Images: Tim Sandall
Most gardeners are aware of the problems caused by invasive weeds, but there are plants - readily available to buy - that can pose just as much of a threat to garden harmony. Gardeners may buy these 'thug' plants unaware that, once established, they can run amok.
Plants that get out of hand in the garden can present a serious headache and involve a great deal of effort to get under control.
One of the simplest mistakes to make is to choose fast-growing plants that soon become too big for the garden. Many conifers make excellent screening plants but, in the case of Leyland cypress for example, do you really want it to grow to 35m (115ft)? Or how about Russian vine (Fallopia baldschuanica)? At a rampant 12m (40ft) it is likely to be just a little too vigorous for most situations - and is sometimes called mile-a-minute for obvious reasons!
Suckers and rhizomes
Poplars are not only too large for most gardens but they readily sucker. This characteristic is shared by several, otherwise excellent, small garden trees. Among them sumach (Rhus typhina) and flowering cherries. Cutting back the suckers will have little effect (in fact, may make the problem worse), but digging down and carefully pulling it away may. Suckering can be inadvertently encouraged where mowing damages surface roots. It also becomes a problem where older trees are felled without the stump being treated with a proprietary stump and rootkiller (Roundup Tree Stump & Rootkiller, Deep Root Ultra Tree Stump & Weedkiller or SBK Brushwood Killer).
The ability to spread by vegetative means can be a great advantage where rapid screening or ground cover is required. Unfortunately, plants with this characteristic do not know when to stop. One of the biggest culprits is Sasa palmata, (left) although its spreading rhizomes can be rivalled by many other bamboos. The common reed (Phragmites australis) is invaluable in stabilizing river banks, but will readily spread. Even grasses such as Phalaris arundinacea can get out of hand planted in the drier soils of the herbaceous border.
Grasses are not alone in spreading by rhizomes or runners. Many gardeners struggle to get Japanese anemone established only to discover it, in time, popping up throughout their carefully tended flower beds - even when they have gone to great pains to dig out the clumps. Other rampant ornamentals that their owners may regret ever planting include Houttuynia cordata ‘Chameleon’, yellow loosestrife (Lysimachia punctata), and golden rod (Solidago canadensis). Modest crevice dwellers such as mind-your-own-business (Soleirolia soleirolii), Leptinella squalida and Pratia pedunculata may invade and take over the lawn. Even the fruit and vegetable garden is not immune from invasion with blackberries, raspberries, horseradish and mint seeming to thrive on total neglect.
Ground and garden cover
Ground cover, by definition, needs to spread but be careful that its territorial ambitions are restricted. The snowberry (Symphoricarpos albus) relentlessly clumps up, while Rubus biflorus layers with ease. Both Vinca major and V. minor (left), can be equally troublesome. Gaultheria shallon (right) and Hypericum calycinum also spread readily.
Much hard labour can be avoided by checking the growth characteristics of plants carefully before planting and using proprietary root barriers to contain invasive plants. Where foresight has not been applied, a programme of spraying using a systemic herbicide containing glyphosate may, in time, get on top of herbaceous weeds, but for woodier plants choose SBK Brushwood Killer. Digging out can be attempted but is unlikely to be totally effective.
There is also the problem of disposal. Temperatures in domestic compost heaps are unlikely to destroy tough roots or rhizomes, although the industrial scale composting of municipal green waste generally will. Burning may be appropriate but check local council guidelines. Be wary of both giving and receiving gifts of vigorous and potentially invasive plants.
Self-seeding
Seed is a method of dispersal employed by all manner of garden thugs from the mightiest tree to the daintiess of garden flowers. Among the former, ash and sycamore are able to deploy their seed over large areas, often to germinate in the most inconvenient positions. Among shrubs, the most efficient of seeders include cotoneasters and buddlejas, although they tend to pose more of a problem in the wider environment than within the garden. This is also the case with Impatiens glandulifera (Himalayan balsam) that is easily controlled by hand weeding in the garden but quickly shades out native vegetation in marshes and along river banks. The secret to controlling these plants is to deadhead immediately after flowering to prevent seed production.
Bulb invaders
Bulbous plants are not without their problems. While many gardeners yearn for carpets of native bluebells, others struggle to restrict their spread by offsets and seed. Native lesser celandine (Ranunculus ficaria) is generally an unwelcomed visitor from beyond the garden gate but many gardeners have regretted introducing the named cultivars that are often as difficult to eradicate. Oxalis debilis is another weed originally introduced for its ornamental qualities, as were Allium paradoxum and A. triquetrum. It is usually difficult to eradicate these bulbous weeds by hand cultivation and some are relatively resistant to glyphosate.
Aquatic weeds
Perhaps the most serious group of garden thugs - and certainly a group 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. In any case, avoid at all costs non-native species such as Crassula helmsii (New Zealand pygmy weed) and Myriophyllum aquaticum (parrot’s feather). Azolla filiculoides (fairy fern) and Hydrocotyle ranunculoides (floating pennywort) are also poor choices. 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.
Tony Dickerson