Pest Repelling Plants for UK Greenhouses: Research-Backed Companion Pairs
Pest-repelling plants can be a useful addition to a greenhouse, but they are not pest-proofing. French marigolds have shown a benefit against whitefly when grown alongside tomatoes from the start. Other companion plants may provide flowers for beneficial insects or simply earn their space as herbs. None should be promised to clear an established infestation.
Key Takeaways
- Use companion planting as support, not a cure. Keep inspecting crops and deal with pests you find.
- French marigolds are worth considering with tomatoes. Research supports a preventative role against glasshouse whitefly, not a guaranteed result.
- Flowers can support beneficial insects. That is different from every plant directly repelling pests.
- Keep the greenhouse usable. Do not crowd roots, block paths or obstruct ventilation.
Expert Tip
Choose one or two companions you actually want to grow. A few marigold pots beside tomatoes are easier to manage than a crowded collection of supposed pest remedies. Keep enough space to turn a leaf over, water properly and remove a troubled plant.
What can companion planting realistically do?
Different planting combinations are proposed for different reasons. A plant’s scent may affect how a pest finds a crop, flowers may provide food for beneficial insects, and a deliberately chosen trap crop may attract a particular pest. These are not interchangeable effects.
A companion can also be worthwhile without a pest-control claim. Basil gives you a useful crop beside tomatoes; flowers add colour around the greenhouse. Choose plants that suit the space and conditions rather than expecting every fragrant leaf to repel insects.
Some combinations, including alyssum with lettuce, have been studied. Other pairings are grown in the hope of similar effects. Treat companion planting as a supporting measure, not a promise of pest control.
French marigolds with tomatoes: the strongest example here
French marigolds (Tagetes patula) are a reasonable choice if you want to try companion planting around greenhouse tomatoes. A published glasshouse study found that growing them alongside tomatoes from the beginning slowed whitefly population development.
The important limitation is timing. Adding marigolds after a heavy whitefly population had developed had little effect in the study. A pot of marigolds is therefore better considered as a companion from the start than as a rescue treatment.
Use manageable pots beside the crop or plant where there is enough room for both. There is no universal one-marigold-to-two-tomatoes recipe to promise. Keep the flowers healthy and the tomatoes accessible, and inspect the undersides of leaves as usual.
Tagetes and root-knot nematodes: a separate question
Marigold research on soil nematodes is not the same as the whitefly result above. Effects depend on the marigold variety, the nematode involved and growing conditions. Planting a few flowers beside a tomato is not equivalent to a carefully managed soil treatment.
If roots are swollen or crops repeatedly struggle in a bed, get the problem identified before choosing a remedy. Do not assume that digging in Tagetes will cure it or protect the soil for several years. The research on marigolds, temperature and root-knot nematodes is more specific than that.
Nasturtiums: useful flowers, but watch the aphids
Nasturtiums are easy to enjoy around a vegetable plot, and gardeners sometimes use them as a trap crop. However, an aphid-covered nasturtium does not prove that nearby vegetables have been protected. Different aphids have different host preferences.
If you try them, put them somewhere you can inspect without reaching through your main crop. Check both the nasturtiums and the vegetables. Deal with unwanted build-up rather than leaving a heavily affected plant because it is supposedly “doing its job”. There is no reliable universal upwind distance that makes this work.
Basil: grow it because you will use it
Basil and tomatoes make good use of a greenhouse when you leave room for each. A pot is handy because you can move it if the tomato canopy starts to shade it too heavily. Give it the growing conditions it needs and harvest it for the kitchen.
Do not rely on basil to reduce aphids or thrips by a fixed percentage. Nor should planting it nearby be promised to change the flavour of the tomatoes. You can enjoy them together on the plate without needing either claim.
Garlic chives, borage and summer savory
Garlic chives
Grow garlic chives as a useful herb if they suit your plans. Pots make them easier to position, but a row by the door is not a dependable barrier that keeps cabbage moths, carrot fly or other pests outside. Check the needs of the crop you want to protect instead.
Borage
Borage is a flowering option for a sunny border near the greenhouse. It can grow large and self-seed, so give it space where it will not lean across the entrance. Flowers for visiting insects are welcome, but they cannot guarantee that predators will arrive in sufficient numbers to clear aphids inside.
Summer savory
Summer savory is traditionally paired with beans and is worth growing if you enjoy using it. Treat claims that it reliably repels bean aphids with caution. Pick it for the kitchen, and keep checking your beans for pests as usual.
What to do when pests are already present
First identify the pest and check how widely it has spread. Look at new growth and leaf undersides, not just the first damaged leaf. Keep checking nearby crops and isolate a newly bought affected plant where practical.
Choose a response that fits the pest, crop and conditions. That may mean removing a small affected shoot, using an appropriate physical control, or asking a biological-control supplier about a suitable product. Follow that supplier’s temperature, timing and compatibility instructions. Buying a beneficial insect is not a guarantee either.
If you decide a pesticide is needed, check that the product is authorised for the intended use and follow its label. Do not substitute a home-made garlic or essential-oil spray for a crop-safe, properly labelled treatment. The HSE’s garden pesticide guidance explains what to check.
A manageable companion-planting layout
Keep your main crop’s needs first. In a small greenhouse, companions in a few movable pots may be more practical than filling every gap in a bed.
- Beside tomatoes: try French marigolds without crowding the stems or blocking watering access.
- In a reachable sunny spot: keep a basil pot if you will use it and can give it enough light.
- Outside the entrance: choose flowers that fit the border, leaving the doorway and vents clear.
- On your regular rounds: check companions as well as crops for pests and remove dead material.
Sow or buy plants to suit their individual instructions and your local conditions. Do not plant tender basil or tomatoes into a cold greenhouse just because a calendar says May. If growing companions from seed, allow enough time for usable plants without rushing them into unsuitable temperatures.
Our greenhouse companion-planting guide covers the broader question of sharing space. For the main crop, start with our greenhouse tomato guide.
Frequently asked questions
What plants repel pests in a greenhouse UK?
French marigolds have useful evidence against whitefly when grown alongside tomatoes. Other companions may support beneficial insects or provide useful herbs. None makes a greenhouse pest-proof.
Do marigolds really keep pests away from tomatoes?
They can help with whitefly prevention, but results are not guaranteed. Research found a benefit when marigolds grew alongside tomatoes from the start, not as a reliable rescue for heavy infestation.
What is the best trap crop for aphids in a UK garden?
There is no single best trap crop for every aphid and vegetable. Nasturtiums are sometimes tried, but monitor both the companion and the crop rather than assuming nearby plants are protected.
Does basil really improve tomato flavour?
Do not expect nearby basil to change your tomatoes’ flavour. Grow it as a useful herb and give both crops enough space. A pest-control benefit should not be promised either.
What kills root-knot nematodes in greenhouse soil?
Identify the nematode problem before choosing a treatment. Some marigold approaches suppress particular nematodes under specific conditions. A few companion plants are not a universal soil cure.
Can companion planting replace pesticides in a UK greenhouse?
Companion planting cannot replace dealing with an established pest problem. It may support prevention, but inspect crops and select appropriate controls when needed; pesticides are not automatically necessary.
When should I sow companion plants for the UK greenhouse?
Follow each plant’s sowing instructions and your local growing conditions. Allow time to establish companions alongside the crop, but do not move tender plants into unsuitable cold conditions.
Do garlic sprays work as well as companion plants?
Neither should be treated as a guaranteed pest treatment. Avoid home-made spray recipes. Identify the pest and, if using a pesticide, choose an authorised product and follow its label.
