Gordon Johnson, Extension Vegetable & Fruit Specialist;gcjohn@udel.edu
Growers with early-planted watermelons (those planted the last week in April or first week in May) have been concerned about the lack of crown set in those fields. Some early set watermelons also have distinctive lobes (are noticeably triangular) and standard seeded pollenizers are showing pinched ends.
These are signs that pollination was lacking during early fruit set. This can occur when there is a lack of pollen – pollenizers have not produced enough male flowers or are delayed in producing male flowers. Another explanation could be a reduction in bee activity during the cold periods we had this spring. Other possible causes would be delays in setting bee hives, damage to flowers, or chemical injury. Early plant stress can also cause abortion of flowers leading to reduced crown set. The earliest set watermelons often are misshaped and have a higher potential for hollow heart.
Management practices to improve early set include use of wind breaks, using pollenizers that produce large numbers of male flowers during the early period, using pollenizers that are the most stress resistant, and placing more bee hives. The University of Delaware vegetable program is currently conducting studies on over 30 pollenizers to evaluate pollen production and pollen viability in early planted conditions (cold weather) and late planted conditions (hot weather).