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Environmental weeds

James Beattie

James Beattie

May 29, 2012

I have always wished for a horticultural equivalent of the joke: Q: When is a cook bad? A: When he beats the eggs. I wish for this because gardening words are by their very nature only gentle or benign, even innocuous. We prune, pollard, dig, churn, plant, nurture, grow, irrigate and pick. We never slap or punch. Nor do we beat.

The words we associate with gardening are always positive and uplifting, or soul nourishing for some. One of the things we gardeners have a hard time with is discussing gardening in negative tones, because we don’t like to think we do any harm. Perhaps the biggest, unintentional negative that can result from gardening is the spread of weeds. Having a joke to lead into a discussion of weeds would be very handy, so if there is a gardening joke out there that reflects this dark side of our craft I’d love to hear it.

Polygala myrtifolia (myrtle leaf milkwort) – a pretty flowering shrub in a suburban garden but an environmental thug when it escapes to coastal areas

I’ve spent the last few weeks working on Phillip Island doing a bit of weed control and ecosystem assessment. Working on Observation Point on the Rhyll Inlet is one of the best offices in the world, but it’s a pity about some of the company. Myrtle leaf milkwort (Polygala myrtifolia) has set up shop there in the past and has flourished to the point of what is now a major infestation. In some areas the only local plants to survive the onslaught of polygala have been the very old silver banksias (Banksia marginata), which stand as pillars of resistance from the monoculture of milkwort below. The experience got me thinking about how gardeners not so much consider weeds but the notion weediness.

I suspect everyone grows a plant that is either an already acknowledged environmental weed (the popular arum lily, Zantedeschia aetheopica, for example) or has the potential to cause weedy infestations if they get away form us. I myself am having a love affair with the genus Oxalisat present, a species of which, commonly referred to as soursob, is probably one of the most hated garden weeds of all time. The genus

An infestation of Polygala at Phillip Island before spraying

Oxalis is a large one, with many redeeming members that get overshadowed by the black sheep of the family. That being said, some of the species I am growing do have the potential to spread in the right conditions, but I have taken precautions to prevent this. Apart from eternal vigilance, the best way to combat the potential weediness of plants we might grow is to know them intimately. Knowing what conditions a plant thrives in as well as information such as the main method of spread are important in keeping them in check (as the saying goes, one year to seed, seven to weed).

The same area of Polygala after spraying

The species of polygala we’re dealing with on Phillip Island is a common garden plant around Melbourne. It will grow without any supplementary irrigation in most environments and flowers most of the year, producing seed in abundance. There is nothing intrinsically wrong with the plant, after all it’s just doing its thing. Where this milkwort crosses the boundary from pleasant plant to wicked weed depends very much on the location of the planting. Polygala likes sandy soils and will quickly spread in coastal ecosystems if left unchecked. Birds will eat the seed it produces, so gardeners should keep in mind that the potential spread of some plants may not be limited to the yards in which they grow. Birds can carry seed far and wide, but growers of polygala in the city areas need not worry too much about this – as with most weeds in bushland areas, residents bordering or close by these habitats that need to be the most vigilant.

In some areas, the old silver banksias, Banksia marginata, are the only plants to withstand the Polygala onslaught

One question I find difficult to answer is how vigilant must gardeners be with the potentially weediness of plants they grow? There are so many factors to take into consideration that a simple, succinct answer is very hard to give. I think most gardeners who know their plants will be very mindful of weediness no matter where they live. Gardening novices, on the other hand, are more likely to be unaware of some plants’ weed potential, so I think education is part of the solution. It is difficult to educate the public on what plant species do pose a greater risk of spread when so many declared invasive species can still be purchased in retail outlets. Montpellier broom, Genista monspessulana, is a terribly invasive weed that you can still buy in many nurseries. Ditto for ivy (Hedera helix) and honeysuckle (Lonicera japonica). This then poses the often divisive question, is banning some plants part of the solution?

With that can of worm juice now well and truly open, perhaps we should all ask ourselves, what does weediness mean to me?

Until next time, happy gardening.


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