Silverleaf nightshade in our sights

Man examining plants

The hard-to-kill noxious weed, silverleaf nightshade, is spreading across NSW. But thanks to some innovative research, control costs look likely to reduce.

GRDC northern panel chairman Norm Marran said that 20 years ago it was estimated that silverleaf nightshade could be found on only about 12,000 hectares in NSW. Now it had spread over an estimated 150,000 hectares. "It is a deep-rooted pest which has reduced grain yields by up to 50 per cent in low rainfall conditions and can also halve autumn-winter pasture production," Mr Marran said.

Just how deep-rooted is demonstrated by agronomist Gerry Hennessy, shown above, demonstrating that nightshade roots can penetrate more than two metres into the soil.

"Summer broadleaf crops such as cowpeas, soybeans, mungbeans and cotton are virtually impossible to grow owing to its competitive ability and the lack of in-crop herbicides," said Mr Marran. "GRDC is helping fund this research which includes testing the combined effect of well-managed competitive pastures with the strategic use of herbicides."

Identifying the culprit

NSW Agriculture weeds agronomist Jim Dellow describes silverleaf nightshade as a plant that grows to about 60 cm, has silvery-green foliage and blue to purple flowers from November to March, with the top growth dying in winter. It should not be confused with other plants such as quena, western nightshade or Narrawa burr.

Mr Dellow said silverleaf nightshade was spread by seed which could remain viable in the soil for well over seven years and also by cultivation which fractured the root system and stimulated germination.

Control

Current control recommendations for small infestations are to apply glyphosate or Tordon 75-D. When used as a high volume spot spray application the cost would be about $21 per 100 litres spray mix and applications over several years would be necessary.

For extensive infestations, the main aim is to contain them and prevent the weed from spreading via animals, water, birds or machinery. Slashing or 2,4-D amine applications can be used to prevent flowering and seed production, but regrowth usually occurs.

Mr Dellow said that GRDC-funded trials had shown that the use of 1.5 L/ha of 2,4-D amine plus 1 per cent crop oil would kill flowering top growth. A Pesticide Order has recently been obtained for the use of this herbicide mix.

Pastures are the other tool. "Research at Gulgong is showing that, in the initial stages, perennial pasture species, such as phalaris, are helping to reduce infestations," Mr Dellow said.

Subprogram 3.15.02 Contact: Mr Jim Dellow 063 913 889