Zucchini, along with cucumbers and other relatives, produce an intensely bitter group of compounds known as cucurbitacins. Wild types of squash and curcurbit crops are so bitter they are almost inedible to humans and most animals. The cucurbits we usually grow in the garden have been selected for low levels of these bitter compounds. However, even carefully selected varieties will produce high levels of cucurbitacins when the plant is stressed by environmental factors such as too little water or too much heat. Larger or overmature squashes or cucumbers will have higher levels of the compounds than smaller fruit. Natural cross-pollination with wild gourds can also increase the bitterness of cultivated squashes.
The best strategy to reduce the bitterness of these cucurbit crops is to water the plants. Remember to mulch the plants to reduce moisture evaporation and, therefore, the need to water. The time of harvest is important, too. Very little time is allowed between peak harvest and overmaturity of cucumbers and squashes, so observe your plants daily and harvest for top quality.
