Non-native plants are those that occur outside their natural range due to direct or indirect introduction by humans. If the introduced plants persist in natural or unmanaged habitats, they are termed ‘naturalised’.

Many naturalised species do not present a problem but some that spread and outcompete native species can threaten ecosystems, habitats or native species. Only where this occurs are the plants termed invasive non-native species. These are considered to be invasive either due to lack of natural control mechanisms (such as herbivores); rapid rate of spread (by seed or vegetatively) or suppression of other species (such as allelopathy or competition for resources). 

Non-native invasive plants can:

• Change ecosystems and habitats and have non-biotic effects, such as reducing or impeding water flow leading to flooding, or changing the
  This describes how acid or alkaline soil is, and is expressed as a number between 0 and 14. Soil with a pH of 7 is neutral, an acid soil has a lower value, and an alkaline soil has a higher one. Most plants grow best in soil that’s within a specific pH range, so it’s useful to know your soil’s pH. Testing kits are easy to use and widely available in garden centres and online.

  pH or the chemical composition of the soil, or lock up nutrients
• Outcompete native plants either by habitat change or by spreading so rapidly as to crowd out slower growing species, threatening the long-term survival of species
• Take a long time to become invasive. Many of the plants now considered invasive have been growing in the UK for over 100 years and for much of that time showed no sign of becoming a problem
• Be expensive to eradicate. It is also very costly to restore degraded habitat, if it can be done at all

  Top Tip

  There are 1,402 non-native plants established in the wild in Great Britain, of which 108 (8%25) are stated to have a negative impact.

There are a number of different regulations at both national and European level in place to help protect our environment from invasive non-native plants.

In Scotland the Wildlife and Natural Environment Act (Scotland) 2011 is now in force making it illegal to plant any non-native plant in the wild in Scotland.

Elsewhere in the UK it is an offence to plant or cause to grow in the wild plants listed on Schedule 9 of the Wildlife and Countryside Act (1981). In April 2014 a ban on sale of five of the worst invasive water plants in the UK came into force (see list below). 

EU Regulation on Invasive Alien Species lists 36 plants. This legislation still applies in the UK. These plants should not be planted or caused to grow in the wild but in addition are banned from sale and gardeners possessing them should undertake measures to control them (see Control).

Below is a list of invasive non-native plants covered by regulation:

Key
EU – applies across EU, including the UK and Republic of Ireland: it is an offence to plant or cause these to grow in the wild. These are also banned from sale and gardeners possessing them should undertake measures to control them
EW – applies in England and Wales: it is an offence to plant or cause these to grow in the wild
NI – applies in Northern Ireland: it is an offence to plant or cause these to grow in the wild
RI – applies in the Republic of Ireland: it is an offence to plant or cause these to grow in the wild

Acacia saligna (golden wreath wattle) EU
Acaena spp. (pirri-pirri bur) NI
Ailanthus altissima (tree of heaven) EU
Allium paradoxum (few-flowered leek) EW, NI, RI
Allium triquetrum (three-cornered garlic) EW, RI
Alternanthera philoxeroides (alligator weed) EU
Andropogon virginicus (broomsedge) EU
Aponogeton distachyos (Cape pondweed) RI
Arundo donax (giant reed) NI
Asclepias syriaca (milkweed) EU
Azolla filiculoides (water fern) EW, NI, RI, Banned from sale in UK since 2014
Baccharis halimifolia (tree groundsel) EU
Cabomba caroliniana (Carolina fanwort, water shield) EU
Cardiospermum grandiflorum (balloon vine) EU
Carpobrotus edulis (Hottentot fig) EW, NI, RI
Cortaderia jubata (purple pampas grass) EU
Cotoneaster bullatus (cotoneaster, hollyberry) EW
Cotoneaster horizontalis (cotoneaster) EW
Cotoneaster integrifolius (entire-leaved cotoneaster) EW
Cotoneaster microphyllus (small-leaved cotoneaster) EW
Cotoneaster simonsii (Himalayan cotoneaster) EW
Crassula helmsii (Australian swamp stonecrop, New Zealand pygmyweed) EW, NI, RI, Banned from sale in UK since 2014
Crocosmia × crocosmiiflora (montbretia) EW
Disphyma crassifolium (purple dewplant) EW
Egeria densa (large flowered waterweed) NI, RI
Ehrharta calycina (purple veldgrass) EU
Eichhornia crassipes (water hyacinth) EU 
Elodea nuttallii (Nuttall’s water weed) EU
Elodea spp. (waterweeds) EW, NI, RI
Fallopia japonica (RHS accepted name Reynoutria japonica) (Japanese knotweed) EW, NI, RI
Fallopia japonica × Fallopia sachalinensis [F. × bohemica] (RHS accepted name Reynoutria × bohemica)(hybrid knotweed) EW, NI, RI
Fallopia sachalinensis (RHS accepted name Reynoutria sachalinensis ) (giant knotweed) EW, NI, RI
Gunnera manicata (Brazilian giant rhubarb) RI
Gunnera tinctoria (giant rhubarb) EU 
Gymnocoronis spilanthoides (Senegal tea) EU
Hippophae rhamnoides (sea buckthorn) NI, RI
Hyacinthoides hispanica (Spanish bluebell) NI, RI
Hydrocotyle ranunculoides (floating pennywort) EW, NI, RI, Banned from sale in UK since 2014
Heracleum mantegazzianum (giant hogweed) EU 
Heracleum persicum (giant hogweed, Tromsø palm) EU
Heracleum sosnowskyi (giant hogweed) EU
Humulus scandens (Japanese hop) EU
Hydrocotyle ranunculoides (floating pennywort) EU
Impatiens glandulifera (Himalayan balsam) EU 
Juncus planifolius (broad-leaved rush) RI
Lagarosiphon major (curly waterweed) EU 
Lamiastrum galeobdolon subsp. argentatum (RHS accepted name Lamium galeobdolon 'Florentinum') (variegated yellow archangel) EW
Lespedeza cuneata (Chinese shrub clover) EU
Lonicera japonica (Japanese honeysuckle) NI
Ludwigia grandiflora (water primrose) EU, Banned from sale in UK since 2014
Ludwigia peploides (floating water primrose) EU, Banned from sale in UK since 2014
Ludwigia uruguayensis (water primrose) EW, NI, RI
Lygodium japonicum (climbing fern) EU
Lysichiton americanus (skunk cabbage) EU 
Microstegium vimineum (Japanese stiltgrass) EU
Myriophyllum aquaticum (parrot’s feather) EU, Banned from sale in UK since 2014
Myriophyllum heterophyllum (broadleaf watermilfoil) EU
Nymphoides peltata (fringed waterlily) NI, RI
Parthenium hysterophorus (parthenium weed) EU
Parthenocissus inserta (false Virginia creeper) EW
Parthenocissus quinquefolia (Virginia creeper) EW
Pennisetum setaceum (crimson fountain grass) EU 
Persicaria wallichii (RHS accepted name Koenigia polystachya) (Himalayan knotweed) RI
Pistia stratiotes (water lettuce) EW, NI, RI
Polygonum perfoliatum (RHS accepted name Persicaria perfoliata) (mile-a-minute weed) EU
Prosopis juliflora (mesquite) EU
Pueraria montana var. lobata (kudzu) EU
Rhododendron luteum (yellow azalea) EW
Rhododendron ponticum (rhododendron) EW, RI
Rhododendron ponticum × Rhododendron maximum (rhododendron) EW, RI
Rosa rugosa (Japanese rose) EW
Rubus spectabilis (salmonberry) NI, RI
Sagittaria latifolia (duck potato) EW
Salvinia molesta (giant salvinia) EU 
Smyrnium perfoliatum (perfoliate Alexanders) EW
Spartina spp. and hybrids (cord-grasses) NI, RI
Stratiotes aloides (water soldier) NI
Trapa natans (water chestnut) NI, RI
Triadica sebifera (Chinese tallow tree) EU
Zostera japonica (dwarf eelgrass) NI, RI

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner. For example, where pests, diseases or weeds pose a serious threat to the wider environment, to important heritage specimens, to habitat, or to native wildlife.

What if I have invasive non-native plants in my garden or on my land?

If you already have these species in your garden or on your land, you are not likely to be prosecuted simply for having them. However, you are advised to control them and, for those 36 plants listed by EU, you are required to take all possible steps to remove them, even if you didn’t plant them. It is also your responsibility to ensure that they are not allowed to spread.

Most of the non-aquatic non-native plants can be suppressed under a weed control membrane or by using non-chemical control methods. If you have a very large area to treat it may be best to obtain the services of a contractor with the appropriate Certificate of Competence or a firm who specialise in the control of invasive weeds such as Japanese knotweed.

Aquatic weeds can be pulled out of ponds or streams by hand. Where spraying is necessary in areas adjacent to, or over, water bodies, consult with the Environment Agency who can supply a list of fully trained contractors. Do not use home weedkillers on or near water.

How should I dispose of these invasive weeds?

There is always a risk when disposing of invasive weeds that you may inadvertently spread them further. Here’s how to reduce that risk;

• Aquatic weeds: compost or bury in trenches in the garden. When dried they may be burnt.
• Himalayan balsam, giant hogweed, and Japanese knotweed: these weeds are regarded as 'controlled waste' under the Environmental Protection Act (Duty of Care) Regulations so if taken off site can only be disposed of in registered landfill sites. Check with your local council for your nearest suitable site. Do not put in your normal green waste or household waste.
• Other invasive plants on the EU list: control with weedkiller or dig up and burn on site. Alternatively, DEFRA advise they can be disposed of as normal green waste through local recycling.

Note that the movement of invasive non-native plants is only permitted as part of responsible disposal. Never dump invasive plants or any garden plant in the wild or at the side of the road.

For further information, see Environment Agency regulatory position statement 178.

What could happen if I don't control an invasive species on my land?

The UK Government introduced new provisions in the Infrastructure Act (2015) to control invasive non-native species in England and Wales. There are two levels of control: a species control agreement and a species control order. In the former the owner of land where an invasive non-native species is present, when approached by the relevant environmental authority, agrees to take action to limit or remove the species.

If the landowner fails to do so, or does not agree, or where it is not known who the landowner is, then the environment authority can take action to enforce the control of the species. This may involve entry of the property by the authority to carry out the control if the owner fails to comply. In the case of an emergency then a species control order may be issued without going through the previous steps. Only those species listed on Schedule 9 of the Wildlife & Countryside Act can be subject to these control measures. Separate Codes of Practice have now been published for England and Wales.

Gardeners should also be aware of the extension of powers under the Anti-Social Behaviour, Crime and Policing Act (2014), which enable local authorities and the Police to address problems where an individual is acting unreasonably in a way that is affecting the quality of life of those living nearby, to be applied to individuals failing to control invasive species, through the issuing of community protection notices. These powers have already been successfully used in a few cases.

1. Follow the guidance in the Horticultural Code of Practice.
2. Avoid using plants known to be invasive, especially in the case of non-native aquatic species.
3. Choose plants for your garden carefully if you live close to sensitive habitat. For instance, avoid Cotoneaster shrubs if you live near downlands or limestone outcrops, as birds will spread the seeds.
4. Do not distribute invasive non-native plants to other gardeners that may damage the wider environment.
5. Take steps to control invasive non-native plants in your garden and to prevent them escaping into the wild.
6. Destroy or dispose of invasive non-native plants in a responsible way.
7. Take part in the citizen science project Plant Alert to flag garden plants you think have the potential to become invasive.