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Flower and fruit set in cucurbits

Closeup of yellow zucchini flower on a vine in a garden.
Zucchini flower

The cucurbitaceae family, which includes cucumbers, melons, zucchini, squash and pumpkins, is notorious for its temperamental flowers and fruiting requirements. Every year we hear from growers who have beautiful green plants, but very little fruit. There are a couple of key reasons that this occurs.

Pollination

Pumpkins, squash, melons, and cucumbers are dependent upon insect pollination. Male flowers bloom about a week before female flowers, and flowers only bloom for a few hours in the morning, so it's crucial that conditions are right for pollination.

  • Bumblebees and squash bees (Peponapis pruinosa) are the best pollinators of pumpkins and squash since they forage in the morning.
    • Squash bees have a life cycle that's perfectly timed with the life cycles of cucurbits.
    • These bees have very fuzzy bodies that accumulate substantial amounts of pollen.
  • Creating habitat on your farm for nesting sites may help boost pollination on your farm.
    • Bumblebees require consistent floral blooms, ideally with native plants, throughout the growing season.
    • Squash bees nest in the soil at a depth of 5-10 inches. Minimize tillage in areas where you grow cucurbits. 
  • Pesticide applications can also negatively affect pollinators. If you're spraying for cucumber beetle or squash bugs, try to time applications to avoid flowering times.
  • See more about vine crop pollination in the Midwest.

Planting density

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

Ideal spacing

Pumpkins

  • Compact/bush varieties: plant 18-24 inches apart with 4-6 feet between rows (4-6 pounds of seed per acre)
  • Miniature pumpkins: plant 2 feet apart with 6-8 feet between rows
  • Vining varieties: plant 2-5 feet apart with 6-8 feet between rows (2-3 pounds of seed per acre)

Other cucurbits

  • Cucumbers: plant 15-18 inches apart with 4-6 feet between rows
  • Watermelons: 3-6 feet between plants with 6-12 feet between rows
  • Muskmelons: 3-5 feet between plants with 5-7 feet between rows 

Watermelons and muskmelons do well when planted on mounds to allow for good drainage. 

Spacing recommendations are from the Midwest Vegetable Production Guide for Commercial Growers

Nitrogen

Too much nitrogen can delay fruiting and cause plants to put energy towards growing vines and leaves rather than fruit.

  • Split fertilizer application is recommended for pumpkins when using quick-release synthetic fertilizers (1/2 N applied at planting, 1/2 applied when plants start to vine).
    • Applying too late (once plants have begun to set fruit) can cause fruit production problems.
  • Make sure to start your season with a soil test, and make fertilization decisions accordingly.
  • For more complete fertilizer recommendations based on your soil test, consult the Nutrient Management Guide for Commercial Fruit and Vegetable Growers in Minnesota.
  • Home gardeners get instructions for fertility directly on soil test results. 

Water and heat stress

  • Water stress, either too much or too little water, can cause cucurbits to abort flowers and fruit.
    • Drought stress can result in a skewed ratio of male to female flowers.
    • Flooding prevents the roots from taking up oxygen and other nutrients.
  • For all cucurbits, high temperatures during fruiting can cause problems. Daytime temperatures in the 90s or nighttime temperatures in the 70s can cause flower and small fruit abortion. 
  • Since flowers only bloom for a few hours, conditions have to be just right for pollination. If flowers bloom during heavy rainfall or very hot conditions, pollinators are less likely to be out pollinating.

For an in-depth discussion of flower and fruit set in cucurbits, listen to a discussion with cucurbit expert Brent Loy, on the What’s Killing My Kale podcast, Season 3, Episode 10.

Author: Natalie Hoidal, Extension educator, local foods and vegetable production

Reviewed in 2021

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