Mature size & growth rate

How big does Great Mullein (Verbascum thapsus) get?

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.

Mature size: Rosette to 60 cm across in year one; flower spike 1–2 m tall in year two.

Indoor size vs how big it gets in the wild

Great Mullein reaches its full size within one growing season — there is no "long-term" size, just how big it gets before you harvest or it dies back. Indoors and in a pot, expect rosette to 60 cm across in year one. In the ground with no restriction it is a completely different plant — flower spike 1–2 m tall in year two. — which is why the pot, the light and the pruning matter so much for the size you actually end up with.

It sizes up fast and once, racing from seedling to full size in a single season; after cropping it is finished, so size is a within-season question.

Growth rate and years to mature

Great Mullein is a moderate grower. Realistically, expect a single growing season — it reaches full size in one year, then is done. Its feeding profile backs this up: fertilising is unnecessary and counterproductive — great mullein performs best in poor soils without supplemental feeding.

Want this turned into the right next pot at the right moment? The pot size calculator and the great mullein repotting guide cover when and how much to size up — pot size is one of the biggest levers on how fast great mullein grows.

How to keep great mullein smaller

You are not stuck with the maximum size. For great mullein specifically, these are the levers, in order of impact:

Choose a compact or dwarf variety of great mullein from the start — that is the most reliable size control for an annual.
Grow it in a smaller container to naturally limit how large it gets.
For some crops, pinching or pruning the growing tips keeps the plant shorter and bushier.
Sow a little later or space plants closer if you specifically want smaller individual plants.

How to grow great mullein bigger or faster

If you want it to fill the space sooner, push the conditions rather than hoping — for great mullein the accelerators are:

Full sun, warm soil and steady water are what drive a crop to full size fastest.
Sow at the right time for your zone so it gets the whole season to size up.
Feed appropriately for the crop and never let it check (stall) from drought or cold.

Light is almost always the ceiling. The great mullein light requirements page covers exactly how bright a spot it needs to grow at its potential instead of stalling.

When great mullein outgrows the room (or the pot)

"Too big" usually arrives as one of these signs for great mullein:

It sprawls beyond its bed or container before harvest — usually a spacing or support issue.
It flops or needs staking once it hits full height.
Once it has fruited or bolted, it is at its final size for good — the next plant is a new sowing.

If it is the pot rather than the room, it is a repotting job, not a goodbye — see the great mullein repotting guide. If you want more of this plant instead of a bigger one, the great mullein propagation guide turns prunings into new plants.

Great Mullein size — frequently asked questions

How big does great mullein get?

Great Mullein reaches rosette to 60 cm across in year one when grown indoors, and far larger where it grows unrestricted (flower spike 1–2 m tall in year two.). It sizes up fast and once, racing from seedling to full size in a single season; after cropping it is finished, so size is a within-season question.

Is great mullein slow or fast growing?

Great Mullein is a moderate grower. Expect a single growing season — it reaches full size in one year, then is done. Great Mullein reaches its full size within one growing season — there is no "long-term" size, just how big it gets before you harvest or it dies back.

How long does great mullein take to reach full size?

Roughly a single growing season — it reaches full size in one year, then is done. Light, pot size and feeding move that timeline more than anything else.

How do I keep great mullein smaller?

Choose a compact or dwarf variety of great mullein from the start — that is the most reliable size control for an annual. Grow it in a smaller container to naturally limit how large it gets. For some crops, pinching or pruning the growing tips keeps the plant shorter and bushier. Sow a little later or space plants closer if you specifically want smaller individual plants.

How can I make great mullein grow bigger or faster?

Full sun, warm soil and steady water are what drive a crop to full size fastest. Sow at the right time for your zone so it gets the whole season to size up. Feed appropriately for the crop and never let it check (stall) from drought or cold.