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Not all of the plants in the garden are as well behaved as we would like them to be. Sometimes they can take over and are difficult to remove. Ornamental or introduced plants can become 'weeds' in the garden when they spread rapidly from their original planting spot by seeding freely or with creeping stems, and can overpower their better-behaved neighbours. Even with more naturalistic gardens they have to be kept in check to maintain any sense of a garden.

Where the spreading roots are a problem they can be removed by digging up infiltrated plants during the dormant season and carefully untangling them. Any regrowth from missed fragments can be eradicated by painting or carefully spraying with, a systemic weedkiller such as Glyphosate. If the culprit is worthy of keeping it can be grown in a large container, in an isolated spot like an island bed or the corner of a paved area.
Some have creeping stolons or stems just below the surface. These can be planted in a bottomless pot or bucket, buried in the flowerbed. This can curb the growth, but they can still spread out over the rim so you need to keep an eye on them, eg. the Mints and Bamboos. For larger clumps bury a vertical membrane such as builder's damp-proof course or a strip of butyl rubber to a level below where the creeping stems emerge.
Seeding problems require vigilance to remove fading flowers, but it is not easy as flowering is often successional with new and old flowers on the same stems. Although some plants produce lots of seeds which germinate quickly in the autumn, the seedlings are killed during the winter so they do not become a problem. Most seedlings are easily uprooted before they grow to any size.
Usually a plant which spreads vegetatively does not produce many seeds and conversely a multi-seeding plant does not have invasive roots or runners so remain as clumps. There are exceptions and these are the ones that cause most problems, eg. Crocosmia x crocosmiflora.

It is easy with hindsight to see that it was a mistake to introduce plants like Japanese Knotweed or Himalayan Balsam and hopefully trialing will avoid such introductions in the future. It is estimated that in Great Britain the annual cost of removing such invasive plants is £1.7 billion. However, there are some invasive plants still being grown and gardeners should act responsibly to prevent their spread to the wild. Shrubs with berries are particular examples as birds can carry the seed beyond the garden and if they are deposited in a wild area the native species can be overcome. Some creeping roots can move beyond boundaries and shrubs like Rhus typhina have become impenetrable thickets along railway tracks. Buddleja davidii has spread similarly when its seed have been wafted along by passing trains, it can also cause structural damage when it germinates, and is allowed to grow in old walls.

Here are some of the plants which can become a problem when they outgrow their welcome. The candidates for a list depend on the individual gardener, the style of garden and the amount of space available. Unfortunately labels rarely warn of their bad habits.
Another thing to beware of is the generous gardener. The reason he or she has some to give away could be that it has produced lots of seedlings or spread to fill its allotted space; the same applies to the donations to plant sales.

Common Bugle (Ajuga reptans) - spreads by surface runners and seeds. Good ground cover in shady areas, but it needs to be kept in check. 'Catlin's Giant' is a larger cultivar which is easier to control.
Goldenrod (Solidago sp.) - creeping stolons form a tough surface mat and some varieties seed around as well.
Creeping-Jenny (Lysimachia nummularia) creeping stems invade surrounding plants.
Crocosmia, Montbretia (Crocosmia x crocosmiflora and C. 'Lucifer') - corms spread by short stolons forming large clumps and infiltrating surrounding plants. It also spreads by seed.C. 'Lucifer' grows to over a metre and flops over, smothering surrounding plants. Digging up and burying the corms deeper every few years can solve the flopping problem as it gives the stems more support.
Cypress Spurge (Euphorbia cyparissias) - creeping rhizomes and seeds allow it to spread fast and infiltrate surrounding plants.
Feverfew (Tanacetum parthenium) - very floriferous herb, but it produces thousands of seeds which remain for many years.
Great Mullein (Verbascum thapsus) - an impressive plant but has similar seeding habits to Foxgloves, so seedlings appear all over the place, but they are easily uprooted and can be moved to a more suitable site when they are small rosettes.
Greater Celandine (Chelidonium majus) - produces lots of seeds, but most are killed by the winter cold and damp.
Heuchera pilosissima - attractive golden foliage, but produces lots of seed.
Himalayan Cotoneaster - (Cotoneaster simonsii introduced in 1865 now naturalised) - birds disperse the seeds - usually from adjoining property - which germinate below the trees and shrubs where they perch. Often they arrive and are allowed to grow until they outgrow their welcome. They can cause structural damage if seeds germinate in old walls, C. horizontalis is noted for this.
Horse-raddish (Armoracia rusticana) - grown for its roots it was introduced in the 15th century it is now in the wild. The deep fleshy roots can be invasive and difficult to eradicate from the vegetable plot.
Houttuynia cordata - creeping rhizomes spread rapidly through a flowerbed and are difficult to remove when they infiltrate other plant root-systems.
Ivy-leaved Toadflax (Cymbalaria muralis) - an attractive little trailing plant which looks great on an old wall. It spreads by an ingenious method of inserting its seed capsule in a crevice and can make its way to the top of a chimney.
Jacob's ladder (Polemonium caerulea) - if the fading flowers are not removed the ground within a six foot radius of the plant can become covered in a mass of seedlings.
Japanese Anemone (Anemone hupehensis var. japonica) - creeping rhizomes spread through flowerbeds, they can penetrate cracks in paving and walls.
Lesser Periwinkle (Vinca minor) - a good ground cover in shade, but creeping stems engulf surrounding plants
Lily-of-the-valley (Convallaria majalis)- like many invasive-rooted plants it can be slow to establish, but once it starts it there's no stopping it.
Mind-your-own-business (Soleirolia soleirolii) - a very low plant, but spreads rapidly and if it reaches the lawn can be a real pest.
Mints (Mentha sp.) - quickly swamp a flowerbed. Unfortunately they do not grow well in containers, but a large bottomless pot or bucket sunk into the ground is one way to curtail their spreading habits.
Monkey Fower (Mimulus sp.) - prolific seeders.
Orange Hawkweed, Fox and Cubs (Pilosella aurantica syn. Hieracium aurantiacum) - similar to Mouse-ear-hawkweed with orange flowers. It spreads by airborne seeds and creeping surface stolons, forming a dense mat.
Pendulous Sedge (Carex pendula) - grows to a large clump and produces lots of seedlings.
Perennial Cornflower (Centauria montana).
Peruvian Lily (Alstromaria aurea) - fleshy, almost translucent tubers spread and are britttle so they are difficult to dig up in one piece. It also produces seeds in explosive capsules. The orange form is the most invasive.
Pirri-pirri burr(Acaena ovalifolia) - low creeping with spherical burrs, now a widespread weed, it is illegal to plant it in the wild in Northern Ireland as it swamps native plants. Native of South America.
Plume Poppy (Macleaya cordata) - spreads by creeping rhizomes.
Poached Egg Plant (Limnanthes douglasii) - prolific seeder.

POND PLANTS - Many plants used in ornamental ponds are not native and have caused problems. Some are fine if kept within the confines of the garden pond, but if a natural stream flows through it they should not be used. Also they should never be discarded carelessly near natural waterways or canals. There is also the possibility that fragments or seeds can be carried away by visiting birds.
The Ornamental Aquatic Trade Association (OATA) has a poster for aquatic retailers to highlight the dangers of non-native pond plants. Also plants best suited to ponds that will not cause a problem should carry a label with the OATA logo to show that they have been approved for use.

Fairy Fern, Fairy Moss (Azolla filiculoides) - a small floating plant native to South America, was introduced to reduce light levels in pond water, so eliminating blanketweed and providing shelter for fish. However in the wild it multiplies rapidly forming a dense red film on waterways.
The weevil (Stenopelmus rufinasus) which is native to Florida feeds exclusively on Azolla species and can be used as a biological control. It has been present in Britain since the early 1920s, believed to have arrived on imported Azolla plants, so it is regarded as being resident and can be used here to clear the weed from waterways. It has also been used successfully in South Africa.

Floating Pennywort (Hydrocotyle ranunculoides) - introduced to help reduce light levels in garden ponds, but now in the wild. Dense growths clog drainage channels leading to flooding, blocking light and oxygen from fish and preventing access for birds to feed. If it is present in a garden pond then it is important not to allow it to escape. The Environment Agency spends about £120,000 a year removing it and recently it has become so invasive in the River Cam that the traditional pastime of punting, so redolent of life in the University City, may have to be halted due to health and safety considerations. It also threatens the open water near the mouth of the river which is important for wildlife and designated as a Site of Special Scientific Interest (SSSI).

New Zealand Pigmyweed, Australian Swamp Stonecrop (Crassula helmsii, Tillaea recurva, Tillaea helmsii) - a pond plant which spreads as small fragments and is smothering out native water plants - it does not appear to seed in our cooler climate. Very difficult to eliminate when it escapes, which has been known to occur when unwanted goldfish are discarded.

Parrot's Feather, Water Milfoil (Myriophyllum aquaticum, M. brasiliense, M. prospernacoides) - used as an oxygenator in garden ponds, but is now becoming a problem by clogging natural waterways It can become so dense that you can walk on it.

Water Primrose (Ludwigia sp.) - introduced from South America it grows in dense mats out from the edges. Bright yellow flowers usually with five petals and willow-like slightly hairy leaves arranged alternately on the tangled stems, are some of the distinguishing features. Two species, L. hexapetala and L. peploides are the worst offenders.

In 2014 a number of these water plants were added to Section 9 of the Wildlife and Countryside Act. These are Australian Swamp Stonecrop, Floating Pennywort, Parrot's Feather, Water Primrose and Water Fern.

Purple Toadflax (Linaria purpurea) - produces many seedlings, but not very invasive.
Red Valarian (Centranthus ruber) - looks good growing in old walls and rocks, but it seeds freely and can become a nuisance.
Russian Vine (Fallopia baldschuanica) - rampant climber which can quickly take over an area, not for the small garden.
Spanish Bluebells (Hyacinthoides hispanica) - it hybridises with H. non-scripta the native Bluebell in Britain, which is scented and smaller. There is some concern that the native species will be lost to the Hybrid Bluebell, Hyacinthoides x massartiana.
Stags-horn Sumac (Rhus typhina) - a small tree with an architectural growth habit and colourful autumn foliage. It produces many suckers which can form a dense thicket as can be found along railway sidings. The suckers should be regularly pulled up when they are small.
Tree of Heaven (Ailanthus altissima) - this native of Asia was brought to England in 1751 from China and until recently was reasonably well behaved. However, with warmer summers it can thrive much better and its rapid growth rate (which gave it its common name) means it can reach over 20 metres quickly. The male trees give off a foul odour and the females produce up to 300,000 seed in a year. Both have extensive roots which can damage drains and foundations as well as producing suckers. The long compound leaves reaching up to a metre in length can be mistaken for Sumac (Rhus sp.) and an alternate name is Chinese Sumac or Stinking Sumac. The wayward habits have already been recorded in France, Spain, Greece, Hungary, Australia and most states in the USA.
Welsh Poppy (Meconopsis cambrica) - seeds freely and once introduced to a garden it spreads around, unless it is diligently deadheaded.
Biting Stonecrop (Sedum acre) - small fleshy leaves and stems. Small fragments can take root. Also seeds freely and appears all over the place including gutters and on tooftops. Also known as Wallpepper.
Wood Spurge (Euphorbia amygdaloides var. robbiae, syn. E. robbiae) - good ground cover in shade but the creeping roots can become a pest.
Yellow Archangel (Lamium galeobdolon) - quickly smothers any surrounding plants by rooting stems. Small fragments of stem can take root. It makes a good groundcover plant for an awkward bank. There are variegated cultivars with silver splashes on the leaves.
Yellow Corydalis (Pseudofumaria lutea syn. Corydalis lutea) - spreads by seeding.

Another page in the Garden Creatures section of this site has information on Alien Species which are causing problems.

Weeds   Weed Prevention   Weed Removal   Weedkillers

   Weed Seedlings   Herbal Remedies