Weeds
Privet narrow-leaf – Ligustrum sinense
Also known as Chinese privet. Narrow-leaf privet is a densely branched evergreen shrub. Originally a garden plant, it is now a serious environmental weed.
Profile
How does this weed affect you?
Narrow-leaf privet is considered to be serious environmental weed throughout Australia. Infestations threaten biodiversity, including endangered plant and animal species and ecological communities. Dense stands of privet prevent other vegetation surviving or establishing.
It is reported that privet pollen causes allergic reactions and hay fever. It is unlikely that the pollen of privet is strongly allergenic; however, cross-reactivity can occur where people who are sensitive to grass pollen can become sensitive to privet, producing allergic reactions. It is thought that the perfume of privet flowers causes these reactions, not the pollen. Reactions occur commonly during spring and early summer when privets produce masses of flowers and pollen. These include allergy-like symptoms such as asthma and irritation of mucous membranes. In one extreme case, hospitalisation resulting in near death occurred after the patient was exposed to privet.
Privet berries and leaves have been reported by overseas sources to be poisonous to humans and livestock if ingested; however, no known cases of poisoning have occurred in Australia.
Privets invade native and plantation forest industries, orchards and pastures in Australia. Costs of control are high and yields are reduced by the presence of privet in these production systems.
Where is it found?
Narrow-leaf privet originates from eastern Asia and has become a widespread weed of disturbed land, pastoral areas and native bushland in coastal NSW. Broad-leaf and narrow-leaf privets occur as widespread weeds in coastal and tableland areas of New South Wales. Within the Sydney area, narrow-leaf privet is the more widespread species.
How does it spread?
Privets reproduce primarily by seed. Mature plants can produce up to ten million seeds each growing season. narrow-leaf privet seeds are thought to survive only 6 months. Masses of seedlings will germinate in ideal conditions, surviving in low light levels until disturbances to the shade canopy stimulate growth.
Privet seeds are commonly spread by fruit-eating birds. Birds such as pied currawongs, silver-eyes and rosellas can spread the seed widely into previously uninfested areas. Privet seedlings often germinate in clusters, as a result of birds regurgitating the seeds. Birds and rabbits assist germination by removal of the soft coating around the seed. Seeds require shallow burial in the soil (to 1 cm) in order to germinate (they do not germinate on top of the soil). Fresh seeds that have passed through the guts of birds or other animals can germinate immediately.
Privets are also spread through the sale of garden plants from nurseries and markets, the dumping of garden waste containing seeds and the sale of foliage in floral arrangements containing fruit and seeds. Seeds can also be spread in flowing water.
Vegetative reproduction commonly occurs in small-leaf privet, but less commonly in the other two species. Small-leaf privet produces root suckers and sprouts, and, it has been suggested, rhizomes.
Growth of seedlings
Privet seedlings growing in dense shade can survive long periods of unfavourable conditions such as low light and water shortage. Growth under these conditions is restricted, but faster than that of the surrounding vegetation, particularly in low light. These young privets can also outnumber native species by ratios of several hundred to one and when growth accelerates with any increase in light (due to activities such as weeding or clearing), the young privets rapidly become dominant.
Flowering and fruiting
Privets appear to have a long juvenile period before reaching reproductive maturity (four years for broad-leaf privet). Privets flower in spring and the fruit ripens in autumn and winter. Small-leaf privet flowers earlier than other privets and can also achieve a low level of flowering and fruiting throughout the year.
What does it look like?
Small-leaf privet is a densely branched, multi-stemmed evergreen shrub 3–5 m tall (occasionally growing to 7 m). The smooth bark is brownish–grey on stems and greyish-green on branches. Young branches are covered in fine, short hairs and small branches have white lenticels.
Oval-shaped leaves occur in opposite pairs and are 1–7 cm long and 0.5–3.5 cm wide. Leaf tips may be pointed or round. The under-surface of each leaf is covered in fine hairs (including the midribs) and both leaf surfaces are mid to dull green. Leaves often have distinctly wavy margins and are held at right angles to the stems.
White, tubular, lobed flowers with mauve to purple anthers occur in branched clusters. Flower tubes are 0.5–2 mm long with 3.5 mm lobes, and are very strongly scented.
Berries are 4–10 mm in diameter, green when young, ripening to dull purple or blue–black. Each berry contains 2 oblong seeds 3–4 mm long. Roots are woody, branching, thickened at the crown and mostly shallow.
Small-leaf privet may be semi-deciduous in colder areas.
| Privet species | Distinguishing features |
|---|---|
| Broad-leaf | Leaves generally 50–130 mm long; flower tube shorter or as long as the lobes; anthers well extended from the tubes; fruits bluish-black, on short stalks. |
| Small-leaf | Lower leaf midribs finely hairy, leaf margins smooth to wavy; flower tube shorter than lobes; anthers mauve to purple, fruits blackish-purple, on hairy stalks. |
| European | Lower leaf midribs not finely hairy; leaf margins never wavy; flower tube slightly longer than lobes; anthers white and barely extended from the tubes; fruits glossy black, on short, slightly hairy stalks. |
What type of environment does it grow in?
Narrow-leaf privet prefers warm, humid environments with moderate to high soil moisture throughout the year. Creeks, gullies and drainage lines are favoured by both species, but seedlings are able to establish in drier areas if run-off water is temporarily available. It occurs in areas with rainfall between 700–1600 mm.
Narrow-leaf privet seedlings can tolerate very low light levels, allowing them to persist beneath dense canopies of vegetation.
Privets have been found growing in a range of soil types, from pure sands through to friable loams, and almost pure clays. However, it is generally agreed that privets thrive on more fertile shale or clay-derived soils found in riparian areas.
Acknowledgements
Authors: Elissa van Oosterhout, J. Mowatt, Leon Smith, Stephen Johnson.
Technical reviewers: Stephen Johnson, Birgitte Verbeek.
References
Hardin, D. (1992), Ligustrum vulgare L. New South Wales Flora Online, Royal Botanic Gardens and Domain Trust, Sydney.
Johnson, S. (2009), Review of the declaration of Ligustrum (privet) species in NSW, NSW Department of Primary Industries, Orange.
Johnson, S. B. (2009), ‘Privet species – are we sitting on species time bombs?’, in Proceedings of the 15th Biennial NSW Weeds Conference, Narrabri, NSW Department of Primary Industries, Orange.
Control
Wide dispersal of seed by birds cannot be controlled; therefore controlling the spread of privet requires the removal of seed trees and young seedlings before they produce seed.
Follow up control and revegetation
Many attempts to control or remove privet have failed because of its ability to regenerate vigorously from root and stem suckers. Follow-up control measures are critical for successful removal. The removal of large numbers of privet bushes from other vegetation can cause enough disturbance that reinfestation occurs. Revegetation with appropriate species, along with ongoing weed control, can assist with preventing reinfestation. Where privet is providing a replacement habitat and food source for fruit-eating birds, control efforts must ensure that removal is undertaken gradually in combination with revegetation with suitable species.
Manual removal
Manual removal techniques such as the original ‘Bradley method’ allow for good control of privet with minimal disturbance to the surrounding vegetation. These techniques involve hand-weeding of small and medium-sized privet plants, where the gaps left by weeding must be similar to those that occur naturally after the death of a native plant. Soil disturbance should also be minimised.
Narrow-leaf privet can be pulled up but the stems are more likely to break from the root system, leaving viable root segments capable of regeneration. They should be dug out and the plants placed upside-down to dry out the roots.
Control with herbicides
Foliar treatments can be made to flushes of seedlings and groups of plants up to 3 m high. Plants must be actively growing, not under heat or moisture stress, and complete coverage of the foliage is necessary to ensure successful control. Foliar treatments are appropriate where infestations contain dense stands of privet and little or no other valuable vegetation.
Basal bark applications are appropriate for treating larger individual plants in amongst other vegetation. Every trunk or stem arising from the ground must be treated.
Stem injection is also appropriate for treating larger individual plants in amongst other vegetation. Stem injection has been found to be the most cost-effective method of control in terms of volume of herbicide and labour costs. It is also most effective in terms of reducing off-target herbicide damage to other vegetation.
Where is it possible or desirable to completely remove whole plants, herbicide treatment of the cut stumps must be carried out in order to prevent regrowth from stumps. Cut-stump application of herbicides is very effective for controlling young plants, suckers or regrowth.
Mechanical removal
Earth-moving machinery may be suitable for removal of dense stands of privet if high levels of soil disturbance can be tolerated. Large areas of seedlings or regrowth can be slashed. These methods will reduce the seeding capacity of a large infestation, but will not eradicate it. Follow-up with herbicide control or manual removal may provide higher levels of control. These areas should also be revegetated with trees, shrubs, ground covers or pastures and repeatedly hand-weeded or slashed thereafter. Mechanical removal is not recommended in steep areas or near water courses.
Fire
Burning is generally ineffective against privet. Privet thickets are of low flammability and bushfires do not readily move through privet-dominated vegetation. Even when fire is very intense, privets are able to regenerate rapidly by sprouting or suckering. There is evidence to suggest that both broad and small-leaf privets can recover after high intensity fires have killed the above-ground plant material (flowering has reoccurred within 3 years). Fire has been used as an initial control measure, followed up by treatment of regrowth with cut stump herbicide applications the following year. Persistent annual cool burns have been shown to eliminate small-leaf and European privet in southern USA, and it is thought that frequent fires probably assist with controlling seedling establishment of privet in infested eucalypt forests and woodlands in Australia.
Biological control
There are no introduced biological control agents available for privet control in Australia.
Reducing nutrient levels
Increased nutrient levels often contribute to the presence of privet infestations. Reducing or stopping the movement of nutrients in water from residential or industrial areas into riparian areas may help prevent establishment of large privet infestations.