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Fertilising guide

How to fertilise Great Mullein (Verbascum thapsus)— schedule & NPK

Also called Great Mullein, Common Mullein, Aaron's Rod, Flannel Plant.

More about great mullein

About Great Mullein

Verbascum thapsus · also called Great Mullein, Common Mullein · flowering

Verbascum thapsus is a biennial native to Europe and western Asia, now widely naturalised across North America. In its first year it forms a flat rosette of large, woolly, grey-green leaves; in its second year it throws up a stout, torch-like spike of yellow flowers reaching 1–2 m. Full sun and sharply drained, even poor soil are the two non-negotiable requirements — waterlogged conditions will kill it. Verbascum thapsus is not listed on the ASPCA toxic plant database and is generally regarded as non-toxic to cats and dogs, though the dense leaf hairs can cause mild skin or gastric irritation.

Growth habit: Rosette-forming biennial; upright, unbranched flower spike in year two.

What fertiliser great mullein actually wants — and why

Great 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 great 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 great 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 great mullein:

Fertilising is unnecessary and counterproductive — great mullein performs best in poor soils without supplemental feeding. 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 great 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 great mullein

Half strength is the safe default for great 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 great mullein first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the great mullein watering schedule.

Signs you are over-feeding great mullein

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for great mullein:

Signs you are under-feeding great mullein

If the symptoms point at watering, light or roots rather than nutrition, the full great mullein care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

Flush the pot of great 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 great 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 great mullein — frequently asked questions

What fertiliser does great 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. Great 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 great mullein?

Fertilising is unnecessary and counterproductive — great mullein performs best in poor soils without supplemental feeding. Fertilising is unnecessary and counterproductive — great mullein performs best in poor soils without supplemental feeding. 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 great mullein?

Half strength is the safe default for great mullein — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.

What does over-feeding great 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 great 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 great mullein?

Flush the pot of great 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.

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