How to deal with spring weeds before they take over your garden

As the southern hemisphere turns to face the sun, it is peak season for colourful wildflowers, exotic annuals and native shrubs.

Frustratingly for gardeners, it is also prime time for weeds.

Lawns, garden beds, and every nook and cranny with a patch of bare earth are relentlessly colonised and overrun by a familiar line-up of botanical pests. 

Some are relatively benign for gardeners and more of an environmental problem.

Others, like the dreaded bindii or bindi-eye, are a literal pain.

What weeds say about your soil

The adage suggests weeds offer a helpful insight into soil health. 

ABC Local Radio gardening expert Rob Pelletier from Heritage Fruit Trees said that was only partly true. 

"Certain weeds favour particular conditions, but they are not a reliable soil test and they are not necessarily 'repairing' the soil,"

Mr Pelletier said.

A better idea, he said, is to create the best possible growing conditions for the plants you want and avoid leaving bare soil for weeds to take hold.

Herbicides can control weeds, though many have side effects that make them less attractive for some gardeners.

Two non-chemical methods Mr Pelletier recommends involve sunlight, albeit in opposite ways.

Occultation involves covering weeds with an opaque object, such as a tarpaulin, to block the light they need to photosynthesise and grow. This is relatively gentle on soil biology. 

Solarisation, on the other hand, uses sunlight and clear plastic to heat moist soil. This has an even greater effect on weeds, weed seed, pathogens and soil organisms, as it heats the soil and cooks the weeds.

Beyond the garden fence

While weeds can be a constant irritant for gardeners, they become an even greater problem when they escape the garden fence.

Gazania, introduced as a pretty ornamental flower from South Africa, shows how a species can become an environmental scourge.

Environmental scientist Kate Blood, a former weeds management program manager with the Victorian government, says the damage they cause is immeasurable. 

"In native grasslands and woodlands, gazanias crowd out indigenous plants that provide essential habitat for wildlife,"

she said.

"They are also becoming a serious problem for farmers, spreading into paddocks and cropping land."

Researchers are searching for practical, affordable and effective ways to manage them. 

Ms Blood says weeds enter your garden or lawn in a variety of ways, but once they become weeds, little can be done.  

"Ever had a random new plant like ivy or lantana pop up in your garden? Chances are its seeds were carried there in bird droppings or arrived through one of the many other ways plants spread — on the wind, in water, on clothing, animals or vehicles," she said. 

"This is how invasive garden plants escape beyond the backyard, finding their way into bushland, waterways and native grasslands.

"What you plant in your garden can have a profound and lasting impact on nature around you, even if you live in a large town or city."

Ms Blood is the Weed Society of Victoria president. Her advice to reduce the spread of weeds in the first place is: 

• Replace invasive garden plants with safer alternatives.
• Select garden plants carefully, checking their weed potential before buying or planting.
• Dispose of garden waste responsibly to prevent invasive plants spreading beyond the garden fence.

"A beautiful garden shouldn't come at the expense of the natural environment around it," she said.

Colonisers, exploders, and persistent perennials

There is no one-size-fits-all solution for the multitude of invasive weeds that pop up during spring. 

Many could be classified into three groups, said Mr Pelletier, each with a different control strategy required. 

"You can look at colonisers which exploit bare or poor ground," he said. 

"Then there are annual weeds that suddenly explode into growth and seed.

"And there are your persistent perennials, which keep coming back from underground parts."

Colonisers

Colonisers are weeds that thrive where desirable plants are struggling, or the soil is bare.

Examples: Capeweed, dandelion, flat-weed, plantain, many thistles.

Control: Remove young plants, improve soil conditions, relieve compaction where needed, and keep soil covered with mulch, lawn or desirable plants.

Spring exploders

These are weeds that often germinate through winter, then grow and seed rapidly during spring.

Examples: Chickweed, cleavers/sticky weed, fumitory, bittercress, sow thistle and annual grasses.

Control: Hoe or pull them while small and, most importantly, before they seed. Mulch afterwards.

Persistent perennial weeds

These weeds survive through bulbs, rhizomes or deep roots, so simply removing the tops rarely works.

Examples: Soursob, onion weed, couch, bindweed and blackberry.

Control: Dig out the underground parts where practical, repeatedly remove regrowth, smother with an opaque tarp, solarise suitable areas, or use an appropriate systemic herbicide.