In the pathway: Understanding companion planting, Managing pests naturally
Banker plant
A plant that sustains beneficial insects near the crop; by extension, in amateur gardening, a trap plant that attracts pests.
Explanation
Two distinct uses: (1) a trap plant that diverts pests (nasturtium for aphids), (2) a refuge plant that shelters beneficial insects outside the peak pest periods (nettle for ladybirds). A concept formalised in professional biological control (Frank 2010) before entering permaculture practice.
Refined definition
The term banker plant comes from biological control in greenhouses: a non-target plant continuously hosts a predatory or parasitoid beneficial insect, which can then reproduce and colonise the main crop as soon as a pest appears. In amateur permaculture, the term is extended to trap plants (which draw pests away from the crop).
Key points
Refuge plants (a reservoir of beneficials)
Nettle: hosts lacewings and ladybirds early in spring, before aphids arrive in the vegetable garden. Borage: nectar and pollen for bumblebees and bees. Yarrow and wild carrot (Apiaceae): shelter parasitoid ichneumons and trichogramma wasps. To plant along the edge of the vegetable garden or in islands.
Trap plants (draw pests away)
Nasturtium: attracts black aphids; watch it closely if broad beans or beans, which they also attack, grow nearby. Mustard: a trap for flea beetles on young cabbage plants. Rocket: a magnet for the mealy cabbage aphid and flea beetle. Once the pests are concentrated, you can either wait for the beneficials to arrive or remove the trap plant.
Practical limits
A trap plant that is too close can encourage infestation. Timing also matters: the nettle must already be hosting ladybirds when the aphids attack. Source: Frank S.D. (2010), “Biological control of arthropod pests using banker plant systems”, Biological Control.
Watch out for
- • Banker plants do not replace a healthy crop: without rotation, mulch and observation, the effect stays marginal.
- • Some trap plants can become invasive (nasturtium in particular in the south), harvest or mow before they set seed.
Sources
- Frank, S.D. (2010). Biological control of arthropod pests using banker plant systems: past progress and future directions. Biological Control, 52(1), 8-16.