Seeds & growth
Runner
A long, trailing stem that a plant such as a strawberry sends out, which roots where it touches the soil to form a new plant — a free way to propagate.
A runner is a long, thin stem that creeps out sideways from a plant, lying along the ground rather than standing up. Where it touches soil it grows roots and a little cluster of leaves, and that becomes a brand new plant — a clone of its parent. The proper name for this kind of stem is a stolon, but in the garden almost everyone just says "runner".
Strawberries: free plants every summer
Strawberries are the classic example. Once they have finished fruiting, usually from June through August in the UK, they put their energy into sending out runners. Each runner can produce several baby plants, called plantlets, spaced along its length. Left alone, they sprawl and root wherever they land, which is how a tidy strawberry bed turns into a tangled mat over a few years.
To make good new plants, choose strong runners and peg the first plantlet on each down into the soil, or better still into a small pot of multipurpose compost sunk level with the bed. A bent piece of wire, a hairpin or a small stone holds it in place while it roots, which takes a few weeks. Once it has a decent root system — usually by late August or September — snip the stem connecting it to the parent and you have a free plant to move wherever you like.
Why removing runners can mean more fruit
There is a trade-off. Every runner a plant makes costs it energy it could have put into next year's crop. If you want the heaviest harvest, pinch off runners as they appear so the parent stays focused on fruiting and building a strong crown. Many gardeners do both: keep a few runners from their best plants to replace tired old ones, and remove the rest. Because strawberries are short-lived perennials that crop less well after three or four years, propagating from runners is the easiest way to keep a bed productive without buying new plants. The same young plants are also ideal if you want to try forcing a few pots for an early crop under cover.
Not just strawberries
Other plants spread the same way. Mint is the best-known: it travels by underground runners (also stolons) and will quietly take over a border, which is exactly why mint is usually grown in a pot to keep it in check. Creeping plants like some saxifrages and spider plants do it too. So a "runner" is really nature's own propagation system — handy when you want more plants, and worth controlling when you do not.
In a UK garden
In a UK garden, strawberries throw out runners from June to August once they have finished fruiting, giving you free young plants to root and pot up before autumn.
Example
Peg a strawberry runner into a small pot of compost sunk beside the parent in July, and by September it has its own roots and can be cut free.