It may be pretty but is it delicious? The dangers of cross-pollination

Jun 23, 2024 | Gardening tips

When your pumpkins ripened, you may have noticed that what grew was not what you thought you had planted. Did you, for example, grow a long, green or bluish-striped pumpkin when you thought you planted a round Kent or even a Queensland Blue? Cross-pollination is the culprit, and that cross-pollination will be from saved seed that crossed in a previous season.

Cross-pollination sometimes produces amazingly good produce and some of it can be visually very attractive. However, it is a lottery and you may find the results bitter or tasteless and the texture is watery or grainy. It seems such a shame to tend to a crop only for this to happen. The photo above is of a ‘tromboncino’ grown from saved seed but it looks like that saved seed was the result of cross-pollination between tromboncino zucchini and Kent pumpkin.

Cross-pollination will not affect plants in the first season as long as you use commercial seed or the flowers have been bagged and hand-pollinated by you to prevent bees and other pollinators accessing the flowers. The bees may be buzzing around transferring pollen between closely related family members, but in the current season, the fruit produced will be whatever you planted. For example, if you plant both pumpkin and zucchini, in season one, your pumpkin will produce pumpkins and your zucchini will produce zucchinis. Cross-pollination relates to the grandparents of a vegetable rather than to its parents.

In season two, however, saved seed from season one will contain the genetics of both parent plants and the progeny will be a cross between the two, just as it is in humans. So seeds from that delicious pumpkin you grew, will be unlikely to give you the same pumpkin in the following season.

Only varieties within one species can cross. The species Cucurbita pepo is a prime example. Within this species, are pumpkins, zucchinis and squash (including spaghetti squash). They are closely related and cross-pollinate with great regularity. Cucumbers (Cucumis sativa) are a member of the Cucurbiteae family too, but are a different species so do not cross with pumpkins, zucchinis and squash. However, cucumber varieties will cross-pollinate with each other.

Other fruits that cross-pollinate include canteloupe and honeydew melon, but neither cross with watermelon. Regular leaf (RL) tomatoes, and potato leaf tomatoes such as the Brandywine varieties can also cross. (When I was growing both one year I got a very bright pink, medium-sized, round tomato, most likely a Grosse Lisse x Pink Brandywine which was spectacular in colour and taste). Regular-leaf tomatoes do not cross with each other. Beans, broad beans and peas can cross but hardly ever do.

Corn is a special case. Corn varieties should be grown in isolation, i.e. plant only one variety. That means one variety must be at least 35 metres from another, but preferably to be absolutely sure, 1 kilometre apart. This also means you must check whether your neighbours are growing corn and determine the variety if you want to grow it from your own saved seed. Popcorn varieties will dominate sweet corn varieties. Corn is wind-pollinated so the pollen can travel a long way on a strong wind.

Brassicas are notorious for cross-pollinating, and to bad effect, usually producing a bunch of leaves in place of a heart in hearting vegetables. Cauliflower, cabbage, kale, Brussels sprouts and broccoli all cross, as do many mustard greens which are from another branch of the Brassica family.

Bees can travel vast distances – up to 8 kilometers – so they can bring pollen to your garden from an astounding array of gardens and plant varieties on their travels. Commercial beehives can be a source of seed contamination. There is a withholding period of 7 days for some crops, meaning for example, beehives that are moved from crop to crop by beekeepers must not be moved from an area where, say brassicas, are growing to another area where brassicas are growing for 7 days. The hives would need to be moved to a neutral zone and wait it out.

When you are traveling in the countryside you may see fully netted areas that provide enclosures with the netting hem weighted down so that no pollinators can enter the isolation area. This is how seed merchants control the genetics of their seed.

If you only use commercial seed, cross-pollination is not an issue. If however you are a seed-saver or you swap seed with friends or select from a seed library, then you need to avoid certain seed or run the risk of a disappointing outcome.

In other words, don’t save seed from cucurbits (pumpkins, cucumbers, gourds, rockmelons, watermelons, zucchini, etc), brassicas (cauliflower, cabbage, kale, Brussels sprouts, broccoli, etc) or sweetcorn unless you are sure that cross-pollination has not occurred.

 Written by Robin Gale-Baker