Yard and Garden: Growing Successful Summer Squash

The summer months are the perfect time to enjoy summer squash from the garden, but successful growers must clear hurdles to gain a bountiful harvest. Here are some tips from Iowa State University Extension and Outreach on the best ways to have a fruitful summer squash crop this year, with help from ISU Extension horticulturists. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

My summer squash are flowering heavily, but aren’t producing many fruit. Why?

zucchiniSquash and other vine crops are monoecious. Monoecious plants have separate male and female flowers on the same plant. Male and female flowers are similar in appearance. However, female flowers have small, immature fruit at their base. Pollen is transferred from the male to the female flowers by bees and other pollinators. When properly pollinated and fertilized, the female flowers develop into fruit. The first flowers to appear on squash and other vine crops are predominately male. As a result, fruit production is poor when plants begin to flower. The squash plants should start producing a good crop within a few weeks as the number of female flowers increases.  

Poor weather and the use of insecticides can also affect fruit set on vine crops.  Cold, rainy weather during bloom reduces bee activity.  Fewer bees visiting the garden results in poor pollination and poor fruit set.  Apply insecticides in the garden only when necessary to avoid harming bees and other pollinators.

The fruit on my summer squash begin to grow, but quickly turn brown and rot. Why? 


The rotting of the small squash fruit could be due to poor pollination or blossom-end rot.  

For squash fruit to develop fully, bees and other pollinators must transport pollen from the male flowers to the female flowers.  If the female flowers aren’t pollinated properly, the fruit will begin to grow and then suddenly shrivel up and die. Bees and other pollinators are less active in rainy weather. Rainy weather could be responsible for poor pollination and rotting of the small fruit.  Drier weather conditions should increase pollinator activity. To prevent the destruction of honey bees and other pollinators, avoid spraying plants with insecticides during bloom. If spraying during bloom is necessary, apply insecticides late in the evening when the honey bees have quit foraging for the day.  

Blossom-end rot is a physiological disorder that occurs on tomatoes, peppers, eggplant and summer squash. On zucchini and other summer squash, the blossom end of the fruit begins to rot and within a short time the entire fruit has rotted. Blossom-end rot is caused by a lack of calcium in the developing fruit. In most cases, there is no need to apply calcium to the soil. Try to maintain an even moisture supply by watering once a week during dry weather. Also, do not over-fertilize plants.  Uneven moisture supplies and excessive nitrogen inhibit calcium uptake. 

When should I harvest my summer squash? 

Harvest zucchini and other long-fruited summer squash when the fruit are 1½ to 2 inches in diameter and 6 to 8 inches long.  Scalloped types are best when 3 to 5 inches in diameter. Fruit should have soft skins (rinds) that are easy to puncture with a fingernail.  Seeds should be soft and edible. Harvest plants frequently for continuous production.