Squash plants do cross-pollinate, but this natural process is governed by strict rules of botanical compatibility. Cross-pollination occurs when pollen is transferred from the male flower of one plant to the female flower of another, usually by bees. This exchange of genetic material is only successful under specific conditions, meaning not all squash varieties grown near each other will hybridize. Understanding these compatibility rules is the first step for any gardener looking to maintain specific varieties or predict next year’s crop.
Which Squash Species Can Cross Pollinate
All cultivated squash varieties belong to the genus Cucurbita, which is organized into several distinct species groups. Cross-pollination only happens between varieties that share the same species classification. This biological barrier prevents unwanted crosses in the garden.
The four primary domesticated species are Cucurbita pepo, C. maxima, C. moschata, and C. argyrosperma. For example, a zucchini and an acorn squash can easily cross because they are both members of the C. pepo species. Gardeners must isolate different varieties of the same species if they plan to save seeds.
Examples of Squash Species
Varieties within the C. pepo group include common summer squash like zucchini, yellow crookneck, and pattypan, alongside winter squash types such as acorn, delicata, and spaghetti squash. The C. maxima species encompasses large winter squash like Hubbard, Buttercup, and Kabocha. Butternut squash, Long Island Cheese, and Calabaza all belong to the C. moschata group, while the cushaw squashes are classified as C. argyrosperma. A butternut squash (C. moschata) growing next to a zucchini (C. pepo) will not cross-pollinate because they are genetically too far apart.
The Difference Between Fruit and Seed Genetics
A common misunderstanding among gardeners is the belief that cross-pollination affects the taste or appearance of the current season’s fruit. The fruit harvested this year is genetically identical to the mother plant, regardless of the pollen it received. This is known as the maternal effect, where the squash’s flesh, shape, and flavor are determined by the plant’s own genetic makeup.
The genetic change from cross-pollination only occurs within the ovules, which become the seeds inside the fruit. These seeds are the F1 generation, representing a hybrid of the two parent plants. If you save and plant these crossed seeds the following year, the resulting plant will produce unpredictable, often bizarre fruit.
Planting F1 hybrid seeds can result in an unexpected mix of traits from both parent varieties, sometimes yielding inedible or bitter fruit. This bitterness is caused by the re-expression of a compound called cucurbitacin, which is typically bred out of commercial varieties. Therefore, the only time a cross matters for the gardener is if they intend to save the seeds for planting in the next growing season.
Preventing Unwanted Crosses
Gardeners who need to maintain seed purity, especially for heirloom varieties, use two primary methods to prevent cross-pollination. The first is physical isolation, which involves separating compatible squash species by a significant distance. For commercial seed saving, this distance is often recommended to be between one-half mile and two miles, though barriers like thick tree lines or buildings can offer some reduction.
Since large distances are impractical for most home gardeners, the more reliable method is hand pollination. This technique requires intervention before natural pollinators, like bees, can visit the flowers. First, identify the female flower by the small, immature fruit swelling at its base and the male flower by its long, thin stem.
Hand Pollination Steps
Before the flowers open in the early morning, the buds of both a male and a female flower must be gently taped shut to exclude insects. The next morning, remove the tape from a selected male flower, which is then picked and its petals are peeled back to expose the stamen, the pollen-bearing structure. This stamen is carefully brushed against the central stigma of the female flower to transfer the pollen. Immediately tape the female flower shut again until the fruit begins to swell.