Which Plants Should Never Be Grown Together?

Not all plant pairings are beneficial — some combinations actively harm one or both plants. The mechanisms vary: some plants exude chemicals from their roots that inhibit the growth of neighbours, some share the same pests or diseases, and some simply compete so strongly for the same resources that neither thrives. Avoiding these bad combinations is as important as seeking out the good ones, and in a small productive garden where every square metre matters, getting this right directly affects what you harvest.

Allelopathic plants

Allelopathy is the suppression of one plant's growth by chemicals released by another — through root exudates, leaf litter, or volatile compounds. Fennel is the most well-known example in the vegetable garden, inhibiting the growth of tomatoes, peppers, beans, and most other crops. Black walnut (Juglans nigra) produces juglone, which suppresses a wide range of plants within its root zone. Some plants have milder allelopathic effects that only cause problems in very close proximity.

Onions and beans

Onions, garlic, leeks, and other alliums are often listed as poor companions for beans (both French and runner beans). The traditional explanation is that alliums inhibit the nitrogen-fixing bacteria on bean roots. The evidence for this is mixed, but if you notice underperforming beans near onions, separating them in your rotation is a simple solution. Conversely, beans are often listed as beneficial near other crops, so use this space for better pairings.

Brassicas and strawberries

Brassicas (cabbages, broccoli, Brussels sprouts) and strawberries are a poor pairing — they compete strongly for nutrients and moisture, and strawberries are said to inhibit the growth of brassicas. More practically, they have very different nutrient and care requirements that make them awkward to grow together effectively. Brassicas prefer firm, consolidated soil while strawberries prefer lighter, more open conditions.

Potatoes and tomatoes

Both are members of the Solanaceae family and share several serious diseases — particularly potato blight (Phytophthora infestans), which affects both crops. Growing them together or in close proximity means that blight on the potatoes quickly spreads to the tomatoes and vice versa. Keep them well separated in the garden and rotate them to different areas each season.