Fertilising guide
How to fertilise Silver Mullein (Verbascum bombyciferum)— schedule & NPK
Also called Silver Mullein, Giant Silver Mullein, Broussa Mullein.
More about silver mullein
About Silver Mullein
Verbascum bombyciferum · also called Silver Mullein, Giant Silver Mullein · flowering
Silver Mullein is a spectacular biennial from Turkey grown for its enormous silvery-white woolly rosettes and tall, branched spikes of sulphur-yellow flowers. The intense silver indumentum makes it one of the most ornamental of all mulleins, catching light dramatically in the garden. It thrives in full sun and sharply drained, poor to average soils, tolerating significant drought.
Growth habit: Biennial; large flat silvery rosette in year one, tall branched woolly flowering spike in year two, then dies after seed set
What fertiliser silver mullein actually wants — and why
Silver 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 silver 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 silver 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 silver mullein:
Little to no fertilising needed or beneficial. Excess fertility reduces ornamental quality. On genuinely impoverished soils, a very light application of general balanced fertiliser in early spring of the flowering year only. 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 silver 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 silver mullein
Half strength is the safe default for silver 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 silver mullein first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the silver mullein watering schedule.
Signs you are over-feeding silver mullein
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for silver 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 silver 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 silver mullein care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Flush the pot of silver 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 silver 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 silver mullein — frequently asked questions
What fertiliser does silver 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. Silver 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 silver mullein?
Little to no fertilising needed or beneficial. Excess fertility reduces ornamental quality. On genuinely impoverished soils, a very light application of general balanced fertiliser in early spring of the flowering year only. Little to no fertilising needed or beneficial. Excess fertility reduces ornamental quality. On genuinely impoverished soils, a very light application of general balanced fertiliser in early spring of the flowering year only. 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 silver mullein?
Half strength is the safe default for silver mullein — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
What does over-feeding silver 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 silver 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 silver mullein?
Flush the pot of silver 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
- Silver Mullein care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water silver 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 thai silk california poppy
- How to fertilise china aster
- How to fertilise matsumoto mix aster
- All 8452 fertilising guides in the Growli library