Every hobby gardener has probably experienced this: the once little plant turns out to be a real weed after just a few years. But there are a few good tricks to keep rampant plants in check.
Many flowering perennials are not quite as gentle as you would like them to be, but turn out to be rampant plants. Columbine and spur valerian (Centranthus), for example, self-seed, the latter even germinating in narrow pavement joints. In the case of the noble varieties of phlox, self-seeding leads to overgrowth: the offspring usually bear the flower colors of the wild species and, in extreme cases, can even displace the cultivated forms because they are more vigorous.
How to control rampant plants?
If possible, cut off all wilted flowers from flowering perennials that should not self-seed before seed ripens. For other wild perennials, however, self-seeding is encouraged. Short-lived species such as foxglove, silverling, and yellow poppy will last you for years this way, even though the individual plants die back after only two years.
Strong partners for stoloniferous species
Stoloniferous species such as garden loosestrife (Lysimachia punctata) are also difficult to deal with. They should be divided regularly and combined in the bed exclusively with species that are also resistant to being subdued, such as cranesbill or lady’s mantle.
Sprawling ground-cover plants
Sprawling ground covers such as ivy, barren strawberry (Waldsteinia ternata) or Japanese lantern (Physalis alkekengi) make it easier to maintain larger groups of shrubs, the plants form a dense carpet that is impenetrable even to weeds. But: low-competition, shallow-rooted shrubs like flowering dogwood (Cornus florida) must be well established or they will die over time. Even ornamental species such as creeping navelwort (Omphalodes) or comfrey (Symphytum) should be treated with caution. They form such a dense carpet that the roots of the woody plants cannot absorb enough water.
When planting, dig a shallow root barrier of pond liner all around the shrub.
How to keep rampant trees in check?
Even trees and shrubs can become a nuisance in the garden. They form runners or spread in the garden by self-seeding, for example the Norway maple. It gets particularly annoying when the seeds germinate in the hedge. You don’t notice them right away and they are very difficult to remove after only two years. Therefore, check your hedge for woody seedlings every time you prune it. The dwarf horse chestnut (Aesculus parviflora) grows 20 to 30 centimeters (8 to 12 in) in width every year and forms several square meters of large clumps with numerous short root runners when it is old.
Plant staghorn sumac only with rhizome barrier
The staghorn sumac (Rhus typhina) is undoubtedly one of the most beautiful colorful plants in autumn, but it can make life very difficult for gardeners with its root runners. What’s more, if you cut off the runners, its urge to spread is really encouraged by the injury to the roots. Therefore, staghorn sumac should always be planted with a rhizome barrier.
In the case of Siberian dogwood (Cornus alba ‘Sibirica’), the outer, low-lying branches form roots very quickly when they come into contact with the ground. In this way and over time, the shrubs can conquer large areas.
Plan rhizome barrier for bamboo
Bamboo is undoubtedly the king of the sprawlers. Stoloniferous species can take over a small garden within a few years, plus the shallow spreading rhizomes are extremely tough. Therefore, either plant the non-proliferating blue fountain (Fargesia) or install a rhizome barrier. This is a plastic sheet about 70 cm (28 in) high and 2 millimeters thick, which is screwed to a ring with a metal rail and buried vertically. Do not choose a diameter too small, otherwise the plants will suffer from drought.