Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Early watermelons are sizing now, later plantings are setting fruit, pickle harvest is underway, and pumpkin planting is finishing up. A common question from growers and crop consultants is how many fruit should a plant carry and what will affect fruit set and fruit “carry” in vine crops.
For watermelons, a healthy, vigorous plant may set 3-7 fruits initially. However, for mid-size and larger watermelons, the plant will only carry 2-4 fruit at any time. Smaller fruited varieties will more fruits per plant but essentially the same amount of pounds as larger types. This is the carrying capacity of the plant and is directly related to the quantity of photosynthates being produced by the plant, mostly in the leaves. Any additional fruits, even if initially set, will be aborted. Once the first fruit ripens and is harvested, additional sets can be carried. To carry the maximum amount of fruit, it is necessary to maintain high plant vigor and good foliage health. This requires paying close attention to irrigation and fertility programs; having excellent disease, insect, and mite control; and having good pollinator activity during pollination and fruit set. If average fruit carry is less than 2 per plant in watermelons, that is a sign that the plants have reduced vigor and are under stress. Repeated fruit set depends on maintaining vine health through the season.
Another factor to consider is where fruit set is occurring. Crown sets are desired in watemelons, especially in early plantings. Crown sets are those that occur on nodes closest to the base of the plant, within the first 8 nodes. Having good crown sets requires that plants have good early growth so that adequate leaf area is produced that can support early set fruit as well as proper pollination (sufficient bees). Lack of crown set is a sign of poor early growth, early plant stress, or of problems with pollination.
With pumpkins the carrying capacity is similar; however, because pumpkins are not repeat harvested as are watermelons, harvest is limited to those fruits set initially. Medium sized Jack-o-lantern types will carry 1-2 fruits, larger types closer to 1. All others will be aborted. Smaller types will carry more depending upon their size in pounds (for example a variety with 5 lb. average will carry 4-7 fruits). Maximum carrying capacity in pumpkins is largely affected by variety (varieties with some heat tolerance will carry more fruits in our climate) and foliage health. Excess nitrogen fertilization will often delay fruit set in pumpkins.
In gynoecious cucumbers grown for once over pickle harvesting, there will be two fruits set on adjacent nodes that are ready for harvest at any one time. These will be set on nodes 2-6 commonly. The pollinizers that make up a small percentage of the population will set pickles every fifth node generally and therefore only one fruit will be ready for harvest. Yield reductions in gynoecious pickling cucumbers occur when there is a loss of set so that fruits are not on adjacent nodes. Parthenocarpic pickle varieties that set fruit without pollination will commonly have 4-6 pickles on 3-5 adjacent nodes ready for harvest at any one time. This allows them to be planted at much lower densities.