Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Early watermelons are being planted at this time. These early plantings often have variable crown sets and more hollow heart in the fruit. In early watermelons, the first flowers develop during colder weather conditions. Cold and other adverse weather conditions such as rainy weather and heavy winds during pollination can reduce fruit set or cause fruit development irregularities.
One issue is reduced pollen production. This is can happen when flowering is delayed in pollinizers, when pollenizers have to be replanted because of plant loss, when there are low male flowers numbers, or male flowers are damaged thus reducing the amount of pollen produced. To avoid these problems and improve early fruit set select earlier flowering pollenizers, select pollenizers with higher pollen production, or plant extra pollenizers to increase the pollen available in the field. Only plant well hardened-off pollenizers and use practices that insure good pollinizer survival during transplanting such as windbreaks, proper plant handling to maintain pollinizer vigor and proper planting to minimize plant damage to pollenizers.
Another issue is reduced pollen transfer. This happens when honey bee flights are reduced in the field due to adverse weather. Other factors related to bees such as reduced bee numbers, poor hive strength, delayed hive placement, and pesticide effects can reduce pollen transfer. To manage bee related pollen transfer issues set extra hives in early fields, work with bee keepers to have the strongest hives in for the earliest plantings, place hives at multiple locations (cold, windy, rainy weather keeps bees closer to the hive), and time placement at first flower opening.
Bumblebees may be an option for earlier plantings because they are more active in cold weather. However, mixing honey bees and bumblebees can be problematic, especially if placed too close to each other. Bumble bees should be placed as far from honey bee hives as possible. Honey bees are very resourceful and a bumble bee colony is a great source of pollen and nectar which honey bees are constantly seeking. Bumble bees are susceptible to honey bee robbing causing a weakened colony and overall loss in productivity from both pollinator species.
A final issue in early fruit set is reduce pollen viability or pollen germination and pollen tube development due to cold weather and high winds causing desiccating conditions. Managing for these conditions is best addressed by high, thick windbreaks (rye) between every row. Extra bee hives and extra pollenizers can also help by increasing pollen production and transfer.