Watermelon with cracks or swirly pattern

Why do some watermelons have cracks or open swirl patterns?

To the average consumer, the inside of a watermelon usually looks like a uniform spread of crisp pinkish red fruit, interrupted only by seeds or the white traces in a seedless melon. 

But, actually? There’s a lot going on in those delicious oblong fruits. Imagine a melon cross section as a pie chart with three pieces. Scientists call each section a carpel (and every type of fruit has at least one and often several carpels). Within each watermelon carpel is a set of essential plant machinery: the locules, which are the little areas in which the seeds nestle (even a seedless melon still has locules); the placenta, which nourishes potential seeds; and the funiculi,* little umbilical cord-like threads that connect the placenta to the potential seeds. 

Normally, regular old watermelon tissue, known as mesocarp, fills in around all these various components, fusing them together so the average watermelon eater doesn’t notice distinctive parts, but rather just sees sweet and juicy melon. 

This can all change, however, if something doesn’t go right in the field, particularly if melons aren’t well pollinated. Not enough pollination means that not enough cell growth is triggered, Gordon Johnson, a retired extension specialist and adjunct professor in plant and soil sciences at the University of Delaware, explained to me. An insufficiently pollinated melon can then fail to connect all of the carpels. Sometimes the outlines of the other parts, i.e. the placenta and the funiculi, are also disconnected or more loosely connected, making them more visible. 

So, the melon has a condition known as hollow heart. And, honestly? I think it’s kind of pretty.

(Other types of produce can develop their own versions of hollow heart; check out what can happen to potatoes and cucumbers.) 

Watermelons with internal cracking often reveal three sections

In a classic case, the watermelon will divide into thirds. You may also see the curls and swirls of the placenta and funiculi, which, to me, resemble handlebar mustaches. Sometimes the cracks appear a little more random and less elegant; especially if you slice the melon lengthwise.

Are watermelons with cracks inside still safe to eat?

Yes! As long as the crack is internal to the fruit (and the exterior isn’t injured), this is purely a physical defect. The inner cracks and swirling pattern don’t indicate microbial growth, nor would they make it more likely for human pathogens to be inhabiting your melon. 

The question is, will it be worth eating?

In minor cases, in which you only see some slight gaps, it has a minimal impact on quality. “In fact,” Johnson wrote in an email, “it makes it easy to cut the watermelon in three sections!”

Penelope Perkins-Veazie, a horticultural science professor at North Carolina State University, said watermelons with mild hollow heart might also be sweeter.

But in more severe hollow heart, quality suffers. The melon will probably be drier. Sometimes hardened tissue forms on the borders of the unfused areas, Johnson noted.  

Can you tell if a watermelon has hollow heart from the outside?

Generally no, but in more serious cases the whole watermelon can look triangle-shaped, lumpy or asymmetrical. 

Mini watermelons are less susceptible to hollow heart

This disorder occurs nearly exclusively in seedless watermelons, which are the vast majority of watermelons sold in the United States. Among seedless varieties, it’s less common in personal or mini sizes, simply because they contain less fruit, Johnson told me. But, he said, when he dramatically limited pollination in experiments, he was able to create hollow heart in mini melons.

Yellow and orange watermelons, and less dense red melons, are more vulnerable to hollow heart

Hollow heart appears more in yellow- or orange-fleshed watermelons, not because of their color, but because they tend to be less dense. “The less dense a variety is, the more susceptible to hollow heart,” Johnson wrote. “Current yellow and orange varieties have low density, maybe newer varieties will be more dense and less susceptible. Low density red varieties are also more susceptible to hollow heart.”

What causes hollow heart in watermelons?

Poor pollination is believed to be the primary cause of hollow heart. 

A flower growing on a seedless watermelon vine must receive pollen from a seeded watermelon variety flower to bear fruit. So, Perkins-Veazie explained, farmers plant seeded watermelons (often orange-sized melons so stuffed with seeds they’re inedible) near their seedless watermelons. They typically plant one seeded watermelon plant for every three seedless ones and bring in bees to ensure the pollen is distributed.  

In 2010, as farmers struggled with seemingly random bouts of hollow heart, Johnson experimented. He planted seeded melons at increasingly long distances from the seedless crops to see if it affected the rate of hollow heart. Indeed, longer distances increased rates of hollow heart. And really, anything that thwarts pollination, from where the seeded watermelons are planted to rainy or cold weather that prevents the bees from doing their thing, can lead to hollow heart. While Johnson said that pollination appears to be the main trigger for the disorder, excessive water and fertilizer can also prompt hollow heart, by triggering an artificial growth spurt that the melon’s inner flesh can’t keep up with. 

*Fun fact!  The word funiculi is related to another fun word, funicular (the cable car that goes up and down mountains), both from a Latin word meaning “small rope.”