Companion Planting – Which Vegetables Help Each Other and Which Don't Get Along?
Not all vegetables make good neighbours. Some repel each other's pests, others compete for the same nutrients or release substances that inhibit their neighbours' growth. Good companion planting is free support for your plants – bad planting choices cause problems you won't understand all summer.
What's actually going on with companion planting?
One plant's influence on another can work through several mechanisms:
- Allelopathy – releasing chemical compounds into the soil or air that inhibit or stimulate neighbouring plants' growth
- Pest deterrence – strong scents from certain plants confuse insects searching for their host
- Attracting beneficial insects – flowering herbs and vegetables draw in pollinators and aphid predators
- Physical improvements – taller plants provide shade for lower heat-sensitive ones; low-growing plants cover the ground like living mulch
- Soil improvement – legumes fix atmospheric nitrogen that neighbouring plants can use
Proven good combinations
Tomato + basil
The classic. Basil releases volatile oils that may repel aphids and spider mites. In practice: plant basil directly beside tomatoes or in the same container.
Carrot + onion
Based on mutual scent masking. The carrot fly locates its host by smell – onion masks it. The onion fly is in turn confused by the scent of carrot. The effect works better when plants are intermixed rather than in separate rows.
The Three Sisters (sweetcorn + beans + squash)
A traditional Native American growing system. Sweetcorn provides a pole for the beans, beans fix nitrogen for both sweetcorn and squash, and squash covers the ground with its leaves, suppressing weeds and retaining moisture. Needs space, but works as a closed ecosystem.
Lettuce + tomato / cucumber
Lettuce appreciates shade in summer heat – taller plants provide natural protection. Lettuce in turn covers the ground beneath, reducing evaporation and suppressing weeds. Works very well in warm summers.
French marigold + vegetables
Marigolds release thiophenes – compounds toxic to soil nematodes. Studies confirm the effect when planted near nematode-susceptible crops (tomatoes, potatoes). The roots act more powerfully than the above-ground part – plant directly between vegetables, not just around the border.
Bad neighbours to avoid
Onion / garlic + beans, peas
The allyl compounds released by onion and garlic inhibit legume growth. Keep them well apart.
Dill + carrot, tomato
Dill releases allelopathic compounds that inhibit carrot germination and growth. It also cross-pollinates with tomatoes when allowed to flower. Sow dill away from these crops, or harvest it before it flowers.
Brassicas + strawberries
Brassicas and strawberries compete for similar nutrients and mutually inhibit each other's growth. Widely reported by gardeners, though the mechanism isn't fully explained.
How to check this when planning?
In a larger garden, remembering all the compatibility rules is impossible. Gardener Planner takes plant compatibility into account when you lay out your beds – after placing vegetables on your plan you can see whether a pairing is beneficial, neutral or problematic, without memorising tables.
A word of caution
Some traditional companion planting advice is poorly documented scientifically – effects are often subtle and depend heavily on conditions. Treat plant partnerships as a bonus, not a guarantee. Good watering, healthy soil and the right variety choices matter far more than perfect companionship.
Check your plant pairings in the app
Gardener Planner shows you which vegetables work well as neighbours – right when you're laying out your beds.
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