Cucumbers, like many other vegetables, contain the natural compound cucurbitacin, which is responsible for the bitter taste. The more stress a cucumber has, the more bitter it becomes.
You can avoid bitter cucumbers with these methods: Plant bitter-free varieties of cucumbers. Mulching will help maintain a constant level of soil moisture. If the cucumber still tastes bitter after harvesting, the Thuringia Consumer Advice Centre recommends cutting off the stem end and working towards the stem when peeling.