Guidance for gardeners
Our gardens have been greatly enriched by the introduction of plants from abroad, but a small number have proved highly invasive in the wider environment threatening natural habitats and native species. By their nature they are usually a problem in the garden as well.
The control of these species is difficult and costly, yet many are widely available with little indication of the damage they can do if they are allowed to escape into the countryside or disposed of carelessly. The threat to native species and habitats from these plants is serious.
For more information on the issues see the RHS Conservation & Environment Guideline Invasive non-native species
Know what you are buying and growing
- Avoid using plants known to be invasive - there are usually many alternative plants better suited to gardens.
- Plant labelling is getting better but the height and spread given may not be that at maturity. Do some research on plants which are unfamiliar to you. Be wary of labels and descriptions that use terms such as ‘vigorous’, ‘spreading’, ‘fast growing’, ‘self-seeds’, ’good ground cover’, etc.
- Consider a non-natives plant’s invasive qualities when you exchange plants with friends. Much of the spread of Himalayan balsam (Impatiens glandulifera) is down to gardeners gifting seeds to friends.
- Beware of introducing an invasive species inadvertently as a ‘hitch-hiker’ - many invasive pondweeds, in particular, are introduced to gardens in this way. It is always a good idea to quarantine newly purchased pond plants to see what else might come with it.
- Many invasive plants are introduced to gardens in topsoil. Always try to see the topsoil before you buy it. Buying topsoil
Plants to avoid
Several non-native species are so invasive that no gardeners would wish to grow them. Examples include giant hogweed (Heracleum mantegazzianum) and Japanese knotweed (Fallopia japonica) and several introduced pond weeds.
Others, such a Himalayan balsam (Impatiens glandulifera), readily self-seed but are quite simple to keep under control in the garden. However, if they escape over the garden fence they quickly colonise stream-sides and other wet areas. For other invasive non-native species, such as Rhododendron ponticum, there is little reason to grow them now as there are so many more attractive hybrids available to the gardener.
However, the decision as to what to grow can be more problematical. There are a number of plants that might be quite appropriate in some gardens in some locations but not in others. If you live near native bluebell woodlands never grow Spanish bluebell (Hyacinthoides hispanica) which hybridises easily and near downland or hill country avoid cotoneasters as birds will spread the seeds.
Aquatic weeds
Perhaps the most serious group of plants posing a major threat to the countryside are several exotic pondweeds. This has partly been a consequence of the explosion of interest in water features and the mistaken belief that oxygenators are somehow essential to the well being of a pond. Generally, the excessive growth of oxygentors poses more of a problem and gardeners are often irritated by the time and effort involved in controlling their rampant growth.
Avoid at all costs non-native species such as New Zealand pygmy weed (Crassula helmsii) and parrot’s feather (Myriophyllum aquaticum) (left). Fairy fern (Azolla filiculoides) and floating pennywort (Hydrocotyle ranunculoides) are also poor choices together with Nuttall’s pondweed (Elodea nuttallii) (right), curly waterweed (Lagarosiphon major) and curled pondweed (Potamogetum crispus).
The really serious problems start however when these plants are released or escape into the wild. Water weeds should be composted or, where this is not possible, buried. As with all surplus material from the garden they should never be disposed of in the countryside.
More information on aquatic weeds
Gardeners and plant escapes into the wild
Although fly-tipping is one way invasive species are spread, most invasives enter the wider environment accidentally. There is an onus on gardeners to keep garden plants within their gardens. An invasive plant in the garden is a problem only for the gardener, however, if such plants escape into the wild the repercussions are more serious and control becomes more difficult. Some gardeners are also guilty of releasing plant material into the wild in the mistaken belief that it is better than destroying the offending plant.
Control techniques
Invasive plants can be difficult to control, but timely action will reduce the scale of the task.
- Dead-heading before the plant has time to seed will restrict spread.
- Hand-pulling: Himalayan balsam can be controlled in this way while giant hogweed can be dug out, but most other invasive species are difficult to control in this way.
- Root barrier fabrics: For plants with strong rhizome systems root barriers can be used to contain their spread.
- Weedkillers: Most invasive plants can be controlled eventually with a sustained programme of weedkiller application but this is costly. For the home gardener products containing glyphosate are usually the most effective.
- Contractors: For large-scale infestations it may be best to employ a contractor with the appropriate Certificate of Competence in the application of herbicides. Where spraying is necessary in areas adjacent to, or over, water consult the Environment Agency (see telephone directory) or the National Association of Agricultural Contractors (www.naac.co.uk) who can supply a list of fully trained contractors.
Responsible disposal
Plant material should never be disposed of in the countryside. Fly-tipping is illegal. Much can be composted or taken to municipal recycling centres but Japanese knotweed should never be composted or taken to recycling centres.
- Composting: Most soft aquatic weeds can be composted in the garden. However, many invasive plants are unsuitable for home composting as the temperatures generated in domestic compost heaps are unlikely to destroy seeds, tough roots or rhizomes, although the industrial scale composting of municipal green waste generally will.
- Burying: Soft aquatic weeds can be buried in trenches in the garden but this technique is unsuitable for most invasive plants. Japanese knotweed is regarded as ‘controlled waste’ under the Environmental protection Act 1990 (Duty of Care) Regulations and, if not burnt, can only be disposed of in registered land-fill sites.
- Burning: Seeds and tough root systems when dried may be burnt, but check local council guidelines on bonfires.
The Horticultural Code of Practice
In March 2005 DEFRA issued a code of practice to draw attention to the problem of invasive non-native plants. The code seeks to inform all users of plant material from gardeners through to garden centres and nurseries to local authorities and developers of the potential damage that introduced species may cause to the wider environment.
A key element of the Code is that plants should be accurately named and described, particularly regarding their growth habit and eventual size. It is also the intention that growers signing up to the code will label plants to indicate possible danger to the wider environment.
The Code also offers guidance on legislation, together with information on the control and safe disposal of plant material. Observation of the Code will make a major contribution to preventing the spread of invasive non-native species in the countryside.
The full Code can be found via www.defra.gov.uk. A summary leaflet (PB10586) is available (telephone 08459 335577).
Action by gardeners: Summary
- Follow the guidance in the Horticultural Code of Practice.
- Avoid using plants known to be invasive, especially non-native aquatic species.
- Do not distribute invasive non-native plants that may damage the wider environment to other gardeners.
- Take steps to prevent the escape of invasive non-native plants into the wild.
- Destroy or dispose of invasive non-native plants in a responsible way. Do not introduce them into the wild or into areas where they may escape into the wild.