Seedless Watermelon Management – Thoughts Ahead of the Season

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Plastic is being laid across the region in anticipation of the first watermelon plantings about a month away.  The following are some thoughts on watermelon management addressing questions received over the past year and in light of recent applied watermelon research.

Managing Fruit Size, Spacing, and Marketable Yield
Some growers have had a problem with producing too many oversized watermelons (which have limited markets) in the last 2 years. Fruit size is best managed by choosing varieties that have been evaluated and selected for filling different size classes. A variety that commonly produces a high percentage of 36 count watermelons may, under certain growing conditions, produce high numbers of oversized melons. Conversely, under heat stress conditions, a predominately 60 count watermelon variety may produce high numbers of undersized melons.

Watermelon yield and size is also affected by planting density. In reviewing the past research on plant density with seedless watermelons, marketable yield of standard sized seedless watermelons was optimized at densities of 8-10 sq ft per plant (1 ft. between plants). For mini-seedless watermelons and small ice-box types optimal yields were at 4-5 sq ft per plant (0.5 ft. between plants). These are much higher densities than commonly used in our industry. Growers must strike a balance between cost of plants and potential yield. Industry standards in our area are between 20-28 sq ft per plant for standard seedless types (3-4 ft between plants) and 12-20 sq ft per plant for small fruited types (2-2.5 ft between plants). These common spacings maximum size potential. Wider spacings do not produce heavier watermelons. Fruit size can be reduced to a certain degree by reducing in-row spacing (increasing plant density). Reductions of average fruit size of 0.5-1.0 lbs per fruit can be expected for every foot of in-row spacing reduced.

Vine Management in Drive Rows and Row Middle Management
There has been interest in alternative vine management techniques to reduce labor costs and manage diseases. Vine turning in drive rows is time-consuming and requires hand labor. An alternative would be using discs to cut the vines which can be done mechanically. In research over the past 2 years we observed that vine cutting had no adverse yield effects as an alternative to vine turning. Of concern is the potential for disease transmission because a wound is made by the disc. This can potentially be mitigated by spraying these wounds with anti-microbial or bactericidal/fungicidal compounds. This will be focus of research this year.

Another interest has been in reducing the potential of Phytophthora capsici fruit rots in watermelons with row middle management. This disease proliferates when row middles remain saturated or have standing water for extended periods of time. High volume rains (more than 2 inches received in a short period of time) and saturated soils are the risk factors. Therefore, the issue with Phytophthora in watermelons is two-fold: getting water off the field a quickly as possible, and how to manage row middles where water accumulates as it runs off the plastic. On flat fields with little or no slope these are major issues. Compaction from traffic between rows and in drive rows makes the problem worse.

Field planning to drain water off of watermelon row middles is a key. Orient beds to improve water movement and then install cross drains at regular intervals to move excess rain water off rapidly. Shaping between bed areas to expedite water removal and eliminate ponding is also important. Subsoiling between plastic beds is another potential practice to improve drainage. Increasing spacing between plastic beds may also reduce ponding by having more soil surface to allow for water infiltration.

Another practice to consider is using planted mulch cover between plastic beds to keep fruits from contacting the soil and to reduce soil splash which can move Phytophthora onto fruits. This would require growing a cover between plastic beds and killing it. Rye windbreaks between every row also serve this function.