What’s Wrong With My Zucchini?

Once zucchinis get started, they’re just about the fastest-producing vegetables in the home garden, from bloom to harvest in less than a week. Photo: insteading.com

I don’t know what’s going on this year, but for some time my inbox has been filled with questions about zucchinis (courgettes). Not the same question, but different ones, so there is obviously more than one thing going wrong with them. Let’s therefore take a closer look at this popular vegetable and answer some of the questions.

What Is a Zucchini? 

A zucchini is a type of squash (Cucurbita pepo). Although squashes are originally from Mesoamerica, the variety known as zucchini was developed in Italy at the end of the 19^th century. The name means “little pumpkin,” from “zucca” (Italian for pumpkin). It can be green, striped or yellow and is usually long and narrow, but there are also round zucchinis. Some old-fashioned varieties are ribbed, but most zucchinis today are ribless. And most modern varieties have spineless leaves.

The plant and its fruit are called zucchini in Australia, Canada and the US, but the French name, courgette (“little squash”) is more common in the British Isles.

The zucchini is a summer squash, that is, one developed for eating in its immature stage, in summer, while its skin is still thin and edible and its seeds still tiny and indistinct. It’s not the only summer squash: others include pattypan, crookneck and straight neck squash. Winter squash, such as pumpkins, spaghetti squash, turban squash and butternut squash, are close relatives, but are harvested in late summer or fall when their skin is tough and inedible and their seeds are large and mature.

The modern zucchini is considered a “bush squash”: it doesn’t produce the long, wandering stems of vining squash, like pumpkins, but rather a short stem rather like a rosette, almost entirely covered with leaves. As such, it takes up much less space in the garden than most other squashes, although it’s still a big plant, reaching about 3 feet (90 cm) in diameter.

Zucchini prefers full sun (or nearly full sun) and rich, evenly moist soil. The more compost you add, the better it tends to grow. 

It likes warm weather and is intolerant of frost and cold nights, so don’t sow it too early in the season. It’s one of the last vegetables to start outdoors. Aim for a minimum soil temperature of 60 °F (15 °C) before starting it outdoors. Even then, the plant really doesn’t gain momentum until the soil reaches 72 °F (21 °C) or so.

Zucchini can be sown directly outdoors or, in short-summer areas, started indoors in biodegradable pots about three weeks before the expected transplant date.

It is important to always practice crop rotation with zucchinis, because they’re sensitive to certain insects and diseases that overwinter in the soil. Most gardeners follow a 4-year program, avoiding planting zucchinis where it or any other cucurbit (melon and cucumber, for example) was grown the three previous years.

Questions About Zucchinis

Here are the most common questions I receive about zucchinis:

1. My zucchini isn’t blooming. Why not?

This usually occurs when the plant lacks light or the soil is constantly soggy. Next year, give the plant full sun and well-drained soil.

2. My zucchini produces flowers but no fruit. Why not?

This is the crux of the matter and merits a more thorough explanation. 

You see, in the vegetable kingdom, most flowers are bisexual, bearing both male and female parts, but this is not the case with squash. It produces separate male and female flowers on the same plant (the botanical term is monoecious). 

They are easy to tell apart: the female flower bears, at its base, an ovary in the shape of the fruit to come. The male flower has no ovary at the base: it is attached directly to the peduncle. At its center is a stamen covered with yellow pollen; if you touch it, the pollen sticks to our finger. 

There are many more male flowers than female flowers overall and, on some days, there may be no female flowers at all, especially at the beginning of the season. So, if you’re not seeing fruit yet, it might just be a question of waiting a bit longer until the female flowers start to appear.

Of course, to produce fruit, pollen must be transported from the stamen of a male flower to the stigma of a female flower. The pollen can come from a male flower of the same plant or that of another zucchini (or another squash).

Normally bees—honeybees or solitary bees—are the main pollinators, but if they’re absent or not visiting the flowers often enough (one study showed that it usually takes 15 bee visits to properly fertilize a female flower!), the fruit won’t be properly fecundated. This can happen during stormy or rainy weather or during heat waves: times when bees are not very diligent about visiting blossoms. Or there may simply not be enough bees in your neighborhood, a problem more and more people are reporting. 

If so, you can pollinate the flowers yourself. You’ll need to check your plant daily and pollinate when a female flower opens. You can’t put it off, because the flowers, male or female, last only one day.

To pollinate a female flower, harvest a male flower (one no ovary at the base), remove the yellow corolla (petals) and use it as if it were a brush, touching the stamen covered with pollen to the center of female flower, covering its stigma with pollen. This needs to be done in the morning or in the early afternoon, because the day’s flowers often close early, especially during hot, sunny weather. And, by the way, one visit is enough. As long as you coat the stigma with pollen, you don’t have to visit 15 times like a bee!

3. Why do my zucchinis fall off before harvest?

They were probably not fertilized or not fertilized thoroughly enough and are being aborted. On a cloudy, rainy or very hot day, pollinate the female flowers yourself.

4. Why do my zucchinis get become so huge?

Zucchinis are designed to be harvested young, when they are not more than 8 inches (20 cm) in length. At this point, their skin is thin and edible and the seeds are only tiny dots. If they are allowed to mature more, they change in taste and texture, the seeds develop (you then have to scoop them out before serving), and the skin toughens and becomes inedible. In addition, if you make a habit of visiting your zucchini plants every two or three days and picking all the fruits that are ready, this constant harvest stimulates the plant to produce more fruits. When s zucchini fruit is allowed to remain on the plant and start to ripen, the plant stops producing new ones.

5. Is it true that zucchini flowers are edible?

Absolutely. And you can harvest them without reducing the harvest. This is because there are many, many more male flowers than female flowers; plus the male flowers become useless once fertilization is carried out. Harvest as many male flowers for the table as you want. Traditionally, squash flowers are eaten stuffed or after dipping them into tempura batter.

6. There are insects in my zucchini plants. What should I do?

Obviously, if the insects are bees or other pollinating insects, just leave them alone and let them do their job. However, zucchini are susceptible to several pests that attack cucurbits (squash, melons, cucumbers, etc.), such as cucumber beetles, squash bugs and squash borers. They tend to be most common in long-standing vegetable beds where rotation wasn’t carried out. 

First, treat visible insects with insecticidal soap or neem oil or hand pick them, but that is a stopgap measure. It’s far better to prevent these insects than to treat them.

For example, you can plant zucchini varieties that are pest resistant such as ‘Black Jack’, ‘Dark Green’, ‘Dark Green Embassy’, ‘Green Eclipse’, ‘President’, ‘Seneca’, ‘Senator’ and ‘Super Select’.

If you had an insect problem the previous year, make sure you do a crop rotation and, moreover, cover the seedlings with floating row cover. It will act as a barrier against pests. Remove the cover when the plant begins to bloom to give pollinators access to the flowers. By this point, the predator has often already gone elsewhere.

If the problem persists despite the above efforts, a more radical treatment may be necessary. The following year, don’t grow any squash or cucurbits (melons, cucumbers, etc.) at all. With no host plant to feed on for an entire year, the insect will either die or go elsewhere. That way you can make a clean sweep for the next year’s crop.

7. The foliage of my zucchini seems covered with white powder. Why?

This is the typical symptom of powdery mildew, by far the most common zucchini disease. It may look horrifying, but it’s rarely very harmful, because it tends to occur at the end of the season when the harvest is almost over, attacking older leaves but leaving the younger ones alone long enough to carry on the necessary photosynthesis. 

Crop rotation will help prevent powdery mildew and, as with most plant diseases, be careful to moisten the soil, not the leaves, when you water. But the best solution is to sow disease-resistant varieties of zucchini. In seed catalogs, look for the letters PM after the name: they indicate varieties that are naturally resistant to powdery mildew. 

Here are some varieties you might want to try: 

‘Ambassador’
‘Astia’
‘Black Coral’
‘Cash Machine’
‘Dario’
‘Desert’
‘Dunja’
‘Emerald Delight’
‘Endeavor’
‘Envy’
‘Esteem’
‘Everglade’
‘Felix’
‘General Patton’
‘Golden Glory’
‘Goldy’
‘Green Machine’
‘Hurakan’
‘Judgment III’
‘Limon’
‘Midnight Lightning’
‘Obsidian’
‘Pascola’
‘Paycheck’
‘Payload’
‘Payroll’
‘Respect’
‘Reward’
‘Sebring Premium’
‘Seduction’
‘Spineless’ King’
‘Spineless Perfection’
‘Spineless Supreme’
‘Yellowfin’
‘Ziti’

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And there you go: the basics of zucchini culture. Don’t be put off by the apparent complications: most people find zucchini to be easy to grow and, indeed, one of the most productive vegetables in the garden.