I love hollow zucchini. I slice them in half, scoop out the center, and stuff them with cheese and herbs.
What I don’t like? Zucchini with hollow heart, where the fruit develops with a hollow center.
Not only are they often funky looking, but sometimes the fruit will even deflate, like a punctured football, if you leave them too long on the vine.
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Hollow zucchini is a disappointing problem that you might not even be able to identify until you heat up the skillet and start to slice up your zukes. That’s when you realize something has gone horribly wrong.
Unlike some issues, which can spell the end of your harvest, you can fix hollow heart and still have plenty of fruits to fill your (and your neighbor’s) refrigerator.
There are three main causes that we’ll go over. Here’s the lineup:
Causes of Hollow Zucchini
To understand why zucchini might develop with a hollow center, we need to do a quick rundown of zuke morphology.
Zucchini are classified botanically as fruits, which are mature ovaries. In all squash, the ovary forms below the floral parts, which is known as an inferior ovary.
Once the ovary matures, or the fruit forms, it is comprised of a mesocarp, or the flesh, the exocarp, or the skin, with seeds at the center.
When the fruit is hollow, it’s usually because the seeds didn’t develop properly, but there can be other causes as well.
In our guide to growing zucchini, we cover how to properly cultivate these plants in your vegetable garden.
And now let’s talk about why the fruits of your labor may end up with hollow heart, and what you can do about it.
1. Lack of or Incomplete Pollination
Zucchini flowers must be pollinated for the ovaries to develop into fruits.
Those that aren’t pollinated or are only partially pollinated will have unusual growth and may lack seeds. Any seeds you do find will be small.
Sometimes the first batch of zucchini fruits produced by the plant will be hollow and that’s because the flowers they developed from weren’t pollinated.
In many areas, spring tends to be wet, with lots of rain, and one thing you don’t see during wet weather is pollinators flitting around from flower to flower.
Frequent transitions between hot days and rainy, cold days can cause the pollen to clump or dry out.
As such, some flowers might not be pollinated properly during the spring.
The good news is that subsequent fruits should be just fine assuming that the pollinators have the opportunity to do their job.
If you want to be extra sure, you can hand-pollinate your summer squash plants. If you’ve never done this before, it’s actually a fun process. I find it meditative, calmly moving from flower to flower with the sun shining on my back and the bees checking out my progress.
To start, find a male flower. These can be identified by the presence of a stamen covered in fuzzy yellow powder – this is the pollen. If you need to move the petals to access the stamen, it’s fine to do so as it won’t hurt the plant.
Either pluck out the stamen or wipe a toothpick over it to pick up some pollen.
Next, identify a female flower. Instead of having a single stamen, the female flowers will have multiple stigma.
Wipe the stamen or toothpick on the stigma, transferring as much pollen as possible, and voila! The flower is pollinated!
You can read more about the process in our guide to hand-pollinating pumpkins.
2. Nutrient Deficiency
A plant that is deficient in nutrients while the fruit is developing won’t have the resources to enable the growth of plump, healthy zucchini.
A deficiency of boron and/or calcium in particular, can lead to hollow zucchini. Unfortunately, this isn’t an easy fix, but you can ensure that you are prepared for next season, if this was the cause of hollow heart in your current crop.
Regardless of whether you suspect pollination problems or water issues (discussed below) is the cause of your woes, I highly recommend you test your soil. It’s cheap and easy and it gives you invaluable insight into the health of your garden.
It can even save you money in the long run because you won’t waste time and money applying nutrients to your soil that it doesn’t need. And as a bonus, it’s better for the environment to only amend when you are sure there is a deficiency because excess fertilizer won’t be leaching into the water table.
Your local extension office provides testing services, or you can purchase test kits online.
I have used MySoil’s test kit in the past and loved it.
You send in a sample of the earth in your garden and within a week or two, they provide detailed results, including the levels of macronutrients and micronutrients, as well as soil pH.
The company then gives you fertilizer recommendations, including the amount per square foot to apply.
Or, you can adjust the size of the area and they will recommend what and how much to apply for your specific space.
I learned that the soil in my garden is naturally low in nitrogen but high in phosphorus and potassium, so I only needed to add nitrogen to my soil. The test paid for itself in fertilizer savings.
You can find MySoil Test Kit available via Amazon.
If your soil is significantly deficient it’s probably too late to save your current crop, but at least you know what to do for next year.
3. Water Issues
Zucchini’s worst enemies are drought and erratic watering.
The absence of consistent moisture can cause the fruits to develop irregularly, with some parts growing quickly and others growing more slowly.
As a result, the interior might not form uniformly, which often results in a hollow center.
If you catch the situation early, you can change up your watering routine to ensure that the soil stays consistently moist.
It should stay just moist, not waterlogged and not dried out. Allow the surface of the soil to dry before adding more.
Applying a layer of mulch helps, too. It reduces evaporation and helps the soil stay moist for longer.
Newly developing fruits should be normal so long as you stay on top of the water situation.
Fill ‘Er Up
The good news about hollow heart is that it generally resolves as the weather improves. You can help by maintaining soil moisture and ensuring your soil has the right nutrients.
So what do you suspect is the source of your woes? Did you have a super wet spring? Or did your plants dry out a bit during a hot spell? Let us know what you’re going through in the comments section below.
And if you need some additional help caring for your zucchini plants, we have several other guides that you might be interested in. Here are just a few:
