How to Prevent Cross-Pollination in Squash

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A few years ago, my mother-in-law's cousin moved into a new house and was delighted to see squash growing in the backyard...That is, until she went to eat it. These squash didn't look like any she'd ever seen before and - more importantly - they didn't taste that great. They were, we realized, the result of cross-pollination - something you probably want to avoid in your garden.

What is Cross-Pollination? 

When insects, the wind, or humans take pollen from one variety of plant and spread it to another, a hybrid is created using cross-pollination. The two plants "mate," so to speak, and their "babies" show characteristics from both varieties. However, cross-pollination can only happen between varieties of the same type of plant. Therefore, contrary to a common myth, cucumbers (Cucumis sativus) cannot cross-pollinate with zucchini...but butternut squash (Cucurbita moschata) can definitely cross-pollinate with zucchini (Cucurbita pepo).

Is Cross-Pollination a Big Deal? 

Sometimes playing around with cross-pollination can be fun. Who knows? You might develop a new variety with excellent traits! However, there are two main reasons you may want to prevent cross-pollination: 

Wind and insects spread pollen around your garden, causing cross-pollination.

1. If you are growing open-pollinated or heirloom varieties* and want to accurately save their seed, or 

2. If you want volunteer plants (i.e. plants that grow from the seeds left behind in the garden by rotten fruit) to produce fruit that's just like the parent plant. 

A common misconception is that cross-pollination will affect the current year's plants. For example, someone once told me that my yellow summer squash and zucchini were planted too close together; the result, this person said, would be weird fruit. However, if yellow summer squash and zucchini cross-pollinate, the parent plants will not be affected. They will continue to produce fruit that's true to the varieties planted. Cross-pollination will, however, affect the parent plant's "babies" (i.e. the seeds in the fruit, which, if planted, will result in a weird cross fruit). 

Cross-pollination never affects the current year's fruit. So if you aren't saving seeds, you don't need to worry about it at all. (In vegetable gardening, there's one exception to this cross-pollination rule: Corn. The parent plants, if cross-pollinated, can grow ears of corn that aren't true to variety.) 

I think there are two reasons people find the concept of cross-pollination confusing. One is that environmental factors - such as irregular irrigation or poor pollination - can create weird-looking fruit that sometimes gets incorrectly blamed on cross-pollination. 

The other, more common, reason is that sometimes volunteer plants pop up in the garden and produce odd fruit. The latter happens because the previous year's squash cross-pollinated and some of the fruit was allowed to rot in the garden. This left cross-pollinated seeds in the soil, which popped up and were allowed to grow. 

Cross-pollination does not affect the current year's fruit.

To prevent this type of mishap, I suggest two things: 

1. At the end of the season, remove all fruit from your squash plants and do not compost it or allow it to rot anywhere near your garden. 

2. Mark the exact planting position of your squash seeds with a stick. If any additional seedlings pop up in that area, you'll know they are volunteers that should be disposed of. (Or at least marked as a new hybrid.)

Separate Lives 

But what if you want to save your open-pollinated squash seeds and have them be true to the parent plants? The most commonly quoted solution is to simply plant them further apart. The problem with this is that to truly work, you'd have to plant them at least a half mile away! Very few of us can do that. (Although the South Dakota State Extension Office website notes, "The presence of barriers such as large buildings, a thick stand of trees, or a hill can inhibit pollinator movement and allow for shorter isolation distances." Emphasis on can.)

However, there are workarounds. You could, for example, cage entire plants and place row cover fabric over them - or you could secure bags over the squash flowers. In both cases, to get fruit, you'd need to hand pollinate: Go out early in the morning and, with an artist's paintbrush or a Q-tip, collect pollen from a male flower's stamen (the penis-like growth in the center of the flower). Find a female flower, which will have a mini fruit growing just below the flower petals, and brush the pollen onto its stigma (the female version of the stamen, found in the center of the flower). For best results, the female flower should be pollinated within four hours of opening in the morning. Re-bag the flowers after pollinating. Bags or other covers can be removed from female flowers once you see that fruit is starting to grow. 

Both summer squash and winter squash are affected by cross-pollination.

As you can imagine, bagging and hand-pollinating all the flowers on a squash plant can be time-consuming. Therefore, consider choosing just a few male and female flowers to hand-pollinate. Bag those, and once the fruit starts growing, gently and loosely tie a string on the the female flower stems so you can later identify the fruit that was hand-pollinated. Save seed only from that fruit. 

You can also almost eliminate the risk of cross-pollination by separating cultivars. There are three main cultivars in the edible squash family: 

Cucurbita pepo (C. pepo): such as pattypan, straightneck, New England sugar pie pumpkin, acorn, delicata, spaghetti, and zucchini squash 

Cucurbita moschata (C. moschata): such as butternut, crookneck, Tromboncino, and Long Island Cheese pumpkin 

Cucurbita maxima (C. maxima): such as buttercup, Hubbard, sweet meat, Red Kuri, and Cinderella pumpkin. 

If you grow one plant from each cultivar, it won't generally matter if they grow close together. However, this method is not 100% effective, so if saving pure seed is important to you, I recommend bagging and hand-pollinating, instead. 


* "Open-pollinated" means the plant will produce "babies" that will be true to the parent plant. "Heirloom" means an older variety of open-pollinated plant. These are in contrast to "hybrid" (or "F1")  plants, which will not produce "babies" that are true to the parent plant.

Understanding cross-pollination is vital when you want to save your own seed.



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