Zucchini FAQ

Three questions and answers about growing zucchini


Why zucchinis can taste bitter


By Sandy Feather ©2008
Penn State Extension

Q. Out of the four zucchinis I planted this summer, three of the zucchini plants were fine, but the fourth produced fruits that were very bitter tasting and inedible. How can I prevent this from happening to my garden zucchini harvest again next year?

A. Zucchini is a member of the Cucurbitacea family, along with cucumbers, pumpkins, squash, melons and gourds. All members of the family produce chemicals called cucurbitacins that can cause the fruits to have a bitter taste. Usually they are present in such low concentrations that you do not taste them. Cucurbitacins are also the compounds that attract striped and spotted cucumber beetles to many members of this family.

Damages Done

These insects damage crops by direct feeding on foliage and flowers, and also by transmitting bacterial wilt. This bacterial disease kills the vines; there is no control beyond controlling the beetles. Lest you think all cucurbitacins are bad, know they are also responsible for the smell and taste of a good cantaloupe.

Environmental Stress

Cucumber fruits can produce high levels of cucurbitacins in response to environmental stress such as high temperatures and drought. Low soil fertility and low soil pH can also contribute to high cucurbitacin levels in cucumber crops.  However, high cucurbitacin levels in zucchini and other summer squash varieties do not appear to be a result of environmental stress, but rather the influence of a single gene. Assuming you grew and cared for all four plants the same way, your experience would seem to bear this out.


Zucchinis are members of the Cucurbitacea family along with pumpkins, cucumbers, squash, melons and gourds.

Plants that produce extremely bitter zucchinis are rare, but it does happen. If you are unlucky enough to have such a plant in your garden, tear it out and do not eat any of the fruit or give it away. Do not save any seed from this plant since it is very likely that the resulting plants would also produce bitter zucchini. A small number of cases of human poisoning from eating minute amounts of bitter zucchini have been reported in the United States and Australia.

There are wild members of the Cucurbitacea family that occur as weeds; they tend to contain very high cucurbitacin levels that render their fruit inedible. All members of this family depend on bees for pollination of the female flowers. If seed production fields are in proximity to wild populations of cucurbits, it is possible that bees could transfer pollen from the cucurbitacin-rich weeds to the seed fields. The resulting seeds would produce bitter fruit because the bitterness gene is dominant.

How can you tell?

There is no way to tell by looking if a zucchini plant will produce these bitter fruits, and no sure way to avoid the problem. Fortunately, it is not very common. If you save your own seed, make sure none of the wild cucurbits are growing near your garden. Prickly cucumber (Echinocystis lobata) and bur cucumber (Sicyos angulatus) are the wild cucurbits commonly found in Pennsylvania.


Why half-grown zucchinis rot on one end


By: Sandy Feather ©2010
Penn State Extension

Q. We are having trouble with our zucchini.  The plants look great but when the fruits reach four to five inches long they start to rot out at the blossom end.  We have tried one dose of Fung-onil (chlorothalonil) and that has not stopped it. Everything else in the garden looks fine so far.  What can we do to stop this problem with the zucchini? 

A. From your description, it sounds as though your zucchini are being affected by blossom end rot. Although we usually associate blossom end rot with tomatoes, zucchini and other varieties of squash are also susceptible. Despite the name, blossom end rot is a calcium deficiency in the developing fruit rather than a disease, so fungicides such as Fung-onil will not help the problem.

Blossom End Rot

Blossom end rot develops due to one or more of the following factors: calcium deficiency in the soil, drastic fluctuations in soil moisture, over-fertilization, or root damage caused by cultivation. Calcium may be present in your soil in sufficient quantity, but is unavailable to the plant due to lack of or excessive soil moisture, or because the fine feeder roots have been severed by hoeing near tomato plants. It is often worse on plants that are carrying a heavy crop of fruit because they have a higher need for calcium.

Soil Test First

To start, have your soil tested to make sure that soil calcium levels are adequate. Soil test kits are available from your local Penn State Extension office for a nominal fee. Send a check made payable to Penn State Extension to Penn State Extension, 400 North Lexington Street, Pittsburgh, PA 15208. Write Attn. Soil Test Kit in the lower left corner of the envelope. You may also pick them up in person, Monday through Friday from 8:30 a.m. to 4:30 p.m. Take the sample according to the directions in the kit, and send it to the soil analysis laboratory in University Park, PA.  Follow their recommendations for limestone and fertilizer applications. Sample of soil test results shown below:


Watering Zucchinis

Maintain even soil moisture by watering when there is not sufficient rain and by mulching your tomato plants. You may use an organic mulch such as straw, or an inorganic mulch such as black plastic. Mulching helps maintain soil moisture, moderate soil temperature, and it also keeps weeds under control.

Precautions when Cultivating

Be very careful about cultivating near squash, tomatoes, peppers and eggplants because you can sever fine feeder roots and make the plant think that it is under moisture stress - which will cause blossom end rot. It is better to hand weed around these plants since it does not cause as much disruption of desirable plant roots.

Easy on the 'N'

Zucchini are classified as light feeders, which means they do not require a high level of nitrogen fertilizer. Too much nitrogen results in a lot of vegetative growth and little or no fruit, and it also can cause blossom end rot because excessive nitrogen blocks the plant from taking up calcium.

There are calcium sprays available that can be helpful with tomatoes suffering blossom end rot, but they may not be as useful for squash and other susceptible fruits.


Preventing wilt damage next season


By: Sandy Feather ©2013
Penn State Extension

Q. Squash vine borers ruined my zucchinis this year. Can you recommend steps I can take to avoid this problem next year?

A. Squash vine borers are important pests of summer and winter squash and pumpkins, and to a lesser extent, cucumbers and melons. They overwinter as pupae in the soil under host plants, and adults become active mid- to late June or early July. There is a single generation yearly in Pennsylvania. Adult squash vine borers are clearwing moths that resemble wasps more than anything. Unlike most moths, they are active during the day. They have a one to one-and-a-half inch wingspan, with metallic green forewings. The rear wings are transparent, with black or brown margins and veins. The body is orange and black.

Colorful squash collection

Colorful squash collection

Life Cycle

Adult female moths lay their eggs on the main stems and sometimes the leaf stalks (petioles) in July and August. The oval eggs are reddish-brown and flat, and usually laid singly or in small groups. The small white larvae hatch in a week to ten days, and bore into the stem where they feed for about a month. The mature larva is a thick, white wrinkled worm with a brown head and is about one inch in length. They exit the stems and burrow into the soil to pupate.

The leaves on infested stems wilt when the borers’ feeding damage destroys the plant’s vascular system. Once that occurs, the plant is unable to take up sufficient water to support the leaves. Upon close examination, you can see frass – sawdust-like excrement - coming from holes in the infested stems.

Prevention is the Key!

Prevention is the best course of action since squash vine borers cannot be controlled with insecticides once they get inside the stems.  Crop rotation is important since squash vine borers overwinter in the soil under their host plants. Avoid growing susceptible crops in the same area from year to year. Also, be sure to remove spent cucurbit crops at the end of the season to remove any larvae that may still be present in the vines. Till up the garden to expose overwintering pupae to winter freeze and thaw cycles and possible predation. Be sure to mulch or plant a cover crop to avoid leaving the soil bare over the winter, which could allow erosion.

Cover transplants or seedlings with a floating row cover (Reemay, Garden Blanket) to exclude adults from laying their eggs on the plants. Allow enough excess material to allow for the growth of the plants and seal the edges with soil. The covers must be removed when the plants bloom because the flowers have to be pollinated in order for fruit to set.


Preventative insecticide applications are another option. Applications must be carefully timed to catch the larvae before they bore into the stems. Trapping adult squash vine borers is the best way to determine when to begin spraying and is relatively easy. The adults are attracted to the color yellow. Simply place a yellow container (pan, bowl or bucket) filled with water in your garden mid-late June. Check the trap daily, and begin making applications when you find adult squash vine borers in the trap. Azadirachtin (BioNeem, others), Carbaryl (Sevin, others), and spinosad (Captain Jack’s Deadbug Brew) are labeled to help control squash vine borer in the home vegetable garden. Repeat applications at labeled intervals will be necessary for control over the egg-laying period. Sprays should be directed toward the base of the plants and stems, rather than leaves.

Damage Control

Even if some borers have gotten into the stems, all is not lost. Find the entry hole, and slit the stem lengthwise with a sharp knife, then remove and destroy the borer. Squash often produce secondary root systems at the nodes on their stem. Push the slit ends together and cover the injured stem with soil. The plant may recover and still provide plenty of zucchini for you to enjoy.


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