Some of the cucumbers I am growing and harvesting taste just fine, but some are really bitter—I wonder if I should even be eating them. What causes this, and are they safe to eat (not that I want to)?
What you are describing actually has a name, toxic squash syndrome. It can affect plants in the Cucurbit family (so not only cucumbers but also zucchini, winter squash, and even melons). Here is a Master Gardener article from the Sequim Gazette about cucurbitacin poisoning. A 2012 factsheet (no longer online) from Oregon Health & Science University about this problem says that the cucurbitacins produced by plants in this family may have benefits for the plants themselves, warding off insects. But in humans, excessive cucurbitacin can cause digestive distress. Wild plants tend to have higher levels of this naturally occurring substance than varieties bred for human consumption. Still, environmental factors (such as uneven watering or fluctuations of heat and cold) can cause normally tasty cucurbits to turn bitter.
A 2007 article from North Carolina Cooperative Extension, “What Makes My Cucumbers Taste Bitter,” says that cucurbitacin is mostly found in the leaves, stems, and roots of the plants but it can spread to the fruit as well. In your cucumbers, the highest concentration is likely to be in the skin and just below the surface of the skin. “Misshapen fruits are more likely to be bitter than well-shaped fruits. Some scientists even think that varying levels of fertilizers, plant spacing and irrigation frequency may also affect cucurbitacin levels. Bitterness seems to vary with the type of cucumber grown.”
Because of the potential for unpleasant side effects, I suggest not eating the rest of a cucumber (or any other member of the Cucurbit family) if the taste is bitter.