Known weeds, and there are plenty of them, cost the Australian economy $4 billion a year. The additional cost of lost biodiversity and land degradation has not been quantified. Unfortunately, there are a further 5,907 foreign species which are in Australia, not yet weedy, but with a history of becoming weeds overseas. These are sometimes referred to as ‘sleeper weeds’ or ‘weeds in waiting’.
Some were introduced as far back as the 1920s and have not yet exploded into the environment. If, however, they are weedy in a country with similar climatic conditions, they are also likely to become weedy here, given the right conditions.
Clearly, not all of these are found in our region but there are some to watch and preferably no longer should be planted. The Weeds CRC (Cooperative Research Centre), now unfortunately closed, developed an Alert List for the highest risk species. It includes rosewood, Tipuana tipu, and Chinese rain tree, Koelreuteria elegans ssp. formosana. These are familiar street trees here and in Brisbane. They produce plenty of seed which germinates readily.
Most of us are aware of the weediness of the umbrella tree but Barung’s attention has been drawn to Syzygium wilsonii, powder puff lilypilly, growing in a patch of native forest. This Australian native is not indigenous to our region but hails from North Queensland. It’s pretty but out of place, and could compete successfully with local species. A monoculture of coffee growing in deep shade within natural forest was also noticed. The fruit is harvested from coffee grown as a managed crop but birds can spread it when it is left on garden plants. Management is the key. If exotic or non-indigenous native plants produce a lot of seed or propagate very easily from vegetative parts, they need to be very carefully managed, or not grown at all. Gardeners living next to bushland need to be especially vigilant.
There are many, many plants we can safely grow, in fact about 30,000 non-weedy species Australia wide. That’s probably enough!
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