Weeds and drought

Drought conditions impact heavily on primary production and an unfortunate side effect of a drought breaking is an increase in the presence of new and existing weeds on a property.

Drought prolongs the viability of weed seeds

Drought creates dry soil conditions that prolong the viability of weed seeds. The fungi and bacteria that break seeds down need moisture to function. In dry soil, weed seeds do not break down, and remain completely viable, as if they have been kept in a paper bag in the cupboard.

Table 1. Longevity of the seedbank of different weed species, assuming no replenishment.
Weed common name Botanical name Seed life
Annual ryegrass Lolium rigidum Short
Barnyard grass Echinochloa spp. Short
Bathurst burrXanthium spinosum Medium
Barley grass Hordeum leporinum Short
Black Bindweed/climbing buckwheat Fallopia convolvulus Medium
Bladder ketmia/wild cotton Hibiscus trionum Medium
Caltrop Tribulus terrestris Medium
Capeweed Arctotheca calendula Medium
Cobbler’s peg/farmer’s friend Bidens pilosa Short
Common heliotrope Heliotropium europaeum Long
Corn gromwell/white ironweed Lithospermum arvense Medium
Cowvine/peachvine Ipomea lonchophylla Medium
Deadnettle Lamium amplexicaule Medium
Dwarf amaranth Amaranthus macrocarpus Short
False castor oil/thornapples Datura spp. Medium
FleabaneConyza spp. Short
Fumitory Fumaria spp. Long
HorehoundMarrubium vulgare Long
Khaki weedAlternanthera pungens Short
Liverseed grass Urochloa panicoides Short
Melons Citrullus and Cucumis spp. Medium
Mexican poppyArgemone mexicana Medium
MintweedSalvia reflexa Medium
Noogoora burrXanthium pungens Medium
Paradoxa grass/phalaris Pharlaris paradoxa Short
Paterson’s curseEchium plantagineum Long
Pigweed Portulaca oleracea Medium
Prickly lettuce Lactuca serriola Short
Quena Solanum esuriale unknown
Saffron thistleCarthamus lanatus Long
Silverleaf nightshadeSolanum elaeagnifolium Long
Skeleton weed Chondrilla juncea Short
Sowthistle/milk thistle Sonchus oleraceus Short
Tarvine Boerhavia dominii Medium
Turnip weed Rapistrum rugosum Long
Variegated thistle Silybum marianum Long
Wild oats Avena spp. Short
Wild radishRhaphanus rhaphanistrum Long
Wireweed Polygonum aviculare Medium
Source: Tony Cook, NSW DPI.
Short-lived seeds (80 to 90% gone after one year, if no replenishment).
Moderate-lived seeds (50 to 80% gone after one year, if no replenishment).
Long-lived seeds (<50% gone after one year, if no replenishment).

Weeds take advantage of drought conditions

Drought can devastate existing vegetation, removing all competition for light, nutrients, moisture and space and affording quick establishment of weeds when conditions become favourable. Weeds will germinate from the seed bank immediately after rain occurs.

Drought also causes mineralisation of nitrogen in the soil, and newly germinated weeds take advantage of these nutrients.

After a drought, weeds that are already on a property may spread to new areas, and weed densities can increase. As well as this, new weeds may have been introduced.

Weeds can be introduced in drought feed

Drought often results in the importation of fodder and grain, and this can bring new weeds onto a property. The following weeds are most likely to cause concern:

  • common heliotrope (Heliotropium europaeum)
  • summer burrs – Bathurst and Noogoora burrs (Xanthium spp.)
  • Amaranthus spp.
  • caltrops (Tribulus terrestris)
  • thistles
  • panic grasses (Panicum spp.)
  • mintweed (Salvia reflexa)
  • Johnson grass (Sorghum halepense)
  • wireweed (Polygonum aviculare)

Some weeds of major concern are introduced when drought feed is brought in from neighbouring states. These include parthenium weed (Parthenium hysterophorus) from Queensland, and bifora (Bifora testiculata) and bedstraw/cleavers (Galium tricornutum) from South Australia (both widespread in the South Australian wheat belt). If grain is purchased from interstate it is important to check for any occurrences of these major weeds, especially in areas where stock have been fed.

If grain is imported from overseas there is a very high risk of introducing serious new weeds, such as kochia (Kochia scoparia) from the United States. By following the mitigation strategies below, this risk can be managed.

Other likely weed contaminants of both grain and fodder are silverleaf nightshade, wild radish (Raphanus raphanistrum) and spiny emex (Emex australis). These species are currently major weeds in NSW, but will spread as a consequence of drought feeding.

Herbicide resistant weed seed can be introduced in drought feed

There is further risk that contaminated fodder and grain could contain herbicide resistant weed seeds - mainly annual ryegrass. Herbicide resistance is widespread in the grain belts of South Australia and Western Australia – and large quantities of grain are often obtained from these areas when there are drought conditions in NSW. Once again, by following the mitigation strategies below, this risk can be managed.

Weeds can be introduced by livestock

New weeds can also be introduced when animals are brought in, either in a restocking program or returning from agistment. Studies have shown that up to 12% of weed seeds can pass through the guts of animals and remain viable.

Weeds can be introduced in contaminated crop seed and on machinery

Weeds can also be introduced to cropping land in contaminated seed at sowing time, and when weed seeds are spread on contract machinery – especially harvesters.

Livestock poisoning can occur after drought

After rainfall following drought conditions there can be an increase in livestock poisoning from weeds. Stock losses are attributed to direct plant poisoning and photosensitisation, with the main problems caused by:

  • Panic species (causing photosensitisation, mainly in sheep)
  • Amaranthus species (causing kidney failure in sheep and cattle), and
  • thistles (causing nitrate poisoning in sheep and cattle).

Cropping areas are susceptible to weeds after drought

In cropping areas the immediate problem is fallow weeds, which quickly rob the soil of both valuable nutrients and moisture. Fallow weeds can be controlled by a combination of cultivation and herbicide. The weeds likely to cause most concern are:

  • common heliotrope (Heliotropium europaeum)
  • summer burrs – Bathurst and Noogoora burrs (Xanthium spp.)
  • caltrops (Tribulus terrestris)
  • Amaranthus species
  • thistles
  • panic grasses (Panicum species)
  • mintweed (Salvia reflexa)
  • Johnson grass (Sorghum halepense), and
  • wireweed (Polygonum aviculare).

In pastoral and tableland areas, weeds that make remarkable recovery and spread include:

Weeds are a threat to drought recovery

Weeds provide an additional financial burden in both lost production and cost of control after a long period of low farm productivity. Drought conditions will often result in serious setbacks for long-term planning and for controlling pastoral and cropping weeds. It is important to maintain vigilance so that weeds do not become a serious threat to drought recovery on a property. An accurate assessment of the situation must form the basis of an effective weed control program after drought.

Weed mitigation following drought

Check fodder and grain

Obtain as much detail as possible about the source of the fodder or grain that is being brought onto the property. Fodder and grain from interstate and overseas pose greatest risks of introducing new weeds onto a property and into New South Wales.

Restrict feeding areas as much as possible

Carefully consider where the grain and fodder are to be fed. Feeding out areas should be restricted to make weeds easier to find and control. A small ‘sacrifice paddock’ may be the best option, located where regular checks can be made after each rain event. Flat, arable areas will allow easy access for most types of weed control (mechanical, chemical, biological or grazing management). When considering herbicide resistance, it is best to avoid feeding in or near cropping paddocks.

Monitor feeding areas

Pay careful attention to all feeding areas. Closely monitor all feeding areas for two years after a drought. Identify all weed outbreaks – even if it is only one plant.

Monitor water points

Native and feral animals may also be accessing livestock feed, so water points should also be monitored for two years after a drought.

Quarantine introduced stock

Livestock are excellent distributors of weed seeds. It is recommended to quarantine livestock for at least 2 weeks. This includes holding livestock returning from agistment. Check holding paddocks for the presence of weeds germinating.

Control weeds quickly after germination

When rain occurs and weeds have germinated, they need to be identified and controlled as soon as possible, and well before they are able to set seed. If possible, delay sowing crops for up to a week to allow the first germination of weeds to be fully controlled. Be aware that the vegetative parts of perennial weeds, such as silverleaf nightshade and blue heliotrope, need to be controlled as well.

Identify any new plants

The most important aspect of controlling newly introduced weeds is early detection and identification. Identification and control of weeds are important for two years after a drought . Keep a close watch for unknown plants and have them identified as soon as possible. Don’t let them establish and set seed. Contact your local council weeds officer for advice and assistance with identifying potential weeds.

Further information

For agronomy advice contact Local Land Services.
For weed identification and advice contact your local council weeds officer.

Acknowledgements

Authors: Bob Tounce, James Dellow, Andrew Storrie, Tony Cook
Editor: Elissa van Oosterhout

References

Primefact 430 (2007) What impact does drought have on weeds?, NSW Department of Primary Industries, Orange.
Primefact 372 (2007) Weed strategies following drought, fire and flood, NSW Department of Primary Industries, Orange.
Primefact 365 (2007) Weeds – a threat to drought recovery, NSW Department of Primary Industries, Orange.