Pollination of Plants in the Gourd Family (Cucumbers, Squashes, Melons, and MORE!)

Southwest Yard & Garden by Dr. Marisa Thompson

Photo credit Erica Perras from https://www.facebook.com/groups/396098954064479/?ref=share. 

Small fruit are developing at the base of the female flower (bottom left), but not on the male flower (top left). Pollen from the stamens in the center of the open male flower (top right) can be translocated to the stigma in the center of the open female flower (bottom right) by pollinating insects or by humans. Photo credit Abrahami, Wikimedia Commons.

Question: I would love to be able to grow both zucchini and watermelon this year, but I am hesitant because of problems with cross-pollination in the past. Any tips?

-          Seed to Supper Online Course Participant, Somewhere in New Mexico

Answer: First of all, for readers who don’t already know about NMSU’s Seed to Supper program through ICAN (Ideas for Cooking & Nutrition), this is a free, online, self-paced beginning gardening course that was first developed by the Oregon State University Extension Service and modified by our own NMSU Food Systems Specialist Sally Cassady to be web-based and New Mexico-oriented (https://ican.nmsu.edu/seedtosupper.html).  

It sounds like the problems you’ve had in the past with zucchinis and watermelons may have been more about fruit set issues—which could include pollination problems—than cross-pollination. Cross-pollination can only occur within plants of the same species. The old gardening tip “don’t plant cucumbers next to squash or melons because they’ll cross-pollinate and form bad fruit” isn’t true. NMSU Extension Vegetable Specialist Dr. Stephanie Walker confirmed: “As long as the cucurbits are different species, it’s very unlikely they’ll cross-pollinate. Zucchini is Cucurbita pepo and watermelons are Citrullus lanatus, so they won’t cross-pollinate to produce viable seed.”

Plants in the cucurbit (gourd) family include melons, pumpkins, squash, and cucumbers. Each of those different cucurbits includes plants of different species and genera (plural of genus). Remember, the scientific names of plants consist of two parts: the genus and the species. So musk melon’s scientific or botanical name is Cucumis melo, with “Cucumis” as the genus and “melo” as the species. Cucumber is Cucumis sativus, so even though musk melons and cucumbers are in the same genus (Cucumis), they are not the same species and won’t be likely to cross-pollinate. Even if they did cross-pollinate, the evidence would not be visible in this year’s crop. If you saved seed from cross-pollinated fruit and grew it next year, you might get something cool and yummy, although it’s more likely to be undesirable. Pumpkins with green bumps could be the result of seeds that were saved from normal pumpkins crossed with green-warted gourds.

It’s not just that plants from different species aren’t likely to cross-pollinate based on their genetics. Dr. Amanda Skidmore, NMSU Extension Integrated Pest Management (IPM) Specialist for Urban and Small Farms, explains that our pollinators are picky too: “Interestingly, different pollinators will visit each plant because of the flower shape and inflorescence. For example, squash bees will visit zucchini, but not watermelon.” Dr. Skidmore encourages gardeners to “take some time to watch and see what different pollinators are visiting the two plants. There will be some overlap (honey bees, bumble bees), but some cool differences too.”

For more information, CLICK HERE to check out our entire NMSU Extension Guide collection for vegetables. Titles include,  H-220 “Starting PlantsEarly Outdoors," H-221 “Spices and Herbs for the Home Garden," and Circulars 457 “Home Vegetable Gardening in New Mexico” and “Growing Zones, Recommended Crop Varieties, and Planting and HarvestingInformation for Home Vegetable Gardens in New Mexico." PLUS MANY MANY MORE!

As retired NMSU Extension Horticulture Specialist Dr. Curtis Smith explained in a 2008 column, cucurbits have “separate male and female flowers on the same plant. You can recognize the male flowers because they do not have a small fruit behind them [more on this later]. They produce the pollen needed to form the fruit, but they do not produce the fruit. The female flower, on the other hand, has a small fruit behind the flower even before it opens. The female flower cannot produce the pollen needed to cause the fruit to develop and is dependent upon insect (or human) pollinators to transport the pollen from the male flower. The male flowers begin forming before the female flowers form. So, it is possible to have cucurbits blooming, but not producing fruit. The time between the first development of male flowers and the female flowers depends on plant variety and environmental conditions. Your problem may just be that the female blossoms have not formed yet. They should be forming soon. However, if you see the female flowers on your cucurbit plants, but they are not "setting" fruit, then the problem may be that you do not have pollinators.”

Smith also offers tips on how to pollinate your cucurbit flowers yourself: “Each morning, collect pollen from the stamens in the center of the male flowers and then transfer that pollen to the stigma in the center of the female flowers. Use a small, soft-bristled paintbrush to do this. If you are successful, you should see small fruit forming within a few days. Leave some female flowers unpollinated (by you), so you can watch for the return of the natural pollinators to relieve you of the early morning effort of pollination.”

A few years ago, I wasn’t convinced that I’d be able to tell the difference between male and female flowers on a squash plant. That is, until I looked closer. Both flower types are huge and bright orange-yellow, but if you look just behind the flowers, you’ll know when you find a female versus a male because there’s a swollen fruit structure developing at the base of female flowers. In some cucurbits, that baby fruit will be more rounded, and in others more like a small pickle.

For more gardening information, including decades of archived Southwest Yard & Garden columns, visit the NMSU Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us on social media (@NMDesertBlooms), or contact your County Extension office (https://aces.nmsu.edu/county).

Marisa Thompson, PhD, is the Extension Horticulture Specialist for New Mexico State University and is based at the Agricultural Science Center at Los Lunas.