Take a two-pronged attack against silverleaf nightshade

Silverleaf nightshade, one of the nation’s most difficult perennial summer growing weeds, can be controlled but not necessarily easily. A new 65-page document, summarising research and practical control is available from NSW DPI. It details control strategies, including herbicide timing, products to use and the need for integrated control, including aspects like competitive pastures and crops. READ MORE: DPI principal research scientist, Dr Hanwen Wu and Dr John Heap (SA) authors of the document, have been involved with silverleaf nightshade (SLN) research for decades. SLN infests hundreds of thousands of hectares in grazing, cropping and horticulture (dryland and irrigation) production areas across NSW, as well as adjoining states. Its main problem is its deep root system (four metres), difficulty to kill, and ability to use soil water that should be available to grow crops and pastures. Hanwen Wu details several control strategies, the best combination depending on issues like infestation level and cropping or pasture systems. SLN, like many perennial weeds, mainly grows from spring to late autumn and is dormant in most areas over winter. It flowers from November to April, and it is common to have several flowerings over summer and autumn. Control of SLN centres around eliminating seed set and weakening and finally killing adult plants. Control is related to SLN root system that uses stored energy to produce shoots in spring with levels depleted to a minimum at flowering. During seed formation levels rise again. At the end of the growing season carbohydrates are exported back to roots from stems. Control programs therefore generally need to be planned over at least a five to 10-year time frame for heavy infestations. One common control strategy advocated by the authors is to reduce the seed bank via winter cropping, combined with constant fallow herbicide control. After a cropping phase of three or more years, a competitive summer perennial grass, if well managed, will compete against any remaining parent SLN of reduced vigour or out compete any new plants. Effective fallow sprays include glyphosate, 2,4-D, fluroxypyr (e.g. Starane) and picloram (e.g. Tordon). Some of these are best used in combination to better control tops and achieve better root penetration. Rarely does one application kill the plants. It’s more a gradual reduction in plant vigour and plant numbers. Dr Wu and colleagues advocate a “dual action” approach to long-term control of silverleaf nightshade. Action one, in spring and summer, is aimed at stopping seeding when SLN plant is at early flowering stage. Options include knockdown herbicides, slashing, grazing, spray/graze or burning. Herbicides choice include glyphosate 2,4-D and fluroxypyr. While picloram can also be used its residual can impact on the following autumn and winter legume pasture component or in the case of crops species like pulses. Grazing should be avoided to minimise the spread when mature berries have formed. Action two is aimed at weakening the root system in late summer or autumn and relies on herbicides (knockdown for boom spraying and picloran for spot spraying). Herbicide choice is influenced by other weeds as well as likely following crops or legume pasture component. While listed as poisonous, appropriate grazing, including spray graze, has been successfully used as part of an integrated control program to prevent seeding. Glycoalkaloids produced by SLN may be broken down in the gut to form nerve toxins such as alkaloids. Sheep are more tolerant than cattle, and goats are reported as unaffected. Because spread is relatively slow, early weed identification is critical, as it can be possible to eliminate SLN before it gets away. Spread is via many methods including root fragments (cultivation), seed (especially livestock eating and passing seed), water, and via hay, grain, and native animals. Drought fodder is an important source to watch for. For spot treatment using picloram, it is important to spray the shoot and the soil for a two-metre radius around it. For information, visit dpi.nsw.gov.au/biosecurity/weeds/weed-control/management-guides/silverleaf-nightshade-best-practice-management-manual-2018 Next week: Maximise reliability of dual purpose crops.