Natural disasters, such as bushfires, give weeds a boost.

Drought, flood and fire may be the ruin of rural life, and cyclones the curse of coastal land and other low-lying communities, but if you’re a weed you love ‘em, says Dr Rachel McFadyen, CEO of the Weeds CRC.

Commenting on the recent floods in northern NSW and central Queensland, Dr McFadyen says that while our agricultural systems, and our native plants and animals, take a hammering from these extreme weather events, they give weeds and other biological invaders the perfect leg up.

Alarmingly, says Dr McFadyen, extreme weather events are likely to become more common as climate change progresses. But for many weeds, that’s great news.

‘In the tropics, cyclones often break open the canopy of a forest and let in new light to the normally dim forest floor – that’s just perfect for weeds.’

‘For thousands of years the local native plants have supplied the only seed available. This meant, given time, that the forest always returned to near-original condition.’

But now, says Dr McFadyen, thanks to people and vehicles, there are often seeds of new foreign species lying around in small numbers ready to out-compete the local plants in the grab for space and light. Birds and animals may also bring in these new  seeds from towns and farmland.

‘Suddenly’, says Dr McFadyen, ‘the invaders get the break they need’.

Furthermore, some weeds are quite capable of keeping the canopy open (e.g. African tulip tree in the far north, madeira vine and cat’s claw creeper in SE Qld, privet in NSW) by smothering the regrowth of native trees. The new conditions with high light levels suit the weeds, and they consolidate their position.

‘With each storm or cyclone event the invaders ratchet up their position’, says Dr McFadyen, ‘and the forest is prevented from repairing itself’.

Fire risk is also predicted to increase greatly under climate change scenarios, and favours weeds by ‘clearing the decks’ of  competitors, says Dr McFadyen. And while fires may burn up many tons of weed seed, and favour the germination of some fire-adapted native species, it also provides the perfect seed bed for weed seeds.

‘If you’re a weed with seed that is blown around by the wind, or have fruit that birds spread, for example, then a bushfire can be good news’, says Dr McFadyen.

‘Chances are that your seeds will be the first to arrive. Your seedlings will be first up when the rain comes, and will have set seed themselves long before any native plants recolonise the burnt ground.’

Some weed species, such as the 4m-tall African gamba grass now rapidly spreading in northern Australia, go even further by fuelling fires so hot that even normally fire-resistant eucalypts are killed. Native eucalypt woodlands, with their many animals and birds, are changed into a pure stand of gamba grass with much lower biodiversity.

‘These new grasslands might be good for elephants’, says Dr McFadyen, ‘but are pretty useless for most Australian animals and birds.’

Drought also tips the scales in favour of weeds. If climate change does result in more frequent and more severe droughts, then weeds will again be the first to benefit.

‘If the drought kills off all the competitors, whether these are crops or pasture grasses, then when the rains finally come the weeds will do what they do best – invade bare ground and grab all the moisture!’, says Dr McFadyen.

‘Those invaders able to cope with drier conditions, and able to quickly move their seed around, (eg parthenium weed, or serrated tussock) will be the weeds we’ll be battling tomorrow’, she says.

Finally, floods can help weeds get a toehold in conditions they like. Climate scientists are predicting more floods – as we are already seeing in the north - as rainfall conditions become more variable. Once again, this will suit the weeds nicely.

According to Dr McFadyen, Mimosa pigra in the NT didn’t really get out of hand until major floods in the 1970s spread the seed from a small initial patch out over the wide flood plain of the Adelaide River. The same happened with Athel pine in the river channels of central Australia, and even prickly acacia in north Queensland.

Any weed that is spread effectively by water is a candidate for trouble in areas where floods become more common, says Dr McFadyen. Examples include willows, giant reed, alligator weed, cabomba and lippia.

'Whether it’s cyclones, fire, drought or flood, weeds are set to become the big winners from climate change’, she says. ‘Extreme vigilance and early action by landholders, agencies and individuals are the keys to keeping these species under control.’

‘Changing climate is new territory for farmers and other land managers, and they will need all the information and understanding that science can provide if they are to stay ahead in the weed war’, says Dr McFadyen.

It is critical that public and private stakeholders join forces and invest wisely in building a new knowledge base, she says. ‘This is the only way we will equip ourselves to meet the new weed challenge now emerging from climate change.’