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Growing Zucchini

Why zucchinis can taste bitter

By: Sandy Feather 2008
Penn State Extension

Q. Out of the four zucchinis I planted this summer, three of the zucchini plants were fine, but the fourth produced fruits that were very bitter tasting and inedible. How can I prevent this from happening to my garden zucchini harvest again next year?

A. Zucchini is a member of the Cucurbitacea family, along with cucumbers, pumpkins, squash, melons and gourds. All members of the family produce chemicals called cucurbitacins that can cause the fruits to have a bitter taste. Usually they are present in such low concentrations that you do not taste them. Cucurbitacins are also the compounds that attract striped and spotted cucumber beetles to many members of this family.

Damages Done

These insects damage crops by direct feeding on foliage and flowers, and also by transmitting bacterial wilt. This bacterial disease kills the vines; there is no control beyond controlling the beetles. Lest you think all cucurbitacins are bad, know they are also responsible for the smell and taste of a good cantaloupe.

Environmental Stress

Cucumber fruits can produce high levels of cucurbitacins in response to environmental stress such as high temperatures and drought. Low soil fertility and low soil pH can also contribute to high cucurbitacin levels in cucumber crops.  However, high cucurbitacin levels in zucchini and other summer squash varieties do not appear to be a result of environmental stress, but rather the influence of a single gene. Assuming you grew and cared for all four plants the same way, your experience would seem to bear this out.


Zucchinis are members of the Cucurbitacea family along with pumpkins, cucumbers, squash, melons and gourds.

Plants that produce extremely bitter zucchinis are rare, but it does happen. If you are unlucky enough to have such a plant in your garden, tear it out and do not eat any of the fruit or give it away. Do not save any seed from this plant since it is very likely that the resulting plants would also produce bitter zucchini. A small number of cases of human poisoning from eating minute amounts of bitter zucchini have been reported in the United States and Australia.


There are wild members of the Cucurbitacea family that occur as weeds; they tend to contain very high cucurbitacin levels that render their fruit inedible. All members of this family depend on bees for pollination of the female flowers. If seed production fields are in proximity to wild populations of cucurbits, it is possible that bees could transfer pollen from the cucurbitacin-rich weeds to the seed fields. The resulting seeds would produce bitter fruit because the bitterness gene is dominant.

How can you tell?

There is no way to tell by looking if a zucchini plant will produce these bitter fruits, and no sure way to avoid the problem. Fortunately, it is not very common. If you save your own seed, make sure none of the wild cucurbits are growing near your garden. Prickly cucumber (Echinocystis lobata) and bur cucumber (Sicyos angulatus) are the wild cucurbits commonly found in Pennsylvania.


Bees in your vegetable garden

Corn earworm

Eggplant flea beetles


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