Mature size & growth rate

How big does Herb Robert (Geranium robertianum) get?

Also called Herb Robert, Robert Geranium, Stinking Bob, Red Robin.

More about herb robert

About Herb Robert

Geranium robertianum · also called Herb Robert, Robert Geranium · flowering

Geranium robertianum is a native annual or biennial wildflower of Europe, North America and western Asia, found in woodlands, hedgerows and shaded rocky ground. It thrives in almost any soil in sun or partial shade and self-seeds so freely that a single plant can colonise a border within a season. The most important care fact is to remove unwanted seedlings early while still small, as the taproot toughens quickly. True Geranium species are not listed as toxic to pets by the ASPCA — this species is considered non-toxic to cats and dogs.

Mature size: 20–40 cm tall and wide

Indoor size vs how big it gets in the wild

Herb Robert 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 20–40 cm tall and wide. A pot, your light levels and a little pruning are what set the final size in a home, far more than the plant's theoretical potential.

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

Herb Robert 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: no feeding required — performs best in lean soils; excessive nutrients encourage lush foliage at the expense of flowers.

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

How to keep herb robert smaller

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

Choose a compact or dwarf variety of herb robert 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 herb robert bigger or faster

If you want it to fill the space sooner, push the conditions rather than hoping — for herb robert 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 herb robert light requirements page covers exactly how bright a spot it needs to grow at its potential instead of stalling.

When herb robert outgrows the room (or the pot)

"Too big" usually arrives as one of these signs for herb robert:

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 herb robert repotting guide. If you want more of this plant instead of a bigger one, the herb robert propagation guide turns prunings into new plants.

Herb Robert size — frequently asked questions

How big does herb robert get?

Herb Robert reaches 20–40 cm tall and wide when grown indoors. 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 herb robert slow or fast growing?

Herb Robert is a moderate grower. Expect a single growing season — it reaches full size in one year, then is done. Herb Robert 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 herb robert 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 herb robert smaller?

Choose a compact or dwarf variety of herb robert 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 herb robert 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.