Hollow heart is a condition in watermelons that causes unfilled spaces in the flesh, and it can also leave growers feeling a bit empty in the wallet.
The condition can severely reduce the marketability of watermelons, and the problem was a main topic during a session on vine crops Feb. 10 at the virtual Mid-Atlantic Fruit and Vegetable Convention.
Seedless watermelon varieties — which total 95% of U.S. production — are particularly vulnerable to hollow heart, as are varieties that don’t have dense flesh.
Inadequate pollination is the primary cause of the problem, said Gordon Johnson, an Extension fruit and vegetable specialist at the University of Delaware.
Seedless varieties see the greatest problems because they do not produce viable pollen. Seeded watermelons have to be planted among the seedless to provide the pollen.
Cold spring weather that reduces the activity of bees can also contribute to hollow heart. Still, producers aren’t completely at the whim of nature.
Seeded watermelon varieties don’t all produce their pollen at the same time. Growers should select an early producer and one with extended pollen production to cover the entire pollination period, Johnson said.
Growers should avoid planting too many early pollen producers because they will be active during cool weather, when pollination is reduced.
But growers also should not skimp on their pollen sources, which can increase hollow heart in varieties with low-density fruit. Johnson recommends one seeded watermelon plant for every three seedless plants.
While the seedless watermelons are the focus of these fields, some growers sell the seeded ones from the pollen plants as a side benefit, Johnson said.
Still, the seeded watermelons should be planted in the rows with the seedless ones, not off to the side, according to Anthony Keinath, a Clemson Cooperative Extension pathologist.
Bees tend to work up and down rows, and they don’t fly very far across rows. If the seeded watermelons were all in one place, a grower would probably get good pollination in only the three to six nearest rows.
“It’s better to put them in the same row and you’ll get more transfer of pollen,” Keinath said.
Also during the conference session, Petrus Langenhoven of Purdue University discussed what consumers are looking for in a melon.
Price and a lack of bruises are two of the biggest factors in shoppers’ buying decisions, and flavor, freshness and ripeness are the most important qualities of the fruit.
Determining when a melon is ripe before picking, however, is often the biggest challenge for growers, said Langenhoven, a horticulture specialist.
“If they don’t slip, it’s very hard to tell. Sugar Cube, for instance, has little green spots, and as the melon matures the green spots become lighter,” he said. “That’s the perfect time to cut that melon.”
Sugar Cube grows well in an open field but doesn’t develop firm flesh or high sugar content when grown in a high tunnel.
There are other specialty melons that thrive under cover, a growing method that can help producers increase acreage and optimize production.
“When cantaloupes and melons are grown on trellises on raised beds with bumblebees for pollination, high tunnels can also minimize the impact of plant disease and foodborne illness,” Langenhoven said.