Weeds, weeds everywhere!

Ask the owner of any home garden if they have weeds and a resounding, “Yes!” is guaranteed. If any council in Australia was asked if it had weeds in its LGA, there would be an equally strong affirmative. As a horticulturalist, Marc Worner might not consider their chosen plants fit the description.

The categorisation of a plant as a weed has no botanical significance. No matter what definition is used, weeds are plants whose undesirable qualities outweigh their good points – basically, any kind of plant growing where you don’t want it. Common characteristics of weedy species include aggressive growth, competition with other plants for light, water, nutrients and space, an ability to grow in a wide range of soils and adverse conditions, and resistance to control measures.

Weeds have a significant impact everywhere they grow, for a variety of reasons.

Weeds are basically any kind of plant growing where you don’t want it.

The NSW government’s Department of Primary Industry currently lists over 325 weeds, and each State body around Australia has its own list. Weeds are a major threat to the unique natural environment, jeopardising the survival of hundreds of native plants and animals. They also impact on the price of food, human health through allergies and asthma, recreational activities and the economy.

Weed control is carried out by home gardeners, primary producers, small landholders, plant retailers and local governments. In fact, legislation is in place across Australia and many other countries to make certain industries and landholders liable for the control of their weeds.

Common choices

A popular landscaping plant worldwide is Agapanthus praecox, subspecies orientalis, which originated in South Africa. It’s well adapted to our climate, and multiplies quickly to form large clumps of broad, strappy leaves. Every flower head produces dozens of long, black seeds that can easily spread into bushland via waterways. Gardeners aware of this choose to purchase sterile varieties from garden centres, and some States declare the original plant as a noxious weed.

English ivy is a fast-growing evergreen creeper that has long been popular for covering unattractive sheds and fences, but it can quickly get out of control. It spreads horizontally, creating new roots where it touches the ground, and can climb up and strangle trees. Ivy is very difficult to remove as its leaves are waxy and therefore resistant to most herbicides, and pulling out the vines often leaves small sections behind to regrow.


Non-chemical means should be the first consideration for small areas needing weed control:

* Pull up weeds by hand or use a mechanical device sold at all garden centres. Try the Fiskars weed puller. It’s simple to use, easy on the back and knees, and an ergonomic handle makes the job even more comfortable. See: fiskars.com.au/;

* Boiling water will severely burn and scorch weeds while killing them directly. Both Weedtechnics’ Satusteam™ and Weedingtech Foamstream offer great equipment for herbicide-free weed control in this area, they’re an excellent option for minimal environmental impact;

* Lemon juice contains high amounts of citric acid, an instant weed killer. The lemons can be either squeezed or sprayed directly onto the weed;

* Baking soda (sodium bicarbonate) can be used effectively to kill weeds naturally by drawing water from the weed’s cells and causing its leaves to fall off;

* Borax is easily absorbed by the leafy parts of the weed and kills it all the way down to its roots.

Foliar spraying is the use of herbicide diluted with water at a specific rate and sprayed over the foliage to the point of runoff – until every leaf is wetted, but not dripping. It’s suitable for shrubs, grasses and dense vines where complete coverage can be achieved, and advantages include speed of application and economy. Disadvantages are mostly the potential for spray drift and off-target damage. Foliar spraying using white vinegar with a 20% concentration of acetic acid works every time. It can be purchased online or at some garden centres (vinegar used in the kitchen has 5% concentration of acetic acid).

Foliar spraying is the use of herbicide diluted with water at a specific rate and sprayed over the foliage to the point of runoff.

TIP: To help the spray stick to weeds, mix the vinegar with two teaspoons of vinegar for every four litres of soapy water.

If the weeds are in a paved area, salt will do the trick. Remember, salt tends to stay in the soil for a while and can have long-term effects on healthy plant life, so it’s best used in paved areas only. Salt may not effect all weeds, but it can dry out some weeds faster than vinegar. You can add salt to the mixture that already has dishwater soap in it. Use cheap table salt or rock salt, Epsom salt (magnesium sulphate) or sea salt.

Salt will do the trick for weeds in a paved area.

These natural herbicides often contain acetic acid, citric acid, or oils such as clove oil, orange oil, or cinnamon oil. Much like homemade weed-control options, store-bought organic and natural herbicides are non-selective, so they will kill both desirable and undesirable plants if spray drift occurs or by human error.


A weed torch using LPG, propane or butane burns a weed directly and destroys its ability to reproduce. Torches are also a lot easier on the back than pulling weeds by hand. What the flame actually does is boil the water in the plant’s cells. The cells collapse and the weed dies. Weed torches only kill the part of the plant above ground, so perennial weeds can return. The solution is to hit them as soon as they reappear. You’ll need to do it several times, but with no leaves for photosynthesis, the roots will eventually die.

TIP: The flame from a weed torch can be almost invisible, particularly in bright sunlight. It’s good practice to have a fire extinguisher or bucket of water handy.

NB: There might still be times of the year in different States when use of a weed torch is prohibited.

CAUTION: You need to keep open flames far from desired plants, trees, any firewood storage area, dry vegetation, houses and other structures, and anything flammable.

Care is needed

Herbicides are commonly used for controlling weeds in agricultural and non-agricultural situations, and there are many types of equipment and techniques available for application.

Herbicides like glyphosate offer an easy way to treat some weeds, but they shouldn’t be used around waterways as they can kill frogs. Frogs have permeable skin which can easily absorb poisons that would have little or no effect on larger animals.

The appropriate option for herbicide use is determined by the size of the infestation, the available resources, access and personal preferences.

Weed control is carried out by home gardeners, primary producers, small landholders, plant retailers and local governments.
A good start

Preparation can help make an environment less suitable for weeds. Cultivate the area with a hoe on a hot, dry day and cover with cardboard or layers of wet newspaper. Then cover the area with an organic mulch like leaf mulch, sugar-cane mulch or compost to a depth of approximately 75mm.

Another option is to plant a dense groundcover perennial plant that will compete with weeds for sunlight and eventually block them out.

There are different ways to kill weeds depending on the type of plant. For instance, if it’s a small shrub or vine with aerial tubers, then stem-scraping can be used. A sharp knife is used to scrape a very thin layer of bark from a 15cm–30cm section of the stem. Herbicide is then immediately applied to the exposed soft underlying green tissue. With some woody weeds the bark can be peeled away, and the exposed wood painted or sprayed with herbicide.

Basal bark spraying is suitable for thin-barked woody weeds and undesirable trees, and is also an effective way to treat saplings, regrowth and multi-stemmed shrubs and weeds in inaccessible areas such as steep banks. It creates little or no spray drift or off-target damage, and will usually control difficult-to-kill weeds at any time of the year. If the bark is not wet or too thick, an oil-soluble herbicide is mixed with diesel and sprayed around the full circumference of the trunk or stem of the plant. The diesel helps move the herbicide through the bark and into the underground storage organs of the plant, slowly killing it. The whole circumference of the stem or trunk must be sprayed or painted with herbicide solution from ground level to a height of 30cm. It’s important to saturate the full circumference of the trunk, and to treat every stem or trunk rising from the ground.

Another commonly used method for the control of a weed infestation is by drilling into the sapwood tissue of the offending plant and pouring the herbicide directly into the hole.

TIP: Apply the herbicide within 15 seconds of drilling the hole, as stem injection relies on the active uptake and growth of the plant to move the chemical through its tissues.

There are options for weed control beyond the use of any natural methods and pesticides and herbicides.


Inaccessible areas such as timbered, rocky and steep locations, grazing areas with sensitive aquatic or environmental needs, and situations where chemical control may be too expensive or not effective must be dealt with differently.

Biological control (biocontrol) involves the introduction of natural enemies – insects, mites or pathogens – of a target weed that will reduce the density of the weed to an acceptable level and will maintain the weed density at that level. It is an economical, self-sustaining and environmentally friendly management technique. Biocontrol does not eradicate weeds, but can reduce populations to acceptable levels, or suppress them to levels where they can be controlled in combination with other methods.

When a biocontrol agent’s population establishes, control becomes self-perpetuating and self-regulating as the agent becomes part of the region’s ecology. The downside is successful programs may take more than 10 years to be effective, and results vary from area to area.

Aggressive growth is a common characteristic of weedy species.
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