The herbs that take over, and how far down the barrier has to go
Nobody plants mint once. They plant it once, and then they have mint, and then they have mint where the parsley used to be. Extension services are blunt about which herbs do this and how to stop them, and the advice includes numbers: how many inches above the soil and how many below.
Two different ways of taking over
It matters which one you are dealing with, because the fix is different.
By rhizome, underground. The plant travels sideways below the surface and surfaces somewhere else. Barriers work. Deadheading does nothing.
By seed. The plant stays where it is and its children do not. A barrier does nothing. Cutting the flowers before they set does everything.
Mint, lemon balm and yarrow spread by rhizome. Feverfew and German chamomile spread by seed. Yarrow, unhelpfully, does both.
NC State Extension Gardener Plant Toolbox, plant pages read 17 September 2026. Utah State University Extension, mint in the garden, May 2020.
The list, and what the sources call them
| Herb | How it spreads | What the extension source says |
|---|---|---|
| Mint, all of them | rhizome | "quite invasive"; "will out-compete most garden plants" |
| Peppermint | rhizome | "an attractive, though aggressive, ground cover"; grow in a pot 12 to 16 inches wide |
| Spearmint | rhizome and rooting stems | stems "root wherever they touch the ground"; "best grown in a container as it spreads rampantly" |
| Lemon balm | rhizome and seed | "spreads aggressively by rhizomes"; best in a container "where it is unlikely to escape into the landscape" |
| Yarrow | rhizome and seed | "can spread aggressively by rhizomes and seeds" |
| Feverfew | seed | "may become aggressive in optimum conditions"; deadhead immediately to control self-seeding |
| German chamomile | seed | "readily self-seeds, unless all flower heads are removed" |
| Oregano | slow clump | "forming a slowly spreading clump", which is the mild end of this list |
The number that makes a barrier work
Utah State University Extension gives the dimensions, and they are more demanding than what people usually bury: a container used to contain mint "should extend 3-4 inches above and 10-15 inches below the soil surface".
Above matters because the stems creep over the rim. Below matters because the rhizomes go under a shallow edging without noticing it. A plastic lawn edging pushed two inches into the soil is not a barrier, it is a speed bump.
Illinois Extension adds the warning that saves the most disappointment: a bottomless pot sunk into the bed does not always work, and mint escapes it. The reliable version is a pot that is a pot, sitting above ground, where you can see what it is doing.
Utah State University Extension, mint in the garden, May 2020. Illinois Extension, herbs, mint page; no date printed on it.
If it already got away from you
Rhizome spreaders do not come out with one pass, because every fragment left behind is a new plant. Expect to go back three or four times over a season, and expect the second flush to look like you accomplished nothing.
The seed spreaders are easier and they need timing rather than effort: cut the flowers before they brown. Miss that window by two weeks and you have a seed bank instead of a plant.
Plan around it instead
The pragmatic layout, which is what most people arrive at eventually: the spreaders live in pots, permanently, and the bed holds the ones that stay where they are put. Sage, thyme, oregano and lavender are clump formers that will not surprise you.
Which of them survives your winter in the ground is a separate question with a map behind it: herbs by USDA zone. And if a chart told you to plant mint next to something because it repels pests, read what the extensions actually say about that first.
Sources, with the date of each
- Utah State University Extension, mint in the garden, May 2020.
- Illinois Extension, herbs, mint; no date printed on the page.
- NC State Extension Gardener Plant Toolbox, plant pages read 17 September 2026; the pages carry no date of their own.
- University of Wisconsin horticulture, chamomile, revised May 2026.
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