Every invasive plant you are likely to meet in Swansea

Swansea is one of the worst-affected places in Britain. Knotweed dominates back lanes, railway embankments, riverbanks and derelict plots across the city, and it is the weed most likely to affect a mortgage or a sale.

Season

Shoots March–April, peak growth June–August, canes die back brown in winter

Where in Swansea

Heaviest along the Tawe corridor, the Clyne and Cadle brooks, disused railway land in Landore and Morriston, and the back lanes of Brynmill, Uplands and Sandfields.

The pink-flowered annual that lines almost every stream in Swansea. One plant throws up to 2,500 seeds and the seed floats, so a single upstream patch reseeds miles of bank.

Season

Germinates March, flowers June–October, dies back with first frost

Where in Swansea

Clyne Valley, Nant Bran, the Tawe upstream of Morriston, Llan brook in Bishopston, and most of the Gower stream valleys.

The one that hurts people. Its sap makes skin blister badly in sunlight, and the burns can recur for years. Children are most often affected because the hollow stems look like play material.

Season

Growth from April, flowers June–July, seeds August

Where in Swansea

Scattered along the lower Tawe, around Llansamlet and parts of the Loughor estuary fringe.

A Victorian garden shrub that has swallowed large parts of Clyne Valley and the Gower woodlands, turning ancient woodland floor into bare, dark, sterile ground.

Season

Evergreen, flowers May–June

Where in Swansea

Clyne Valley Country Park, Clyne Gardens fringes, Bishopston and Nicholaston woods, and the Kilvey Hill plantations.

Birds carry the berries from gardens straight onto Gower's limestone cliffs and grasslands, where cotoneaster smothers some of the rarest plant communities in Wales.

Season

Flowers May–June, red berries autumn into winter

Where in Swansea

Mumbles Head, Langland and Caswell cliffs, Pennard, Oxwich and Rhossili slopes.

Sometimes picked as wild garlic. It carpets shady verges, churchyards and hedge banks across west Swansea and Gower, replacing native bluebells and celandines.

Season

Leaves from December, flowers March–May, dormant in summer

Where in Swansea

Uplands and Sketty verges, Brynmill Park edges, Mumbles road banks, and lanes across south Gower.

A thorny dune invader. It binds and over-stabilises mobile sand, which is exactly the wrong thing for Swansea Bay and Gower dune systems that depend on bare sand.

Season

Flowers June–September, hips autumn

Where in Swansea

Crymlyn Burrows, Swansea Bay dunes near the SA1 fringe, Whiteford and Broughton Burrows.

The orange garden crocosmia that has escaped into hedge banks and coastal paths all over Gower, forming solid corm mats that nothing else grows through.

Season

Foliage spring, orange flowers July–September

Where in Swansea

Coast path banks from Mumbles to Rhossili, Bishopston and Pennard lanes, Clyne Valley path edges.

Increasingly planted as a garden screen, running bamboo behaves like knotweed: its underground rhizomes travel metres under fences, patios and drains, and removal is expensive. Clumping types are far safer, but sellers rarely say which is which.

Season

New shoots April–June, evergreen canes year round

Where in Swansea

Garden-boundary plantings across Sketty, Killay, West Cross and the Mumbles suburbs; increasingly reported on terraced lanes where a single planting has run the length of a back lane.

The purple butterfly bush that seeds itself into every wall, bridge joint and chimney in Swansea. It is a genuine problem for buildings: roots widen mortar joints and brick cracks, and a mature buddleia can lift coping stones off a bridge parapet.

Season

Flowers June–September, seeds into autumn; deciduous

Where in Swansea

Bridge parapets and retaining walls along The Strand and Oystermouth Road, railway land behind High Street station, chimneys across Sandfields and Townhill, and every demolition site in the city.

The common estate hedge that has escaped into Gower and Clyne woodlands. A mature cherry laurel casts shade so deep nothing grows beneath it, and it suckers freely from cut or dumped material.

Season

Evergreen; white flower spikes April, berries autumn

Where in Swansea

Clyne Valley, Bishopston and Nicholaston woods, Kilvey Hill, and old estate boundaries across Mayals and Blackpill where hedges have gone wild.

A garden escape with pendulous maroon-and-white flowers that has jumped from borders into Swansea's lanes and woodland edges, forming dense bamboo-like stands from dropped berries.

Season

Shoots April, flowers July–October, dies back in winter

Where in Swansea

Hedgerows and woodland edges around Clyne Valley, Mayals and the Gower lanes; increasingly along the old railway paths.

A pond-garden plant with striking yellow arum flowers that has escaped into wet ground and streamside margins, where it forms dense crowds that swamp native marsh flora.

Season

Flowers March–May, large leaves to autumn

Where in Swansea

Pond margins and wet hollows in Clyne Valley, ditches on the margins of Parc Wern and Hendrefoilan, and streamside gardens across west Swansea.

A tiny aquarium plant that is one of Britain's most destructive pond invaders. A single fragment regrows into a new mat, and once established in a pond or ditch it is close to impossible to remove without draining.

Season

Grows year round, flowers May–October

Where in Swansea

Pond and ditch systems across Clyne Valley and the Lliw reservoir margins; a serious risk to Gower's freshwater pools if fragments arrive on boots or dogs.