Which Plants Repel Whitefly?

Whitefly is one of the most persistent pests in the greenhouse and, during warm summers, outdoors on brassicas and tomatoes. The tiny white insects congregate on the undersides of leaves, suck sap, excrete honeydew that causes sooty mould, and reproduce rapidly. In greenhouses the problem intensifies because natural predators are absent and conditions suit the pest perfectly. Companion planting in the greenhouse is an underused but effective part of whitefly management.

Basil — the Most Effective Whitefly Companion

Dense plantings of basil near tomatoes, peppers, and cucumbers in the greenhouse have been shown to reduce whitefly numbers significantly in several horticultural trials. Basil releases linalool, estragole, and other volatile compounds that interfere with the whitefly's ability to locate host plants. For greenhouse use the key is density — a few individual plants scattered around will have minimal effect, but dense clumps of basil every 50–60 cm along the growing row create a strong enough aromatic environment to make a real difference. Ocimum basilicum 'Genovese' and 'Red Rubin' both have good evidence for this effect.

French Marigolds in the Greenhouse

French marigolds interplanted with tomatoes in the greenhouse are a classic companion combination and one that specifically targets whitefly. The marigolds produce a root chemical called thiopene that repels whitefly from nearby plants. They also attract parasitic wasps (Encarsia formosa) that are commercially used as biological whitefly control. Growing marigolds throughout the greenhouse provides habitat for these parasitoids even when they have not been introduced commercially. Use Tagetes patula, not T. erecta, as it is the smaller species with better evidence for whitefly deterrence.

Nasturtiums as a Whitefly Trap Crop

Nasturtiums are strongly attractive to whitefly and can serve as a sacrificial trap crop in the greenhouse. Position nasturtiums away from the plants you want to protect — near the ventilation openings where incoming whitefly will encounter them first. Monitor the nasturtiums weekly and remove heavily infested plants before the whitefly population builds enough to migrate to your main crops. This is particularly effective early in the season before predator populations have built up.

Lemon Balm and Catnip as Aromatic Deterrents

Lemon balm and catnip both release lemon and camphor-like volatile compounds that whitefly finds unattractive. In a greenhouse with limited air circulation these volatile compounds accumulate to higher concentrations than outdoors, making aromatic companions more effective. Position pots of lemon balm near ventilation openings so the scent reaches incoming pests before they land on your crops. Catnip in flower is also attractive to predatory hoverflies that enter through ventilation openings and feed on whitefly nymphs.

Outdoor Brassica Whitefly — Thyme and Savory

Brassica whitefly outdoors responds to companion planting differently from greenhouse whitefly. Creeping thyme and summer savory planted around outdoor brassica beds reduce colonisation. The most effective outdoor approach combines aromatic companions with physical barriers — fine mesh over brassicas prevents whitefly from establishing in the first place, and companions reduce the pressure outside the mesh. Inspect the undersides of brassica leaves regularly from midsummer and remove infested leaves promptly.