Isolation
distances

Two varieties of the same species flowering at the same time will cross. The distance needed to prevent it depends on how the pollen travels.

Self-pollinating crops

Lettuce, tomatoes, peppers and beans pollinate themselves before the flower even opens. Isolation of three to ten metres is usually enough, and often none is needed if you grow one variety per species.

Insect-pollinated crops

Brassicas, carrots, onions, basil and coriander rely on insects. Bees routinely forage 500 metres and further, so the honest figure for brassicas is 800 metres to a mile. In a small garden, the practical answer is to grow only one variety of each species for seed per year.

Wind-pollinated crops

Spinach, beetroot, chard and corn release pollen into the air. Corn is the extreme case: isolation is measured in fields, typically 400 metres to 1.6 kilometres, and home gardeners achieve it with timing rather than distance — sow so varieties tassel three weeks apart.

The time trick

Where distance is impossible, use time. Two varieties of the same species can be grown for seed in the same garden if their flowering periods do not overlap. That is how most small-scale seed savers manage brassicas.