Improving Early Fruit Set in Seedless Watermelons

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

The first watermelon plantings have gone in across the region. Markets for early watermelons are normally the strongest so early planting is often more profitable. However, fruit set is often below desired levels in the earliest plantings and crown sets in early plantings often have quality issues such as higher levels of hollow heart.

The following are some considerations for managing watermelons to maximize early fruit set:

1) Get plants off to a good start with a minimum of stress. In early plantings always plant on a warming trend where temperatures are expected to increase and skies are mostly clear. Black plastic mulch will then allow soils to accumulate heat and roots will be able to establish more quickly. Use every row rye windbreaks (or clear row covers if windbreaks have not been planted) to reduce heat losses and protect plants. Plant well hardened off plants and train transplanting crews to handle plants carefully with a minimum of damage. Provide adequate water at planting and avoid putting excess starter fertilizers in transplant water which can cause salt stress on plants. Manage early fields more intensively by monitoring irrigation and fertigation programs so that stress is reduced throughout the growing period. Extra nitrogen can delay flowering so there is a fine balance between promoting growth and initiating flowering. Avoid practices that put extra stress on plants and be careful of phytotoxicities with misapplication of foliar fertilizers, fungicides such as copper products, and herbicides (proper shielding when spraying row middles, follow label guidelines for herbicides). Manage windbreaks so that mites do not infest watermelons when they are terminated. Manage insecticide applications so that bees are not affected during flowering (see pollinator protection information on labels).

2) Manage pollinizer-seedless combinations for maximum pollination potential. Loss of pollenizers after planting will reduce fruit set. This has been a problem in the past when pollenizers were not hardened off properly because they were seeded later in the greenhouse. In-row pollenizers should be used to achieve best early fruit set. Pollenizers should be chosen so that they are flowering adequately as the seedless come into flower. Pollen is the key for early fruit set and earlier flowering pollenizers should be used to improve crown sets. A case can be made also for increasing the number of pollenizer plants for the earliest plantings. A 1:3 ratio of pollenizer to seedless should be the minimum used and extra pollenizers that flower early could be planted at intervals to provide additional pollen. Another issue is the vigor of pollenizers. Make sure that pollenizers have good disease packages. In fields with a history of Fusarium wilt, Fusarium resistance in both pollenizers and seedless is needed. If at all possible, place early plantings in fields with little or no history of watermelon production to avoid soil borne disease stress.

3) Manage pollinators so that pollen is transferred effectively and in adequate quantity. Consider placing extra hives in early plantings. Have hives set when pollenizers are 10% in bloom so bees start to work fields immediately. If there are not enough bees when first female flowers open, you will lose much of the crown set. Avoid having flowering crops nearby that are more attractive to bees and could siphon off bee activity. Fruit set is often reduced when weather conditions at first flowering is rainy and windy or night temperatures are cold. Honey bees rarely work when the temperature is below 57°F and don’t fly when the temperature is below 55°F. They do not forage in rain or in wind stronger than 12 mph. Cloudiness also reduces flight activity, especially near threshold temperatures. A cold spell in June can reduce fruit set significantly because of reduced bee flights. While honey bees can work over a 2 mile distance, a case can be made for placing honey bee hives at more than one location in or around the field in early plantings to address shorter flights in bad weather. Bumblebees are stronger fliers that can fly in heavier winds and are active at lower temperatures. Placing bumblebee hives throughout the field may improve early fruit set. Growers should be cautioned not to place bumblebee hives near honeybees because the honeybees will place stress on and rob from the bumblebee colonies if both honey bees and bumblebees are used.

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