Why garden plants escape

A garden is a small, tidy patch of the world, but plants have no interest in boundaries. Seeds blow over fences on the wind, berries are carried by birds, and fragments of root or stem wash down drains into ditches, streams and rivers. Most of the plants we grow are perfectly well behaved, but a handful are quietly built for escape: they set seed in enormous quantities, spread by runners or bulbs, or regrow from the smallest piece of root left in the soil.

It is rarely anyone's fault. Many of these plants arrived as fashionable ornamentals long before anyone realised how they would behave in a British climate, and some are still sold without much warning. The good news is that a few sensible habits around planting, cutting and composting make an enormous difference — both to your own plot and to the hedgerows, verges and waterways beyond it.

Read the label, and know the rules

Plant labels are more revealing than they look. Words like vigorous, spreads readily, self-seeds freely and good ground cover are often polite warnings. If a plant carries one of those descriptions, treat it as something to watch rather than something to plant and forget.

There is also a legal dimension worth knowing. Certain non-native species are listed on Schedule 9 of the Wildlife and Countryside Act 1981, and it is an offence to plant them, or cause them to grow, in the wild. Japanese knotweed and Himalayan balsam are the best-known examples. You are not expected to be a botanist, but if you discover one of these on your land, it is worth checking your local council's guidance before you touch it — a little knowledge here prevents a great deal of trouble later.

When buying pond plants especially, ask whether the species is known to be invasive. Aquatic escapees travel faster and further than anything in a border.

The usual suspects

These are the ones most often seen jumping the garden fence in Britain:

• Himalayan balsam — a tall annual with exploding seed pods that can fling seeds several metres, and it loves damp ground beside rivers.
• Japanese knotweed — spreads by rhizome fragments and is genuinely difficult to eradicate; never move soil from an affected area.
• Spanish bluebell — crosses with our native bluebell and gradually dilutes it in the wild.
• Cotoneaster — several species are spread by birds onto chalk downland and limestone grassland, where they shade out small native plants.
• Rhododendron ponticum — casts heavy shade and carpets woodland floors with seedlings.
• Three-cornered leek and montbretia — both spread by bulbs or corms and can form dense colonies in the west and south-west.
• Aquatic thugs — floating pennywort, water fern, parrot's feather and New Zealand pigmyweed all regrow from tiny fragments.

Keeping things in bounds

Control is mostly about interrupting the plant before it reproduces. Deadhead flowering plants before the seed sets and drops, and put the heads straight into a bag rather than on the ground. With buddleia, cutting the faded flower spikes off after flowering stops the seed scattering along walls and railway lines, while still leaving the blooms for butterflies.

For anything that spreads by runners, plant it in a container or sink a rigid root barrier into the soil. Never tip pond water, or surplus pond plants, into a stream, ditch or wild pond — that single act is responsible for a surprising number of invasions. Choose clumping bamboos rather than running ones, and give thuggish perennials a firm boundary they cannot creep past.

Composting and disposal

A domestic compost heap rarely gets hot enough to kill tough seeds, rhizomes or bulbs, so be selective about what goes in.

• Do not compost seed heads, roots of running perennials, bulbils, corms or any fragment of knotweed.
• Drown it first. Submerge chopped material in a bucket of water, weighted down, and leave it for six weeks or more until it turns to sludge, then compost or bin it.
• Dry it out. Leave roots and stems on a hard surface in the sun until they are brittle and clearly dead.
• Use your council's green waste service where it accepts the material; commercial composting reaches temperatures that home heaps cannot.
• Never fly-tip garden waste in a layby, woodland edge or field corner. That is exactly how the worst invasions begin.

If you are dealing with Japanese knotweed, contact your council before moving anything, as disposal rules vary and may require a licensed carrier.

Better choices for the same job

Happily, there is almost always a well-behaved alternative that does the job just as beautifully.

• Instead of buddleia: lavender, hemp agrimony, field scabious, marjoram or wild privet — all excellent for butterflies and bees.
• Instead of cotoneaster: hawthorn, blackthorn, guelder rose or wild privet for berries and nesting cover.
• Instead of Spanish bluebell: native English bluebell, bought from a reputable British-grown source.
• Instead of floating pennywort: water forget-me-not, marsh marigold or a native water lily.
• Instead of running bamboo: a clumping bamboo, or a mixed native hedge for screening.

Gardening with a bit of care for what lies beyond the fence is one of the quiet pleasures of a British plot. Plant thoughtfully, cut and compost attentively, and your garden becomes part of the solution rather than the start of the next problem.