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Pollination in the Vegetable Garden - Cucurbits

Karl Foord, Extension Educator - Horticulture

Karl Foord

Exhibit 1: Cucumber flowers

Exhibit 2: Cucumber flowers

Exhibit 3: Male squash bees in squash flower

Exhibit 4: Male squash bees in squash flower

The plant family Cucurbitaceae contains a number of our favorite garden plants. This includes: cucumbers, watermelon, muskmelon, pumpkins, squash, and gourds. This group is particularly fascinating in terms of its flower morphology. These plants are called monoecious because they have separate male and female flowers on the same plant (Exhibit 1). These flowers are most easily recognized by the shape of the stem below the flower. The stem below the female flower looks like a smaller version of the final fruit (Exhibit 2), and the stem below the male flower remains a single slender stalk.

Cucurbit flowers are short lived flowers that open a few hours after sunrise and are often closed by midday or early afternoon. Both male pollen viability and female stigmatic receptivity are at their highest when the flower opens and for the next few hours. Both pollen viability and stigmatic receptivity decrease significantly as the day progresses. It is important for the female flower to be pollinated as early in the day as possible.

In cucurbits there is one key concept: The quality of the fruit is a function of the number of seeds in the fruit. The number of seeds produced is a function of the number of viable pollen grains deposited on the stigma. The number of pollen grains deposited is a function of the number of visits by pollinators as well as by the type of bee visiting the flower. Expressed as an equation:

Quality of Fruit ~ # of seeds ~ pollen grains deposited ~ # of bee visits & type of bee

Because both male and female sexual parts are not in the same flower, pollen must be transferred from the male flower to the female flower. The pollen is too large and sticky to be transferred by wind and thus requires insect transfer. Bumble bees tend to deposit more pollen per visit than squash bees and squash bees tend to deposit more pollen per visit than honey bees.

To produce quality fruit watermelon need on the average 1,000 grains of pollen deposited, whereas pumpkin, cucumber and cantaloupe need between 300 and 400.

These flowers need to be visited multiple times (@ 10 - 12) by pollinators to achieve satisfactory pollen transfer and seed set.

Like the solanaceous crops in the previous article, these plants prefer higher temperatures and are sensitive to frost. Daytime temperatures in the high 70s or low 80s degrees and nighttime temperatures close to 65 degrees are optimum for seed set and growth. Poor fruit set or misshapen fruit can sometimes be the result of poor weather which has limited pollinator activity leading to poor pollination and insufficient seed set.

Another very interesting thing about squash flowers is that there is a bee that has evolved as a pollinator specific to squash flowers. Surprisingly enough this bee has been named a Squash Bee (Peponapis pruinosa). Another fun thing is that the male squash bees will often spend the night and parts of the day in the protected space created by the squash flower petals (Exhibits 3 & 4). So if you choose to grow squash you get the potential added treat of watching these very interesting bees.

In addition squash bees are solitary ground nesting bees who will often dig their ground nests in the garden near the squash plants. If you find holes in your garden that are a half inch in diameter, it could be the entrance to the squash bees nest. The linked article in the above paragraph has a picture of the entrance to a squash bee nest. If you encounter such a nest in your garden, one option would be to avoid tilling around this nest and encourage next year's squash bees. If you continue to plant squash every year you could end up with permanent residents.

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