Hollow Heart in Watermelon

Gordon Johnson, Extension Fruit & Vegetable Specialist; gcjohn@udel.edu

The first watermelons will be transplanted in the field the last week in April. One problem with seedless watermelons that can cause significant loss of marketable fruits is hollow heart. This interior separation of fruit storage tissue is most common on crown sets in the first harvests.

In the past, the cause for hollow heart was thought to be related to rapid growth of the fruit where the rind expanded faster than the internal flesh leading to separation of the three internal fruit compartments and an open area between. Excess nitrogen and over-watering along with favorable growing conditions were implicated in higher incidence of hollow heart.

There is growing evidence that hollow heart is not directly tied to nitrogen and water management but is related to pollination and weather conditions during pollination. Plant hormones are thought to be important in this effect. Several researchers have found no increase in hollow heart with increases in nitrogen; even in varieties know to have hollow heart problems. It is thought that with inadequate pollination, there is reduced release of the plant hormone that controls the development of storage tissue leading to hollow heart.

The first flowering and fruit set in watermelons often occurs in periods of stress with cold conditions. Cold, rainy weather during pollination will also reduce bee flights and may be a causal factor. In addition, some varieties are more susceptible to hollow heart, although hollow heart is wide spread across varieties in some years.

What can watermelon growers do to reduce hollow heart? First, it is important to choose varieties that are less prone to hollow heart for early plantings. Second, make sure that pollinizers in early plantings are in synch with the seedless varieties (there is plenty of pollen for the early sets). A higher pollenizer to seedless ratio may be warranted for the earliest plantings. Third, make sure that you have strong honey bee colonies and consider increasing colonies in the early plantings.

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