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The unique pollination systems of cucumbers, melons, and squash

You've likely all seen the images of what grocery store shelves would look like without pollinators. While many fruit and vegetable crops require pollinators to set fruit, cucumbers, melons, squash, and other plants in the cucurbit family have one of the most complex pollination systems of any garden vegetable. This article covers some basic information abut the complex and very cool pollination systems of cucurbits. 
For a much more in-depth discussion of flowering, pollination, and fruit set dynamics, listen to our recent episode of What's Killing my Kale.

Flowering dynamics

Image: Natalie Hoidal
Pumpkins, squash, cucumbers, and melons are monoecious, meaning they produce both "male" and "female" flowers on the same plant. However, these flowers emerge at different times. In general, male flowers bloom one week or so before female flowers. However in zucchini and summer squash, female flowers tend to bloom first. 

In zucchini and summer squash, early female flowers will sometimes produce a fruit without the pollen of a male flower; this is why we sometimes see very small or oddly shaped fruit early in the season. 

In a perfect world, pollinators would visit the male flowers to pick up pollen, then visit female flowers to deposit it. However, the world is rarely perfect, and pollination doesn't always work out this way. 

Sensitive flowers

Cucurbits have very sensitive flowers. Most squash and pumpkin flowers only last for about four hours, so conditions have to be just right for pollination. Conditions like high temperatures can cause the plants to produce only male or only female flowers. Drought tends to result in more male flowers. Heat and drought, and also too much water can also simply cause the flowers to die, or to wilt even more quickly than their already short blooming period. In these scenarios, pollinators may not be able to reach the plants in time, or if they do, they may not be able to effectively transfer pollen from plant to plant. 

A specialist pollinator

Squash bee, Susan Ellis, Bugwood
While cucurbits can be pollinated by any insect pollinator, they are most frequently pollinated by bumblebees and squash bees, which forage in the morning when flowers are open. The squash bee in particular is a very special insect. Squash bees (Peponapis pruinosa) co-evolved with cucurbits, and so their lifecycles are perfectly aligned. They come out in the summer around the time that cucurbits are flowering, and have adapted their feeding habits based on the flowering systems of these crops. In order to encourage squash bees and bumblebees in your garden, try to limit the amount of tilling and digging you're doing around your garden, and leave some bare soil areas for nesting. Growing flowers all summer long is also a great way to support bumblebees in your garden.

Even in a perfect garden setting with plenty of bee habitat and resources, we still sometimes see limited pollination in cucurbits. Extreme heat and heavy rain can cause pollinators to "stay inside", or to seek shelter. Given the short lifecycles of cucurbit flowers, bad weather can mean that a flower doesn't get pollinated at all.

Again, for a much more in-depth conversation about flowering dynamics in cucurbits, listen to our recent What's Killing My Kale episode - a conversation with Dr. Brent Loy, emeritus professor of plant genetics and cucurbit breeder extraordinaire at the University of New Hampshire.

Additional resource: Vine Crop Pollination by Dr. Marla Spivak, U of MN
Author: Natalie Hoidal, UMN Extension Educator

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