Why are Cucumbers Bitter?

It's the rare gardener who hasn't experienced growing a bitter cucumber and wondered why it happened. Few things are as frustrating as tending your vegetables all season long, only to finally harvest them and find out they don't taste very good when you get them to the table. Cucumbers are known for being prolific, sometimes to the extreme. But what good is a bounty of cucumbers if they aren't tasty?

The actual cause of bitterness in cucumbers (which also occurs in other gourds, like zucchini,) is due to the compound cucurbitacin. It's always present in cucumbers, but the levels change based on the vegetables' growing process. The higher the level of cucurbitacin, the more bitter the cucumber will be.

Unfortunately, you can't tell if a cucumber is becoming bitter while it is still growing and you have time to do something about it. Usually, the bitterness is caused by some climate or soil condition that could be easily solved. Thus, it's important to take preemptive steps to keep them from becoming bitter in the first place.

What Makes Cucumbers Bitter?

Cultivated cucumbers all contain cucurbitacin B and cucurbitacin C, compounds that are supposed to make their leaves bitter and less tasty to munching animals. These compounds are usually confined to the leaves and stems of the plants, parts of the plant humans don't eat, so we are not aware they are there. It is only when they move into the fruits that we start detecting a bitter taste. Usually, it is not the whole fruit that turns bitter. More commonly, the bitterness will be concentrated at the stem end and the area right under the skin.

Preventing Bitter Cucumbers

There is still some disagreement about what causes the bitterness to spread into the fruits, but it seems to point to types of stress while the cucumbers are growing. So although we cannot correct the problem after the fact, we can try and avoid the following 3 growing conditions that are potential culprits of bitter cucumbers.

1. Dry Conditions: Long periods of hot, dry weather can contribute to bitter cucumbers. Keep your cucumbers well watered to offset the tendency for them to turn bitter. Give them a deep soaking of at least an inch of water per week, more during extreme dry spells. Consider adding drip irrigation to ensure the plants receive adequate, consistent moisture. Stick your index finger into the soil an inch deep. If it's moist, no need to water. If the soil is dry, time for a drink. Add mulch to the area around the roots when temperatures rise to help conserve soil moisture and prevent weeds. When temperatures rise to mid-90s, consider adding a shade cloth to provide filtered light to the plants during the hottest part of the growing season. Extremely hot temperatures stress the plants, which encourage bitterness.
2. Lean Soil: Another factor in bitter cucumbers is lean soil and a general lack of nutrients. Cucumbers are heavy feeders, and a soil rich in organic matter will go a long way toward producing less stressed, better tasting cucumbers. Add compost when preparing beds, and then side-dress the plants throughout the growing season. If your soil is less than ideal, give your cucumbers balanced fertilizer every 4 to 6 weeks. Consider planting cucumbers in raised beds or mounds--both options allow the soil to warm earlier in the season, helping young plants avoid stress brought on by cool soil. Less stress = less bitterness.
3. Too Much Competition: As heavy feeders, cucumbers dislike competition for water and nutrients, whether it's from other cucumber plants or weeds. Keep the bed weed-free, and space the cucumber plants appropriately when sowing seeds in the spring. Plant trellised or caged cucumbers 8-12 inches apart, and increase the distance for mounded plants to 3 feet apart. With plenty of room to grow and available nutrients, well-spaced cucumbers tend to avoid bitterness.
4. Lack of Sun: Overcast areas, like the Pacific Northwest, have reported bitter cucumbers due to lack of sun. Again, you can't control the hours the sun will shine, but you can plant in a spot that gets as much sun as nature will allow. If it's cool and damp, as well as overcast, growing your cucumbers undercover, like a polytunnel, will amplify the available heat and light. In the garden, avoid planting cucumbers in areas that will be shaded by taller crops, unless you live in an extremely warm climate--a little afternoon shade may help avoid bitterness during the hottest days.

So even though cucumber plants grow rather easily, and you can get a prodigious harvest from a couple of plants, to get quality as well as quantity, you need to provide them with good growing conditions: plenty of sunshine, regular water, and rich soil.

Finally, look for varieties that are well-suited to your area and that are labeled "non-bitter". Some reliable varieties are 'Marketmore 97', 'Diva', 'Eversweet' (any variety with "sweet" in the name), 'Long Green Improved', and the heirloom 'Lemon'.

As with all plants, edible or otherwise, the real trick to healthy, productive cucumber plants is to research what growing conditions the plant prefers and do your best to provide them. Even a few days of stress can cause a ripple effect of damage. Ornamental plants will probably recover, but you only get one chance to get it right with cucumbers and other edible plants. That's why it is so important to put some thought into choosing both your growing site and your edible varieties.

What to Do with Bitter Cucumbers

If you find yourself with bitter cucumbers, don't automatically reach for the compost bucket. Peeling the fruit should improve the flavor. Cut off the first inch or two from the stem end and test the flavor. If it's still bitter, try a slice toward the center of the cucumber and see if it is sweeter. You should be able to salvage more than enough for a salad.

And as soon as you notice a bitter cucumber, take the precautions above to ensure the rest of your harvest doesn't suffer the same fate.