Summer is in full swing and so is harvest in the vegetable garden with the first summer squash, zucchini and cucumbers. In many small towns, soon you won’t be able to leave your car parked with the windows rolled down or you might come back to find the front seat full of zucchini or cucumbers!
Both cucumbers and zucchini are members of the Cucurbit family, which also includes pumpkins, melons, squash and gourds. Bitterness is a common problem of zucchini and cucumber and very frustrating to gardeners who have carefully tended the plants for several weeks, only to find the vegetables too bitter to eat. What causes this bitterness?
Cucurbitacin
All cucurbits produce chemicals called cucurbitacins, which cause the vegetables to taste bitter and served as a defense against plant-eating wildlife. Cucurbitacin also contributes to the musky scent of cantaloupe. Naturally, higher cucurbitacin concentrations cause vegetables to be more bitter. Wild cucurbits, including wild cucumber and buffalo gourd, contain such higher levels they are inedible.
In commercially cultivated cucumbers and zucchini, plants have been bred with low cucurbitacin concentrations making the vegetables more tasty for us.
The best size for harvesting regular cucumbers depends on how they will be used - sweet pickles, dill pickles or fresh slicing.
Mild Bitterness
Environmental stress - high temperatures, wide temperature swings or too little water - can cause mild bitterness. This is a common occurrence, especially in later summer, and is caused by higher cucurbitacin levels triggered by environmental stress. Uneven watering practices (too wet followed by too dry), low soil fertility and low soil pH are also possible stress factors.
If environmental stress is the cause, it would be natural for all plants in the garden to have the same level of bitterness. At least until the weather cools or the gardener steps up their watering. Then bitterness levels on newly developing vegetables would decline.
Over mature or improperly stored cucurbits may also develop mild bitterness. Proper harvest size of cucumbers is determined by how they will be used – sweet pickles 1.5-2”, dill pickles 3-4”; fresh slicing 7-9”; burpless 1-1.5” diameter and up to 10” long. Store at 50-55°F, 90-95% humidity for 10-14 days.
Zucchini should be harvested when 1.5” diameter and up to 4-8” long. Store at 40-50°F, 90% humidity for 5-14 days.
In most cases with mild bitterness, it is not severe enough to prevent gardeners from eating the cucumbers or zucchini. Removing the outer skin is usually enough to moderate the flavor.
Bitterness in zucchini can be caused by environmental stress (mild bitterness) or plant genetics (severe).
Severe Bitterness
However, occasionally a gardener finds a cucumber or zucchini growing in their garden that is extremely bitter. Eating these vegetables can cause severe stomach cramps and diarrhea lasting for several days.
This happened to an eastern Nebraska gardener I assisted several years ago, who’s symptoms were similar to twenty-two cases of human poisoning by bitter zucchini reported in Australia from 1981-1982, and in Alabama and California in 1984. The variety of zucchini grown in Nebraska was ‘Black Beauty’ zucchini and the variety implicated in Australia was ‘Blackjack.’ Very small amounts (3 grams) of the bitter zucchini were ingested.
Of 12 zucchini grown in the Nebraska garden only 1 plant produced very bitter fruit. Since all plants in the garden originated from one seed packet, were planted in the same location and received the same amount of water, simple environmental stresses could not be the culprit.
Unlike cucumbers, extreme bitterness in zucchini and summer squash is not influenced by the environment, but is completely controlled genetically by one dominant gene. How do these plants with extremely bitter fruits happen? High cucurbitacin levels are still present in wild cucurbits, including weeds like buffalo gourd. Since cucurbits are pollinated by insects, especially bees, cross-pollination of cultivated plants by wild cucurbits (in the form of weeds growing near a seed production field) can cause problems if seeds from those plants are saved and planted in the garden the following year.
Fruits from the parent plant would not be affected and would not exhibit bitterness. Only seeds resulting from the cross-pollination with a wild cucurbit or gourd would carry the gene for extreme bitterness. Plants grown from these seeds could express the dominant gene for fruit bitterness. Rarely, mutations in seed-production fields could also result in seeds that carry the dominant bitterness gene.
What should you do if you find extremely bitter zucchini in your garden? Well, you’re unlucky since these plants are rare, but don’t eat them and don’t give them to your neighbors! Discard them.
Finally, don’t save seed from plants that produced extremely bitter fruits. If you like to save your own seed, be sure to save fruit only from flowers that have been isolated to ensure that pollen came only from other domesticated, non-bitter cucurbits and not from gourds or wild cucurbits.