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Become a Smarter Gardener in 2019: Why so few pumpkins and squash?

Squash blossom. Photo: Natalie Hoidal, UMN Extension
We’ve heard multiple reports of pumpkin and squash plants not producing well this year. Some plants have no fruit at all, and others produced fewer fruit than usual.

If this happened in your garden, there are a few explanations.

1. Pollination

Pumpkins and squash are dependent upon insect pollination. They are monoecious, which means that each plant will produce both male and female flowers. 

However, these flowers emerge at different times. Male flowers bloom about one week before female flowers. In an ideal situation, bees and other pollinators would visit the male flowers and coat themselves in pollen. When the female flowers bloom a week later, those same pollinators would visit and pollinate them. 

However, most squash and pumpkin flowers only last for about four hours, so conditions have to be just right for pollination. The time lapse between male flower bloom and female flower bloom means that conditions have to continue to be favorable for an extended period to ensure good pollination.

  • Bumblebees and squash bees (Peponapis pruinosa) are the best pollinators of pumpkins since they forage in the morning, and because squash bees have a lifecyle that's perfectly timed with the lifecycles of cucurbits. Cucurbits include pumpkin, squash, melons, cucumbers, and zucchini. 
  • Creating habitat (i.e. growing flowers throughout the growing season) for bee nesting sites may help boost pollination. For bumblebees, this means consistent floral blooms, ideally with native plants throughout the growing season. For squash bees, which nest in the soil at a depth of 5-10 inches, this means minimizing the amount of tilling or digging you may do around your pumpkin patch.
Keep in mind that pesticide applications can also negatively impact pollinators. If you're spraying for cucumber beetle or squash bugs, try to time applications so as to avoid flowering times. 

Bad weather for pumpkins...

This year, we experienced many days with heavy rainfall as well as significant temperature fluctuations. These environmental conditions can result in a failure to pollinate. Even if pollinators are present in your area, they are unlikely to come out and pollinate when conditions are too wet, too windy, too hot, or too cold. 

While we can’t control the weather, there are a few things we can do to encourage better pollination in next year’s crop.

2. Planting density

If your plants are too close together, they'll experience competition for both light and nutrients. When plants have to compete for sunlight, photosynthesis is reduced, which in turn reduces growth and development.

Ideal spacing for pumpkins is:
  • Compact / bush varieties: plants 18-24 inches apart with 4-6 foot rows (4-6 pounds seed / acre)
  • Miniature pumpkins: plants 2 feet apart with 6-8 foot rows
  • Vining varieties: plants 2-5 feet apart with 6-8 feet per row (2-3 pounds seed / acre)

3. Too much nitrogen or nitrogen applied at the wrong time

Too much nitrogen can delay fruiting, and cause plants to put energy towards vegetative growth rather than reproductive growth (growing vines and leaves rather than fruit).

Make sure to start your gardening season with a soil test, and make fertilization decisions accordingly. If you need help interpreting your soil test, talk with your local Master Gardeners. 

4. Water stress

Either too much or too little water can result in low yields. Drought stress can cause the plant to produce more male flowers then female flowers, and only the female flowers can make fruit. 

If conditions are too wet, the plant can become stressed and even drown. If you are growing pumpkins in a low area that gets a lot of water, consider making raised beds or growing pumpkins in mounds. By elevating your planting surface, you allow excess water to drain away from the roots. 

5. High temperatures

High temperatures during fruiting can cause fruiting problems in pumpkins and squash. Daytime temperatures in the 90s or nighttime temperatures in the 70s can cause flowers and small fruits to die, or to fall off the vines. 

Unfortunately, there is not much that we can do in these situations since we don’t control the weather. Making sure that plants have enough water is one way to help them make it through heat spells. 

Using shade cloth during heat spells can also help to keep your garden a few degrees cooler. 

Gummy stem blight in squash.
Photo: S. Jensen, Cornell University, Bugwood.org
6. Diseases

If your plant has produced small fruit that then shriveled up or fell off, it may have a disease. UMN Extension's "What's wrong with my plant" tool provides a nice overview of many of the diseases affecting squash and pumpkin fruit. 

However, disease symptoms are not always easily identified using pictures alone - consider sending plants to the UMN Plant Disease Clinic for diagnosis. 

Author: Natalie Hoidal, Extension Educator in Fruits and Vegetables

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