Bitterness in Zucchini Squash and Cucumber

  • by Sarah Browning and Dr. Laurie Hodges
  • Department of Horticulture
  • University of Nebraska
  • Lincoln, NE 68583

A common problem found in zucchini squash (Cucurbita pepo) and cucumber (Cucumis sativus) is bitterness, and it can be very frustrating to gardeners who find the vegetables too bitter to eat! Both cucumber and zucchini are members of the Cucurbit family, which also includes pumpkins, melons, squash and gourds. All cucurbits produce a group of chemicals called cucurbitacins, which cause the vegetables to taste bitter; the higher the concentration of cucurbitacin, the more bitter the vegetable will taste. In commercially cultivated cucumbers and zucchini squash, the amount of cucurbitacin is in such a low concentration that it usually cannot be tasted and, aside from bitterness, this chemical does provide other attributes such as the musky scent of cantaloupe.

Mild bitterness is fairly common in cucumbers resulting from higher levels of cucurbitacin triggered by environmental stress, including high temperatures, wide temperature swings or too little water. Uneven watering practices (too wet followed by too dry), low soil fertility and low soil pH are also possible stress factors. Over-mature or improperly stored cucurbits may also develop a mild bitterness, although it is usually not severe enough to prevent gardeners from eating them.

Occasionally, a gardener will find a zucchini growing in their garden that is extremely bitter, as was the case in 2003 for one Dodge county, Nebraska gardener. Eating these vegetables caused severe stomach cramps and diarrhea that lasted several days. These symptoms were similar to 22 cases of human poisoning by bitter zucchini reported in Australia from 1981 to 1982, and in Alabama and California in 1984. The cultivar of zucchini grown in Dodge county was ‘Black Beauty’ and the cultivar implicated in Australia was ‘Blackjack’. In each case, very small amounts (3 grams) of the bitter zucchini were ingested. Extreme bitterness has also been documented in summer squash (Cucurbita pepo).

Of 12 zucchini squash plants grown by the Dodge county gardener, only 1 plant produced very bitter fruit. Since all plants in the garden originated from one seed packet, were planted in the same location, and received the same amount of water, simple environmental stresses could not be the cause. In fact, unlike cucumbers, extreme bitterness in zucchini and summer squash is not influenced by environment, but is genetically controlled by a single dominant gene.

Crookneck squash has had fewer complaints. Only one out of 3 million seeds sold. One explanation of low complaints is that most people spit it out before ingesting, perhaps only people that do not have bitter-sensitive taste buds or who use a lot of spices really get sick.