Pollenizer Systems and Spacing for Seedless Watermelon Revisited

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

There are four pollinizer systems that have been successful for seedless watermelons. The original research with seedless production showed that for standard size seedless watermelons a 1:3 ratio of pollenizers to seedless maximized yields and field space. A 1:2 ratio did not increase yield. A 1:4 ratio gave similar results often to a 1:3 ratio. However, if there were any pollinizer losses, the reduction in pollen production had a much greater yield effect. For example, a 20% pollinizer loss in a 1:3 ratio results in a final ratio of about 1:3.8; in contrast, a 20% pollinizer loss in a 1:4 ratio results in a final ratio of 1:5 which can be pollen limiting.

Pollenizers can be planted in several configurations:

  1. Pollenizers are planted in separate rows between seedless rows
  2. Pollenizers are planted every fourth plant in the seedless row at even spacings
  3. Evenly spaced seedless plants with the pollinizer placed between every third and fourth seedless plant in-row
  4. Every third plant is co-planted with seedless and pollenizer in the tray and then planted in-row

Research has shown that the in-row pollinizer planting method (3) and the co-planted pollenizer method (4) have the highest yield potential per area planted.

One issue with in-row pollenizer planting is the need to have a separate pollinizer planting operation at the same time the seedless is being planted. This has led to problems with mixing up pollenizers and seedless plants by planting crews. One way that this can be avoided is by spraying a white particle film clay product on the pollenizers to “color code” them so that crews can tell them apart from the seedless. Research at UD has shown that this coating has no effect on pollinizer performance as new leaves that are produced are normal green in color.

Another way that this issue has been addressed is to switch to co-planted pollenizers. In this program, every third plant double planted with a seedless and a pollinizer plant. The planting crew then pulls plants in order from the tray and the correct ratio (1:3) of pollinizer to seedless is planted without needing a separate planting operation. This eliminates the need for separate planting trays of pollenizers to keep track of and reduces by ¼ the number of trays to be carried in the field.

With seedless spacing, research has shown that with standard seedless types (36-60 count seedless), a 3-foot spacing between plants give the best yield and economy (plants used). Closer spacing had the potential for higher yield but did not justify the higher plant cost while wider spacing (4 ft. between plants or greater) sometimes reduced yield or increased hollow heart.

In mini-watermelons (under 8 lbs), the standard recommendation hast been to plant at a 2 ft spacing between plants. However, other research has shown that yield and size grades were optimized at a 1 ft in-row spacing.

Research on pollenizers for seedless watermelon production in several production regions including Delmarva, Georgia, and Indiana have shown some interesting results. The bottom line is that pollenizer selection can be as important for overall yield, fruit quality, and early crown set as the triploid seedless variety selected.

Research at the University of Delaware and the University of Georgia showed that early flowering differed with pollenizers and seedless varieties and that some combinations were better matched than others.

An interesting point to consider is that currently, no one pollenizer is perfect for achieving high early sets, high later sets, reduced hollow heart, and total over all yields. In addition, some standard seeded and special pollenizers are better suited for in-row use than others.

The following are some points on how to get achieve the best results for seedless watermelon production with pollenizer choice:

  • For in-row and co-planted systems, choose only those pollenizers that provide good male flower production but that are not overly competitive. Most special pollenizers work well, but fewer standard seeded types are adapted to these uses (Stargazer, Mickylee, hybrid icebox types). In contrast, the more vigorous seeded types are well suited for separate bed systems (such as Sangria, Estrella).
  • Advances have been made with special pollenizer breeding and newer generation pollenizers have better disease packages and more extended flowering. If one pollenizer is being used, consider these new varieties (SP-7 from Syngenta or Wild Card Plus from Sakata as examples).
  • Consider using two pollenizers in a field. Choose a good early flowering type for effective early yield and long flowering type for sustained yield. Field surveys have shown good results where this type of combination has been used.

In fields where diseases are a concern such as second year fields, or those that have had shorter rotations, use only pollenizers with good disease resistance packages. For example, research in Indiana has shown that some pollenizers are much more susceptible to anthracnose and Fusarium wilt than others.