Cucumbers plants that are stressed during the growing season may produce fruit that is bitter flavored. Commonly a lack of water or temperatures too cold or too hot cause cucumbers to bear bitter tasting fruit.
But some cucumbers may have a slightly bitter flavor by nature. Cucumbers contain organic compounds called cucurbitacins that can cause fruit to taste bitter. Low levels of cucurbitacins are not detectable, but high levels make fruits taste bitter. Cucurbitacin levels may increase with environmental stress during the growing season.
Tips to Avoid Bitter Tasting Cucumbers:
To avoid bitter flavored cucumbers, plant varieties that have very low levels of cucurbitacins or give cucumbers optimal growing conditions. Here are suggestions for optimal cucumber growing and also a list of cucumbers that are usually not bitter tasting:
Site. Plant cucumbers in a sunny spot in soil rich in organic matter and well drained. Raised beds or mounds are ideal for growing cucumbers; the soil will warm early in the season and stay warm. Work several inches of aged compost and aged manure into the planting beds ahead of sowing or transplanting. During the season, sidedress plant with aged compost. Compost is nutrient rich and moisture retentive.
Give cucumbers plenty of room to grow; trellised or caged cumbers should be spaced 8 to 12 inches (20-30 cm) apart. Space hills for growing cucumbers at least 3 feet (91 cm) apart.
Cucumber Planting. Sow seed or set out cucumber transplants after all danger of frost has passed and the soil has warmed to 60°F (16°C). Frost can stress cucumbers. If there is a danger of frost once cucumbers are in the garden, protect plants with floating row covers.
Water. Give cucumbers plenty of water; do not let the soil go dry especially while they are flowering and fruiting. Water stress during the early stages of growth will cause bitter-tasting compounds to concentrate in the fruit. Water cucumbers deeply once or twice a week or place plants on a drip so that the soil stays moist but not wet. Use your finger to measure soil moisture; the soil should not be dry deeper than 3 inches below the surface.
Mulch. Once the soil has reached 70°F (21°C), reduce soil moisture evaporation by mulching plants with an organic mulch or black plastic. Mulch will also reduce weeds which compete for soil moisture and nutrients.
Protect cucumbers from high temperatures. Temperatures consistently in the mid-90s or warmer can stress cucumbers. Provide filtered afternoon shade to help cool the garden; plant cucumbers to the south of tall crops such as corn or sunchokes or place a frame and shade cloth with a 40 to 50 percent block of sunlight over cucumbers.
Cucumber Harvest. Pick cucumbers at their optimum size and pick them frequently. Cucumbers should be ready for picking 50 to 70 days after planting. When the cucumber drops its flower at the blossom end of the fruit, the fruit is ready for harvest. Cucumbers are less tasty when they grow too big.
Know the mature size of the cucumbers you are growing: about 6 to 8 inches (15-20 cm) for American slicers, 4 to 6 inches (10-15 cm) for Middle Eastern types, 3 to 5 (7-13 cm) inches for pickling types; 8 to 12 inches (20-30 cm) for Asian varieties.
Serving. Bitterness concentrates in the stem end and skin of the cucumber. Peel the fruit and cut off the stem end by an inch or two to reduce bitterness at serving time. Rinse your peeling knife after each slice so that you do not spread the bitter taste.
Cucumber varieties. Choose cucumber varieties that are not bitter flavored. The level of curcurbitacins in cucumbers varies by variety but also from plant to plant and even fruit to fruit on the same plant. (An enzyme called elaterase also present in cucumbers can reduce the amount of cucurbitacins but the amount of elaterase can vary from season to season and plant to plant as well.)
Cucumber varieties with low levels of cucurbitacins include Jazzer, Holland, Lemon, Aria, and Marketmore 97. Keep a garden journal and note varieties you have grown that were not bitter tasting.
Best cucumber growing tips at How to Grow Cucumbers.