trick or treat

Companion planting can help keep insect pests off your valued plants, says MARTYN ROBINSON

As southern gardeners start to think about planting their brussels sprouts and other brassicas, it pays to plan ahead and pop in some decoy plants. Nasturtiums fit the bill perfectly. Larvae of the cabbage white butterfly love to munch on brassicas, especially the tasty young seedlings, and are notorious for devouring them, but they will feed on nasturtiums instead if they are nearby.

Now, you might say, “But these plants look nothing alike, and butterflies have good vision, so why can’t they see the difference?” Well, butterflies find their food plants by scent, and these plants smell very similar. Nasturtiums belong to the brassica family, and the mustard oil they produce is more appealing to the butterflies than the same oil produced by cabbages and the like, so the butterflies are more inclined to lay their eggs there.

This is great for the vegie gardener but not so good if you want to grow a crop of nasturtiums. The peppery oil that these plants evolved to dissuade insects from eating them has made it easier for the few that do eat them to find them more easily… and it makes us want to eat them, too!

There are many other companion plants that can be used around the garden, reducing the number of pests or diseases that attack your more valued plants, or benefiting them in other ways. Marigolds and daisy bushes growing near roses attract ladybirds and hoverflies, which both feed on aphids, the bane of rose growers. Mint, plectranthus, abelia, buddleja and lavender attract native blue-banded bees, which do an excellent job of pollinating your tomatoes, chillies and eggplants.

Martyn gardens mainly on Sydney’s Northern Beaches