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Portrait of Rhododendron

Nature · Shrubs

Rhododendron

Neophyte · introduced 1763 · evergreen to 8 m · flowers May–Jun · woods and heath, west and north

Rhododendron ponticum — Ericaceae

Rhododendron ponticum is Britain's most damaging invasive woody plant: a big evergreen shrub from the Iberian Peninsula that Victorian estates planted under trees as game cover and that now forms dense, shady thickets across western woods, heaths and Atlantic temperate rainforest. It is a member of the heather family, and thrives on the same acid soils and fungal partners. Removing it is a slow, costly job of cutting, herbicide and years of follow-up.

How to spot them

  • ▸Large evergreen shrub, often 5 to 8 m, with big, leathery, smooth-edged, dark green leaves paler beneath, in whorls at the tips
  • ▸Rounded trusses of bell-shaped mauve-purple flowers in May and June, with greenish-brown spotting in the throat
  • ▸Dense, arching thickets that cast such shade that almost nothing grows beneath, with hard woody seed capsules
  • ▸Acid soils in high humidity, mostly in the wetter west and north of Britain and Ireland
  • ▸Look-alikes: cherry laurel has toothed-edge leaves with a marzipan smell when crushed and white flowers; garden hybrids have red, pink or white flowers, and ponticum is used as their rootstock

How to know it

A huge evergreen shrub with mauve-purple trussed flowers in early summer that forms dense, dark thickets beneath oak woods in the wet west

Rhododendron — R. ponticum.

Introduced by Conrad Loddiges in 1763, it was planted by Victorian estates as game cover and as rootstock

Rhododendron ponticum.

DNA work shows that nearly all invasive bushes in the British Isles come from the Iberian Peninsula, and are hybrids

Rhododendron ponticum.

A single large bush can shed up to a million seeds in a season, and its shade and leaf litter smother the ground beneath

Rhododendron ponticum.

Woodland managers treat it by cutting and by drilling stems and injecting herbicide, which kills large plants within about six months

Rhododendron — stem injection.

Imported plants of this genus can carry Phytophthora ramorum, the pathogen behind the widespread felling of larch

Rhododendron — a host of ramorum.

It thrives on acid, humid ground, helped by a fungal partner in its roots, which is why it does best in western Britain and Ireland

Rhododendron — an Ericaceae.

Quick-fire facts

  • •Rhododendron ponticum, family Ericaceae; ponticum refers to the Pontus region of the Black Sea, though the invasive British plants come from Iberia
  • •First introduced as seed by Conrad Loddiges in 1763 (Forest Research)
  • •Victorian estates planted it under woodland canopy and on heathland as game cover, and used it as rootstock for hardy Asian hybrids (Forest Research)
  • •Most if not all invasive plants derive from the Iberian Peninsula and are hybrids with R. maximum and R. catawbiense (Forest Research)
  • •Grows best on acid soils in high humidity, mainly in the west of Britain and Ireland; ericoid mycorrhizal association aids nutrient uptake (Forest Research)
  • •Reduces numbers of earthworms, birds and plants; mature bushes make up to a million seeds a season (Forest Research)
  • •Described as Scotland's most threatening invasive non-native plant; treated with chainsaws, herbicide, heavy machinery and repeat visits (Forestry and Land Scotland)
  • •Choking Britain's temperate rainforest, which covers just 1% of UK land; it is listed in law as an invasive non-native species, so planting it in the wild is prohibited though sale is not (Woodland Trust)
  • •Injecting herbicide into stems of large plants kills them in about six months, cutting herbicide volume and cost (Forest Research)

Drill it — 7 cards

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