How to Keep Vegetables From Cross-Pollinating

The recent article Gardening Myth: If You Plant Hot Peppers and Sweet Peppers Together, It Will Alter Their Taste, which discussed what happens when you grow hot peppers and sweet peppers together, led to many questions, particularly about the isolation distances needed between various crops to prevent undesirable cross-pollination. If you want to harvest vegetable seeds, you shouldn’t let pollen from one variety fertilize another, otherwise, when the seeds germinate, you’ll have a hybrid vegetable on your hands, one mixing the traits of both its parents instead of the desired variety.

Of course, cross-pollination is of no importance if you don’t intend to save the plant’s seeds. Cross-pollination doesn’t change the taste, shape, color or other aspects of the vegetable of the current season, so if all you’re doing is growing veggies to put to feed your family, there’s no need to be concerned about the effects of cross-pollination… with a few exceptions.

It’s when you want to harvest seeds to sow in future gardens that the purity of the variety is important. Your goal then is simple: you want to prevent the pollen from one variety from fertilizing the flowers of another variety. As they say in the gardening world, you want to keep your variety “true-to-type.”

The Exceptions

If cultivating two varieties of the same vegetable nearby doesn’t normally alter its taste and shape the first year in the vast majority of cases, there are two major exceptions:

Sweet corn (maize) undergoes a double fertilization: the pollen carried by the wind fertilizes both the ovule that will give a new plant, but also the endosperm, the grain that we eat. So yes, the taste and even the color and texture of the grain can be negatively affected by the presence of another corn variety nearby. Read Grow Sweet Corn in Isolation to better to understand this unique situation.

The other common exception is the English cucumber (greenhouse cucumber). It is parthenocarpic, that is to say, it produces fruits without being fertilized (that’s why its seeds never fully develop). But if it is pollinated by a normal cucumber nearby, it will produce bumpy, irregular fruits with seeds. That’s why it is usually grown in a greenhouse to avoid cross-pollination. Read English Cucumbers Don’t Like Company for more information.

Preventing Cross-Pollination

There are several ways of preventing cross-pollination in vegetables.

Growing them inside a greenhouse or a screened cage of some sort, even just under floating row cover, will be enough to prevent insects or wind from transferring pollen … but then you would have to pollinate them manually.

If two related varieties don’t flower at the same time, that will also ensure that they won’t cross.

However, the most widely used method of preventing cross-pollination is isolation by distance: grow the plants far enough from any other variety of the same plant that there is little risk of pollen transfer.

Isolation by Distance

The recommended isolation distance varies greatly depending on the vegetable being grown. Wind-pollinated vegetables need a greater distance, for example, because their pollen is often light and can be carried far away. That’s the case with beets, for example. But also insect pollinators will travel greater distances to pollinate certain desirable flowers than less interesting ones … and some plants are simply more open to cross-pollination than others.

You can often increase the effectiveness of isolation by distance (and thus decrease the distance required) by planting densely in the zone between the two strains. Not only will dense vegetation help trap much of the windblown pollen, but it offers pollinators other sources of pollen so that they no longer need to travel long distances to feed. Thus, cultivating variety X at one end of a vegetable garden and variety Y at the other, with a good mass of different plants in between, can help reduce the risks of cross-pollination. A forest between the two would be even better!

For many vegetables, however, the most decisive factor still remains the distance normally traveled by the usual pollinator (wind, bee, butterfly, hoverfly, etc.).

Also, some vegetables have closed flowers, ones not accessible to wind or insects, and thus almost always self-pollinate—tomatoes, peas and beans, for example—so the distance you need to respect can be 0 feet. This will ensure a purity of about 95%. (On very rare occasions, an insect will succeed in penetrating a closed flower carrying the pollen of a different variety, hence the small percentage of contamination.) In other words, you can estimate only about one plant in 20 will not be true-to-type.

This is where the reason why you harvest seed comes becomes important. If you’re harvesting seed for a seed bank or for commercial purposes, you’ll want it to be absolutely true-to-type and the isolation distance will be greater than for the home gardener who simply wants to harvest his own vegetables and who will probably be more intrigued than disappointed by the occasional presence of a vegetable that isn’t true-to-type.

Recommended Isolation Distances for Home Gardeners

What follows are vegetable isolation distances recommended for home gardeners. They should give seeds at least 95% true-to-type (19 plants out of 20), a percentage normally considered acceptable for a family vegetable plot. For those who intend to contribute to a seed bank, on the other hand, these distances will likely be insufficient. You’ll need to strive for 99.9% purity and for that, I suggest you contact the seed bank for their recommendations.

Note that this list blends distances coming from several sources, so it is quite possible that the information does not exactly match what you have seen elsewhere. And that’s to be expected. In this field, there seems to be no consensus: opinions vary enormously!

1. Amaranth: 100 ft (300 m)
2. Arugula, rocket salad: 650 ft (200 m)
3. Basil: 150 ft (50 m)
4. Bean: 0 ft (0 m)
5. Beet: 5 miles (8 km)
6. Broccoli: 1 mile (1.5 km)
7. Brussels sprouts: 1 mile (1.5 km)
8. Cabbage: 1 mile (1.5 km)
9. Carrot: ½ mile (800 m)
10. Cauliflower: 1 mile (1.5 km)
11. Celery: 1 mile (1.5 km)
12. Chinese cabbage: 1 mile (1.5 km)
13. Chives: ¼ mile (400 m)
14. Coriander, cilantro: ½ mile (800 m)
15. Corn, maize: 100 ft (30 m)
16. Cucumber: ¼ mile (400 m)
17. Dill: 1 mile (1.5 km)
18. Eggplant: ¼ mile (400 m)
19. Fennel: ½ mile (800 m)
20. Garden huckleberry: 0 ft (0 m)
21. Gourd: ¼ mile (400 m)
22. Ground cherry: 325 ft (100 m)
23. Hyacinth bean: 50 ft (15 m)
24. Kale: 1 mile (1.5 km)
25. Leek: 1 mile (1.5 km)
26. Lettuce: 25 ft (8 m)
27. Lima bean: 1 mile (1.5 km)
28. Melon, cantaloupe: ¼ mile (400 m)
29. Mustard: 1 mile (1.5 km)
30. Okra, gumbo: 855 ft (250 m)
31. Onion: ¼ mile (400 m)
32. Parsley: 1 mile (1.5 km)
33. Pea: 0 ft (0 m)
34. Pepper: 30 ft (10 m)
35. Potato: 30 ft (10 m)
36. Radish: 650 ft (200 m)
37. Scarlet runner bean: ½ mile (800 m)
38. Soya: 25 ft (8 m)
39. Spinach: 5 miles (8 km)
40. Squash, pumpkin, vegetable marrow: 125 ft (40 m)
41. Strawberry: ½ mile (800 m)
42. Sunflower: ½ mile (800 m)
43. Swiss chard: 5 miles (8 km)
44. Tomatillo: 0 ft (0 m)
45. Tomato: 0 ft (0 m)
46. Turnip: ½ mile (800 m)
47. Watermelon: ¼ mile (400 m)