympic 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 olympic mullein first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the olympic mullein watering schedule.
Signs you are over-feeding olympic mullein
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for olympic 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 olympic 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 olympic mullein care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Flush the pot of olympic 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 olympic 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 olympic mullein — frequently asked questions
What fertiliser does olympic 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. Olympic 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 olympic mullein?
No feeding required in poor soils; this is the preferred condition. In moderately fertile garden soil, a light dressing of slow-release balanced fertiliser in spring of year two supports flower spike development. Over-feeding produces fast, weak growth and reduces the striking silver colouring of the leaf rosette. No feeding required in poor soils; this is the preferred condition. In moderately fertile garden soil, a light dressing of slow-release balanced fertiliser in spring of year two supports flower spike development. Over-feeding produces fast, weak growth and reduces the striking silver colouring of the leaf rosette. 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 olympic mullein?
Half strength is the safe default for olympic mullein — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
What does over-feeding olympic 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 olympic 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 olympic mullein?
Flush the pot of olympic 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
- Olympic Mullein care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water olympic 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 greater knapweed
- How to fertilise centaurea 'amethyst in snow'
- How to fertilise yellow foxglove
- All 8452 fertilising guides in the Growli library