White weeping broom was brought to Australia as an ornamental shrub. It was first recorded in South Australia in 1841. Like many of the broom plants, it invades nutrient-poor to fertile, well-drained soils where it can fix nitrogen and form a scrub layer that can outcompete and shade out native plants. This species is possibly the most drought tolerant of the exotic brooms in Australia, making it a particular threat in dry regions and during drought years. It may infest grazing land and prevent access to stock. It is also probably the least palatable to stock of the exotic brooms.
White weeping broom is native to northern Africa and western Sahara, Sicily and the Middle East. In Australia white weeping broom has become naturalised in South Australia and is growing around Perth, WA.
White weeping broom reproduces from seed. Each plant produces hundreds of seed pods and up to thousands of seeds on larger plants. The seeds drop when the seed pods split open, and can be further spread by water. A hard seed coat renders most seeds dormant initially, but as the seed coat wears away germination can take place. Seeds remain viable in the soil for several years.
White weeping broom is a Mediterranean shrub that grows to about 3 m tall and may reach 6 m across. Plants are grey-green with slender, drooping branches. Young plants are wispy with a single stem and strong taproot.
The leaves, which are very small (about 5 mm long) and narrow (only 1 mm wide), are quickly dropped and the plant remains leafless for most of the year.
Flowers are 8–10 mm long, white and pea-like, appearing close to the stems in clusters of 3–15. Each flower tube contains ten stamens, the pollen bearing stalks that are the male reproductive parts of the flower.
The hairless grape-shaped seed pod (10–15 mm diameter) contains one or two kidney-shaped seeds, which are about 6.5 mm long and may be yellow, green, brown or black in colour.
Stems of young plants are covered with long soft hairs but become hairless with age. An extremely similar looking and closely related species, Retama monosperma, is a popular garden plant in Australia and also a potential weed.
In its native range, white weeping broom grows in grasslands in the Mediterranean region and is a common feature of deserts and grasslands in the Sahara. White weeping broom has several adaptations, including its lack of leaves, that reduce water loss and make it ideally suited to the warm dry conditions of its native range.
CRC for Australian Weed Management: Andy Sheppard (CSIRO/Weeds CRC), John Virtue (SA DWLBC/Weeds CRC), Jason Emms (University of Adelaide/Weeds CRC), Sandy Lloyd (Agriculture WA/Weeds CRC) and John Thorp (National Weeds Management Facilitator).
Because there are relatively few white weeping broom infestations, and it can potentially be eradicated before it becomes established, any new outbreaks should be reported immediately to your local council weed officer. Do not try to control white weeping broom without their expert assistance. Control effort that is poorly performed or not followed up can actually help spread the weed and worsen the problem.
White weeping broom is known in Australia only from several small populations and a preferred method of control has not yet been developed. However, experience in controlling broom (Cytisus scoparius), which is a major weed in southern Australia, can be applied to this species. Any control of white weeping broom should be undertaken cooperatively with your local council.
No herbicides are registered for control of white weeping broom. In the case of Cytisus scoparius, chemical control is effective in the short term but is expensive and needs to be followed up for many years until the seedbank has been depleted. There is also a risk of damage to non-target species.
Physical removal is an option for isolated plants, especially if they have not seeded, although seedlings are hard to hand pull. Monitor the area in summer and remove any young plants that may have germinated.
Bulldozing infestations into heaps and burning the resulting weed mounds has been a common method used to control broom but it only provides a temporary solution. Bulldozing causes massive soil disturbance and physical movement of plants, not only burying seeds but also spreading them beyond the original infestation. In at least one place this practice and a lack of follow-up treatments has exacerbated the Cytisus scoparius problem. Cutting stems off near the ground with saws will stress the plant but cut plants resprout vigorously, so cutting alone will not kill them. Because white weeping broom has a more continuous growth than some of the other broom species, it can be treated throughout the year whenever it is growing actively. It should be killed before it sets seed. Permits to mechanically clear vegetation may be required if native species are likely to be affected.
Fire effectively kills plants and can help to break seed dormancy. Experience using fire to control other species of broom indicates that it kills a large proportion of seeds but lightly scorched plants may resprout. Follow-up chemical treatment after fire will probably be needed for many years until the seedbank is depleted. Usually though, fire is not recommended to control broom in Australia due to the risk of out-of-control fires and because it leaves the land initially unusable, with many burnt stems remaining in the ground. Permits may be required to light fires.
Once an area of white weeping broom has been treated, it will be necessary to monitor the treated area for many years and destroy new plants.
Hand-pulled plants can be placed into large plastic bags and left in the sun. Seeds present on pulled plants should be cut from plants, collected in bags and placed in the household rubbish. Take care not to spread seeds beyond the current infestation. If removed weed material cannot be accommodated on the site, (eg mulched or dried in the sun), it should be bagged and removed to tip facilities. Seeds should not be included in vegetation to be used for the production of garden compost. If the plant is being removed from gardens, dispose of waste by carefully bagging all material and putting it in the household rubbish. As seeds are difficult to destroy, it is advisable to dispose of plants when they are carrying seeds. Never give plants to other people or dump plants on vacant land, over back fences or in bushland areas. Contact your local council for specific advice before attempting to dispose of white weeping broom.
White weeping broom (Retama raetam) is not declared in NSW under the Noxious Weeds Act 1993.