How to Tell the Boys from the Girls (Squash, That Is)

It's a question that is asked a lot in the late spring and early summer by vegetable gardeners: "Why am I not getting any squash? After all, there are flowers on the plant. And any fruit that does develop tends to fall off."

According to retired UC Vegetable Specialist Hunter Johnson: "Squash, melons and cucumbers have a flowering habit which is unique among the vegetable crops. They bear two kinds of flowers, male and female, both on the same plant. In order for fruit set to occur, pollen from the male flower must be transferred to the female flower. The pollen is sticky; therefore, wind-blown pollination does not occur. Honeybees are the principal means by which pollen is transferred from the male to the female flower. Other insects cannot be depended upon for pollination."

Not only will a shortage of bees result in poor pollination of your squash, it may only be partial pollination. This results in misshapen fruit, low yield and the demise of immature fruit.

Rain, low light, or cold and hot temperatures can limit bee activity. Also, avoid using insecticides that are harmful to bees; it will say so on the product label.

There are other reasons why you may be having problems with your summer squash production: not enough sunlight (plant them in full sun) and planting too early in the season. Squash need very warm soil temperatures to thrive, ideally 70-95 degrees (F). Here in the lower elevations of California, sow seeds directly in the garden in May.

Another reason for early season squash fruit failure, according to Hunter Johnson: "All of the early flowers are males. Female flowers develop somewhat later and can be identified by the miniature fruit at the flower base. However, In hybrid varieties of summer squash,  the first flowers to appear are usually females, and these will fail to develop unless there are male squash flowers -- and bees -- in the nearby area."

But lack of bees is the primary reason for poor fruit set in squash, melons and cucumbers.

If you have a shortage of bees, consider putting in bee-attractive plants. 

So, what's a bee-less gardener to do? Pollinate by hand!

Transferring the pollen from the male flower to the stigma inside the female flower is easy to do, if you follow a few guidelines:

Know your squash flowers. The male flower has a long slender stem, along with a penis-like structure in the inside of the flower. The female flower has a very short stem, along with a miniature fruit (the ovary) at the base of the flower.

 Ronde De Nice Zucchini Flowers. Male on the left, female on the right.

Inside the zucchini flowers, male and female. Note the cute yellow condom on the male.

You could break off the petals of the male flower and then use a small artist's brush to transfer the yellow pollen from the male squash penis (OK, OK, it's technically a "stamen") to the stigma of the female flower.

American River College Horticulture Professor Debbie Flower has an even easier way: "Just peel away the male flower petals, cut off the flower and then twirl the male flower inside the female flower."

The best time to do this? In the early morning. Use only freshly opened flowers; they're only "in the mood" for one day.

What about cross pollination among these cucurbits? Not a worry, unless you save seed for next year, says Johnson: "A common misconception is that squash, melons, and cucumbers will cross-pollinate. This is not true; the female flowers of each can be fertilized only by pollen from that same species. Varieties within each species, however, will cross-pollinate. Thus, zucchini squash will cross with crookneck or acorn squash, and similarly among varieties of cucumber, and among varieties of muskmelon. When more than one variety of a particular cucurbit is grown in the garden, they will readily cross, and seed saved from these plants will produce fruit which will be different from either of the parents."