Mark Humphrey, of Whanganui, asks :-

I have recently removed a heap of mature agapanthus clumps and found no animal life in, on or under the plants. Are the plants noxious to the usual insects etc that attack my garden vegetables? I soaked some leaves in water and tried the infusion on a green caterpillar but it appeared not to affect it in any way. If the plant is noxious to our garden pests, could the vital chemical be used as a natural control? I am aware that South Africa hosts an insect that feeds on agapanthus.

Lynley Hayes, a weed biocontrol expert with Landcare Research, responded.

Numerous cultivars of agapanthus are available. The large-growing form Agapanthus praecox subsp. orientalis is becoming a weed here of increasing concern for some regional authorities and environmental organisations, spreading readily to areas where it is not wanted and displacing other plants.

You comment that recently you removed some mature agapanthus clumps and observed no animal life in, on or under the plants. Agapanthus is reported to contain compounds known as saponins (which typically foam when added to water like soap), as do many other plant species. Saponins are reported to be toxic to cold-blooded organisms and insects at particular concentrations. Saponins also act as a repellant so this could explain your observed lack of insect life under the clumps. Pest control products containing saponins have been developed overseas, but I am not aware of any available in New Zealand. However, many home gardeners make up their own soapy water sprays against insects like aphids.

Some insects attack agapanthus in South Africa, where this plant is native. Like all plants agapanthus has its own natural enemies that have co-evolved with it over thousands of years and can therefore tolerate its chemical defences. These natural enemies could be researched as a means of controlling agapanthus in New Zealand in the future if it becomes a more serious problem.

In the meantime scientists at Landcare Research are working to develop sterile “eco-friendly” cultivars. They have successfully developed some sterile cultivars which are currently being tested by the nursery and garden industry, and it is hoped that soon it will be possible to enjoy this popular garden plant without harming the environment.