What Not to Plant Together: The Complete Incompatibility Guide

Most companion planting guides focus on what to plant together. Equally important — and less frequently addressed — is what not to plant together. Some plant combinations are actively harmful: one plant inhibits the other through chemical compounds in the soil, shared diseases create cross-infection risk, or competing demands deplete the same resources. Getting these separations right can prevent some of the most frustrating and inexplicable garden failures.

This guide focuses on incompatibilities where the evidence is strongest, explains the mechanism behind each, and gives practical separation distances.

Level 1: Evidence-Backed Incompatibilities (Avoid These)

Garlic and Onions with Beans and Peas

Mechanism: Allicin and related sulphur compounds released by alliums inhibit the Rhizobium bacteria that colonise legume root nodules. Without healthy Rhizobium populations, beans and peas cannot fix atmospheric nitrogen — their most valuable contribution to soil health. A bean planted next to garlic is nutritionally deprived and fails to enrich the soil for subsequent crops.

Separation: Keep at least 60 cm between alliums (garlic, onions, shallots, leeks) and any legume (beans, peas, lentils, clover).

Note: This is a frequently violated pairing in mixed garden beds. If you are growing beans in the same raised bed as alliums, the nitrogen-fixation benefit of the beans is compromised even if both crops grow adequately.

Tomatoes and Potatoes

Mechanism: Both belong to Solanaceae and share Phytophthora infestans — the oomycete responsible for late blight. Late blight spreads via airborne sporangia that can travel significant distances. A single infected potato plant can infect tomatoes several metres away under the right humidity conditions.

Separation: Grow tomatoes and potatoes in different parts of the garden, separated by at least 3–5 metres. In small gardens where this is not possible, the rotation strategy matters more — never grow either in the same soil two years running.

Applies also to: Potatoes near aubergines and peppers (same family, same blight susceptibility). Tomatoes near aubergines and peppers can stay together as the disease dynamics are the same between them, but the collective Solanaceae patch should be moved each year.

French Marigolds — Wrong Type Near Lettuce and Brassica Seedlings

Mature French marigolds are good companions for many crops, but their root exudates that kill nematodes can also inhibit germination of lettuce and some small-seeded crops if planted too densely in the same immediate area. Do not sow lettuce or other fine seeds directly next to established marigold roots — transplant seedlings instead, which are past the vulnerable germination stage.

Level 2: Strong Traditional Evidence (Take Seriously)

Fennel with Almost Everything

Mechanism: Fennel releases allelopathic compounds including anethole and fenchone from its roots and decaying foliage. These inhibit the germination and growth of a very wide range of plants. Affected species include tomatoes, peppers, basil, beans, brassicas, and many herbs.

Separation: Grow fennel at least 1.5 metres from all vegetable crops, or ideally in its own isolated container. Fennel also self-seeds aggressively and hybridises readily with dill — if you grow both, keep them separated or you will lose the distinct flavour profiles of both.

Exception: Dill is also in the same family (Apiaceae) as fennel but the allelopathic effect between them is bidirectional — they suppress each other and cross-pollinate. Keep them well separated.

Brassicas with Tomatoes

Mechanism: Both are heavy nitrogen feeders. In the same bed they compete directly for the same nutrients. Root exudate interaction between the two families may also be inhibitory, though the research is less definitive. Additionally, brassicas attract caterpillar pests that can expand to tomatoes when their host plant is stripped.

Separation: Separate beds whenever possible. If you must grow both in a small space, ensure excellent soil amendment with compost to reduce competition, and keep at least 60–90 cm between the two plantings.

Sunflowers with Potatoes

Mechanism: Sunflower root exudates are allelopathic to potatoes — they inhibit potato growth and yield. Sunflowers are also broadly allelopathic to several other crops and should be treated similarly to fennel: beautiful in the right place but best kept to their own section of the garden rather than interplanted throughout vegetable beds.

Separation: At least 1.5 metres from potatoes. Sunflowers as standalone plants at the garden's edge, away from vegetable beds, avoid most allelopathic conflicts while still providing pollinator benefits.

Level 3: Black Walnut — A Category of Its Own

Black walnut trees (Juglans nigra) deserve their own section because their incompatibility is more severe, wider-ranging, and persistent than any other commonly encountered garden plant.

Mechanism: Black walnut produces juglone (5-hydroxy-alphanaphthoquinone), a potent allelopathic toxin. Juglone is released from all parts of the tree — roots, leaves, nut hulls, bark — and persists in the soil for years after a tree is removed. It is toxic to a wide range of plants including tomatoes, peppers, potatoes, apples, blueberries, rhododendrons, azaleas, and many others.

Separation: Keep vegetable gardens at least 15 metres from any black walnut tree. This represents the typical root spread radius of a mature tree. Even fallen leaves allowed to decompose on garden soil introduce juglone.

Resistant plants: Some species are tolerant of juglone including beets, beans (interestingly), carrots, corn, squash, most grasses, and many established native trees. When gardening near an unavoidable black walnut, restrict plantings to known-tolerant species.

After tree removal: Juglone persists in the soil from decomposing roots for 2–5 years after a black walnut is removed. Do not plant sensitive species in former black walnut areas until the residual root material has fully decomposed.

The Most Problematic Incompatible Pairs: Quick Reference

What the Evidence Does Not Support

Some incompatibility claims in garden literature are folklore without scientific basis:

• "Cucumbers and tomatoes should never be together": No strong evidence exists for this. The two have different pest complexes and different water needs (tomatoes prefer drying between waterings more than cucumbers), but no chemical incompatibility.
• "Basil and sage are incompatible": Widely repeated but weakly evidenced. The practical issue is their different water needs, not allelopathic inhibition.
• "Peppers and fennel": True — but the problem is fennel's general allelopathy affecting peppers, the same as it affects tomatoes, not a specific pepper-fennel interaction.

When in doubt about a claimed incompatibility: search for experimental evidence rather than garden tradition. Many incompatibility claims trace back to one source repeated across gardening books without original research.