Experts and BHG readers answer.
Hollow fruits are a common problem. They usually result from a combination of nutrient deficiency and irregular watering (when the soil gets very wet for a while, then very dry). Keeping the soil consistantly moist, but not wet, will help. Too much nitrogen in the soil may make the fruits grow so fast that they "open up"-the inside can't keep up with the rest of the fruit. Reducing the amount of fertilizer would help if this is the case. Also, some cucumber varieties are more prone to the condition than others. Check the descriptions in seed catalogs or on seed packets to identify selections that better resist hollow centers. The hollow centers have little effect on the quality of the fruit, although affected ones are sometimes bitter as a result of the moisture stress.