Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
We have had a number of reports of growers concerned with poor fruit set in watermelons or significant abortion of fruits in watermelons. A healthy, full sized triploid watermelon plant will normally carry 2-3 fruits at any time (minis watermelon varieties will carry more). This carrying capacity is limited by the amount of photosynthates (sugars) being produced by the plant. Plants that have healthy foliage and that are vigorous will carry the most fruits.
A watermelon plant will produce many more female flowers than will be set, and will set many more fruit than it will carry to harvest. Aborted flowers and small fruit are a normal occurrence. However, delayed set, where early female flowers or fruit are aborted, or reduced set where, on average, less than 2 melons are carried per plant in the field, are indications of problems with pollination or with plant vigor and health. As plants are harvested, as long as vine health is maintained, the plant will continue to carry 2-3 fruits (another fruit replaces the one that was harvested).
A female watermelon flower will need around 500-1000 pollen grains to be fertilized effectively. This will require a minimum of 8 visits by a honey bee. Research has shown that over 20 visits may be required to achieve full set and full size.
The crown set in watermelon is fruit that set on one of the first 8 nodes of the plant. This is often the most profitable, especially early in the season. This year (2013), crown sets in watermelon may be off due to poor weather during early flowering. Bees flights are reduced significantly in rain and when winds are 15 mph or greater. Cloudy weather also reduces bee activity. Bees also do not fly much below 55°F, so on cold mornings, as we often had in June, bee activity will not pick up until later in the morning. Unfortunately, female watermelon flowers open early in the morning, are most receptive before 10 am, and then close in the afternoon.
In addition, in early mornings and during poor weather, bees usually visit plants closest to the hives. As the temperature rises or the weather improves, the bees will forage further from the hive. This means that in bad weather watermelons closest to the hives will have the best set and furthest from the hives will have the worst.
This year another problem is that some watermelon fields have lost significant pollenizer plants due to poor weather conditions during or after planting. This means that pollen will be limiting. Research has shown that were pollen is limiting, fruit numbers will be reduced with distance from a pollen source. In fields with limited pollen, expect reduced fruit set or reduced fruit size in areas where pollenizers are missing.
Watermelon growers can manage crops for improved pollination, fruit set, and fruit carry several ways:
● Increase the number of honey bee hives for early watermelon crops.
● Consider bumble bees for early crops as they fly in more adverse conditions than honey bees.
● Place hives in several locations in a field rather than just on one edge. While bees will fly over a mile, the best pollination activity is closest to the hives.
● Pay attention to early plant vigor to improve crown sets by reducing environmental stresses. The best way to do this is with rye windbreaks between every row, having proper soil conditions so that a good crowned bed can be produced with plastic tight against the soil during plastic laying (this improves heat transfer), using high quality transplants that are hardened off properly, transplanting on a warming trend, and eliminating herbicide injury (making sure herbicides are washed off of plastic and avoiding damage during shielded row middle applications).
● Ensure that pollenizer plants are of high quality and replace any pollenizers that were lost after planting as soon as possible.
● Maintain plant health and vigor to improve fruit carry. Do not plant second or third years in the same field. Pay attention to disease, insect, and mite control programs to eliminate or reduce pest pressures. Maintain good fertility programs but do not apply excess nitrogen.
● Have ample sources of pollen by planting pollenizers at a ratio of one pollenizer per every 3 seedless plants. Use the most effective pollenizers as shown by local trials. In-row pollenizers should have limited competitiveness with the seedless melons.