Bitterness in cucumbers is caused by the compounds cucurbitacin B and cucurbitacin C. The cucurbitacins are normally found in the leaves, stems and roots of the cucumber plant. The cucurbitacins spread from the vegetative parts of the plant into the cucumber fruit when the plants are under stress. Hot, dry conditions are usually responsible for bitterness in cucumbers in Iowa.
Bitterness does not accumulate uniformly in the cucumber fruit. The cucurbitacins are usually concentrated at the stem end of the fruit and just under the skin of the cucumber. Bitter cucumbers can sometimes be salvaged by cutting off the stem end and peeling the remainder of the fruit.
Cucumber varieties differ in their tendency to be bitter. Varieties that usually experience few problems with bitter fruit include Sweet Slice, Sweet Success, and Marketmore 76.
Watering cucumber plants once a week during hot, dry weather may also be helpful.