Over the past 30 years Astron has been on many a front line in the war against environmental weeds in the Pilbara region of Western Australia. The battles have taken our weed control technicians to most areas of the region, including the major towns of Karratha, Port Hedland and Newman, to offshore islands such as Barrow Island, Thevenard Island, Airlie Island and Varanus Island, and to the vast majority of mine sites throughout the Pilbara region.
In the northwest of the State, weed control is usually undertaken over autumn and winter when rainfall promotes seed germination and growth. The commencement of weed control activities in the region is generally triggered by the first significant rainfall in the year. Sometimes this can be as early as Christmas if a tropical low or cyclone results in early season rainfall. However in the average year weed control activities usually commence around March and continue through to August or September when temperatures being to rise, rainfall abates and the drying soil conditions result in weed senescing.
For the effective management of weeds the timing of control is a critical consideration. It is important that resources and activities are deployed when the target species are actively growing but have not yet begun to set seed. For chemical control this promotes active uptake of the chemical and prevents the further spread of seed. In the northwest our experience tells us that chemical weed control should ideally be undertaken two to six weeks after germination, i.e. a significant rainfall event. Sunlight and soil temperature and of course the target species all play a big part in whether your weed control program needs to react faster to rainfall or can afford to react more slowly. For example, on the Islands of the northwest of our state we have witnessed Buffel grass go from germination to seed set in as little as 3 weeks after a summer rainfall event!
However, a key point for successful weed control is the ability to react to an event and being flexible in our deployment of resources. As a case in point, this year we have noticed that in some locations rainfall patterns have resulted in the need to continue and sustain weed control activities later in the year than usual.
|Roadside spraying of Buffel grass on a gas pipeline alignment in WA.||Buffel grass is a prolific seeder especially in good seasonal conditions.|
In Onslow, a small town situated on the coast, 1386 kilometres north of Perth, the significant winter rainfall that occurred in June and July (see graph below) has resulted in an extended weed season. Some environmental weeds (such as Buffel grass and Kapok) are expected to have a much longer growing season and because the rain occurred during the cooler winter months, the entire lifecycle will be slower than if the rainfall came in summer. Germination has been slower, flowering is only just commencing and seed set will arrive once the soil conditions dry out. Situations such as this require weed control activity to be conducted over a longer period of time or over successive control events to ensure that the control program is effective.
Favourable seasonal conditions such as what is being experienced over much of the northwest this year presents land managers with both an opportunity and a risk.
The timing and significance of the rainfall in Onslow means that good seed bank germination results in abundant weed growth and with slower growth due to cooler conditions, weed contractors are likely to have more time to control the target species. An effective control program during a year such as this will go a long way to depleting the weed seed bank in the soil and that’s a huge opportunity that’s not always apparent in the Pilbara.
On the flipside a season like this presents a big risk. Many populations will be left uncontrolled for a variety of reasons, whether it be lack of funding, lack of organisation, lack of professional advice or sometimes simply an ineffective strategy. Weeds will be left to grow unabated and significant volumes of seed will set into the environment, adding to the future pressure on biodiversity and adding significant future weed control costs for subsequent land managers.