How invasive plants escape from gardens
One means by which invasive plant species reach bushland is the dumping of rubbish containing seeds and plant material that regenerates vegetatively. As the rubbish breaks down, the soil is enriched with nutrients that encourage weeds to invade and deter the growth of native species.
People also unintentionally spread plants into bushland when seeds are carried on clothing, shoes, pets, and car tyres. If someone takes green waste to the tip in an uncovered trailer, seeds and cuttings can fall off at the roadside.
Natural agents also aid the spread – winds carry seeds; waterways transport seeds and vegetative material; and birds eat berries then excrete the seeds. With such diverse means of dispersal, the only way to stop certain plants becoming environmental weeds is not to cultivate them in the first place, and to get rid of existing invasive plants.
Invasive plants common in gardens
A large number of garden plants are current or potential environmental weeds, and often cause problems in domestic situations, too. Listed below are some of the most problematic, together with methods of eradicating them from gardens.
Please note:
- Where herbicide is suggested, nursery and garden centre staff can recommend the most suitable product for a particular plant. Always read the label before using.
- Where felling of a tree is suggested, it is recommended to employ a tree removal specialist who has the proper equipment and insurance.
- Where digging or pulling out plants is suggested, dispose of them responsibly by recycling through a local council collection service or taking them to the tip.
Arum lily
All parts of the arum lily (Zantedeschia aethiopica) are toxic. Poisoning symptoms include swelling and burning of lips, tongue and throat, stomach pain and diarrhoea. According to Australian hospital records, arum lily causes a significant number of child poisonings. It is also harmful to pets and livestock.
Arum lily is a now Declared Plant for the whole State of Western Australia. Since 1 September 2006, it has been illegal to sell or trade arum lily plants, seeds or tubers. However cut flowers can still be sold.
Arum lily has already invaded thousands of hectares of pasture paddocks, forest and wetland throughout the south-west and is still spreading, despite the efforts of community groups and landholders.
Although vegetative spread is slow, arum lily produces many berries (each containing four to five seeds), which are dispersed by birds or water. Arum lily forms dense monocultures. Gardeners who want to keep growing it should cut off the seed-heads before they mature. This way, the flowers can be enjoyed without allowing the plant to reproduce.
There are two ways to eradicate arum lily from the garden. It can be dug out, although with large clumps this is hard work, and any small daughter rhizomes left behind will produce new plants. Herbicide like glyphosate is effective when applied between June and September, but a follow-up application may be necessary after a year or two.
Blue periwinkle
Blue periwinkle (Vinca major) has invaded shady sites, including creeklines and woodlands, from Perth to Albany. In the garden its rampant growth can become a big problem because the trailing stems can take root wherever they touch the ground.
Eradicating it from the garden is hard. Herbicides like glyphosate only work when used at very high rates and with a wetting agent. Once the blue periwinkle is dead, it can be pulled or dug out.
Eastern States’ wattles
Cootamundra wattle (Acacia baileyana), Sydney golden wattle (A. longifolia) and Flinders Range wattle (A. iteaphylla) are not a problem in their own natural geographic locations where local seed-eating insects keep them in check. But this natural control does not occur in Western Australia where these wattles reproduce prolifically by seed. If fire sweeps through bushland that they have invaded, it promotes faster germination of their seeds.
These and other invasive Eastern States species are often labelled as ‘native’ so make sure you shop at a reputable plant nursery and do some research before you buy. This website may be useful.
Wattles are nitrogen fixers. When they grow in an area that has been disturbed — for instance, by someone driving in to dump garden rubbish — the combination of disturbed soil and extra nitrogen encourages exotic grasses to invade. As the grasses dry out in summer, they increase the risk of bushfire. Fire stimulates more wattle seeds to germinate. The resultant wattles fix more nitrogen in the soil, more exotic grasses invade and act as fuel for further fires — and thus a destructive cycle is set in motion.
Pruning the flowers before they set seed is not a practical option once these wattles reach a certain size. The only sure way to prevent seed spreading is to fell the tree.
Elephant ears and taro
These plants belong to the same family (Araceae) as the arum lily. Elephant ears (Alocasia brisbanensis) are popular in gardens because of handsome foliage and a capacity to thrive in shade. However, its fruits are poisonous to people and pets, though they can be eaten safely by birds, which then disperse the seeds.
Tubers of taro (Colocasia esculenta) are commonly eaten in Polynesia, but they are edible only when boiled or baked in the correct way — otherwise they are poisonous.
As taro tolerates permanent inundation, fancy forms have become popular for ornamental ponds and swamp gardens in partial shade. However, taro also tolerates dense shade and intense sunlight. This flexibility allows it to naturalise easily in a wide range of situations.
Taro is increasingly causing concern in the Swan and Moore River catchments and is very common on parts of the Canning River. Flowers are infrequent and seed is uncommon and of low viability, but taro spreads aggressively by corms and stolons. Its ability to inhibit the growth of other plants makes it undesirable in the garden.
Elephant ears and taro can be removed by pulling or digging out, but taro in particular is likely to resprout. If this occurs, follow up with herbicide.
Fountain grass
Many grasses produce pollen that can trigger asthma and hay-fever, so garden owners who suffer from these allergic reactions would be wise not to grow them.
Fountain grass (Pennisetum setaceum), from eastern Africa, is undesirable for another reason. It invades bushland and roadsides around Perth and from Geraldton to York. The fluffy heads contain many seeds that are transported long distances by the wind.
Clumps can be dug or pulled out of the garden, but first cut off the seed-heads and seal them in a plastic bag to prevent seed blowing away.
Gazania
As low maintenance, water-wise gardens have increased in popularity, so has the hardy South African gazania, which thrives in beachside situations. There are now many cultivars available.
However, the ‘wild’ species Gazania linearis has invaded road verges and bushland in coastal areas (notably the Perth metropolitan area), the southern wheatbelt and the goldfields.
This gazania spreads by rooting along its stems and from seed carried by wind or water. Pulling or digging out is the best way to remove it from gardens.
Lantana
Lantana (Lantana camara) is a Weed of National Significance and is prohibited from sale or trade across Australia. It is a particularly serious environmental weed throughout eastern Australia and is toxic. It has naturalised from Geraldton to the south coast, particularly along the rivers, creeklines, and in wetlands around Perth.
This vigorous, drought-tolerant plant has flowers that are plain yellow, or combinations of yellow, red, pink, orange or cream, followed by berries that birds eat and spread to bushland via their droppings.
In the garden, too, this lantana can become a problem when it forms dense, tangled thickets up to three metres high. Pulling it out is difficult for most people because, even if it is killed with herbicide, the woody stems remain and are covered with hooked prickles. It might be necessary to employ a garden maintenance contractor to remove it, using proper equipment.
Some yellow-flowered forms are often mistakenly labelled as sterile, or as the groundcover form Lantana montevidensis which is permitted but only has purple or white flowers, never yellow.
Morning glory
Two related vines that cause enormous problems for gardeners are the blue morning glory (Ipomoea indica), and mile-a-minute (l. cairica) which has pinkish mauve flowers. Both species are commonly called morning glory.
In domestic situations morning glory can invade gutters and eaves, cover most other plants, and jump the fence to wreak similar havoc in neighbouring properties. Apart from its rampant vegetative growth it also seeds prolifically. Morning glory has invaded bushland from Geraldton to Albany and is common along rivers and creeks in the Perth area, where it blankets trees and shrubs.
Removing morning glory from the garden requires persistence. A glyphosate herbicide can knock it back, but it may be necessary to repeat the applications many times before the vine dies.
Pampas grass
Pampas grass (Cortaderia selloana) is not a practical choice for gardens because of its huge size and dangerously sharp leaves — in New Zealand, where it is a serious weed, its common name is cut-throat grass.
It has invaded sunny, swampy sites from Perth to Albany, where local councils made it the subject of an eradication campaign. Pampas grass could become a major weed of Western Australian wetlands. Each plume can hold up to 100 000 seeds, which the wind carries great distances.
Removing a large specimen from the garden is virtually impossible for the average gardener, who would need to wear protective overalls at the very least. Employing a back-hoe operator to dig it out is an alternative option.
A small specimen could be cut down to ground level and then, when regrowth reaches between 50cm and 1m, the new growth can be treated with a glyphosate-based herbicide. Do not attempt to burn out a plant close to a house or other important structure.
Sweet pittosporum
Sweet pittosporum (Pittosporum undulatum) is native to New South Wales, Queensland, Tasmania, and Victoria. It produces large quantities of orange berries that are eaten by birds, which then spread the seeds via their droppings.
Simply cutting it off at the base will only result in the stump resprouting, so the whole tree needs to be removed professionally.
Victorian tea-tree
The home range of Victorian tea-tree (Leptospermum laevigatum) includes New South Wales, Queensland, South Australia, and Tasmania. It was an early introduction to Western Australia, mainly for use as a hedge plant, especially in coastal settlements. Many coastal areas of southern Western Australia are now seriously threatened by dense monocultures of Victorian tea-tree. Seed is distributed by the wind, while stem layering facilitates further spread.
In the garden, felling the tree should usually suffice to remove it, but it has been observed to resprout. If this happens, treat the stump and new shoots with a 'tree and blackberry killer' herbicide.
Resisting invasive plants
Good gardens evolve over time with planning and hard work. If you reject invasive plants when first planting your garden, not only will you be helping the environment but also saving your own gardens from big problems in the future.
With television programs and magazines showing radical make-overs and quick fixes, it may be tempting to seek ways to create an instant garden. But short-cuts sometimes lead to long-term disappointment. Some plants which are promoted as growing vigorously, providing rapid cover, or being indestructible can take over both gardens and bushland and then be hard to remove.
When buying new plants, ask nursery and garden centre staff to recommend non-invasive plants, including native species. Check that any plants labelled as ‘native’ are actually from Western Australia.
Unfamiliar pests and weeds
The Department of Agriculture and Food, Western Australia (DAFWA) is on the lookout for animal and plant pests, diseases and weeds that could pose a threat to agriculture and the environment.
If you discover something unfamiliar please send a photo to the Pest and Disease Information Service (PaDIS) by email: info@agric.wa.gov.au or phone them on Freecall: 1800 084 881.
Please read the sending samples for identification web article before sending samples to the Pest and Disease Information Service, 3 Baron-Hay Court, South Perth, 6151, WA.