Could swan plants become a weed of the future?
Swan plants are essential to monarch butterflies — but they could also become a weed problem if we don’t grow them responsibly.
Dr Trevor James, a respected weed ecologist with the Bioeconomy Science Institute, has identified swan plants as a potential weed of the future. In some areas they are already naturalised, and their windborne seeds can travel well beyond our gardens.
What has changed?
In the past, monarch caterpillars provided remarkably effective natural control of swan plants. Anyone who has raised monarchs will remember caterpillars stripping plants bare — often leaving gardeners desperately searching for more swan plants! But monarch numbers are now low in many parts of NZ. One contributing factor is predation by introduced social wasps, which readily take monarch eggs and caterpillars.
Fewer caterpillars means less grazing of swan plants. More plants can mature, produce seed pods and spread their seeds on the wind. It is an unintended consequence — or collateral damage — of an ecosystem that has become unbalanced.
How swan plants spread
Swan plants (Gomphocarpus fruticosus and G. physocarpus) originate from Africa and have been in New Zealand for well over a century.
When a seed pod ripens, it splits open and releases numerous brown seeds. Each seed is attached to a tuft of silky white hairs called "coma", which acts like a tiny parachute and allows the seed to travel far and wide on the wind.
That means plants grown for monarchs in our gardens can potentially establish on roadsides, reserves, farmland and other places where they are not wanted.
Become a Responsible Swan Plant Grower
We can continue growing swan plants for monarchs while taking some simple steps to prevent their unwanted spread:
1. If you want to save your swan plant seed for next year, then the use of these seed bags (or similar) will ensure that the seed is at optimum ripeness when you collect it, and it won’t get carried away on the wind.
2. If you do not want the seed keep deadheading the plant(s) to encourage leaves (much preferred by monarchs) and prevent the plant from flowering OR going to seed.
3. Consider planting tropical milkweed (Asclepias curassavica) if you are in a warmer region, or swamp milkweed (A. incarnata) in cooler areas. Seeds for sale in our online Shop.
4. We discourage people scattering unwanted seeds on empty sites. I am guilty of having done this in the past, but the times have changed. I know that there are people who aren’t fond of the plant and until such time as we can control or reduce the number of pest wasps, it’s important that we be more responsible about the way we grow it on.
We can help monarchs AND prevent a future weed problem
There is no need to stop growing swan plants. They remain vital to the monarch butterfly because New Zealand has no native milkweeds on which monarch caterpillars can feed. What needs to change is how we grow them.
By controlling seed dispersal, supporting healthy monarch populations and becoming a Responsible Swan Plant Grower, we can provide food for one of NZ's best-loved butterflies without allowing its host plant to become a problem.
