Grafted Watermelon Production in Southcentral Alaska

First 2 graftings were 7 watermelon cultivars – Marmeladnyi (Tatiana’s Tomatobase), Blacktail Mountain (Seed Savers Exchange), Katanya (Baker Creek), Golden Crown (Osborne Seed Co.), Bozeman (Adaptive Seeds), Cream of Saskatchewan (Fedco Seeds), Small Shining Light (Seed Savers Exchange), and a variety supplied by another grower Alaska grafted onto Flexifort rootstock (Osborne Seed Co.). Damping- off (Pythium) disease occurred in the healing chamber. The source was potentially the seed of one of the watermelon cultivars that had been provided by a local seed producer. Disease samples were not sent to a lab for diagnosis (UA extension agent is no longer available in Anchorage so sample submission is not simple).  Rob had never experienced an outbreak of Damping-off at this level.  About 60% of the planting, about 350 plants, were lost. A third grafting was done on June 10 with two watermelon cultivars, Blacktail Mountain (Seed Savers Exchange) and Marmaladny (Tatiana’s Tomatobase), grafted on Tetsukabuto rootstock (Kitazawa Seed Co.). This was successful, 90% of grafted plants survived, and plants were transplanted to the high tunnel June 28.  Rob believes application of Bacillus amyloliquefaciens strain D747 (Trade Name Double Nickel 55) at grafting and once more 5 days later enabled successful graft healing and establishment.  

Seeds were sown individually in 606 cell packs inside Rob’s house.  Bottom heating was utilized with a temperature probe inserted into the soil and the temperature set to 83 F.  For germination, plants are started on shelves 12 inches below a bank of 4- 4 foot long T5 fluorescent grow lights that are each 54 watts and run for 16 hours each day. They are then transferred to a grow room, also inside Rob’s house (Fig. 1). Lights in the grow room are 1000 watt metal halide bulbs, and light is on for 14-16 hours. Air temperature is 75-80 ^oF and relative humidity is 35-40%.  The lights travel along a track which allows, Rob to grow more plants in the space.  He tries to average 1300 foot candles of light measured at the highest leaf surface.

Plants are grafted in an indoor room using Xacto knives, bamboo skewers, and clips (Fig. 2). Grafting method is hole insertion as Rob feels his fingers are not nimble enough to do the one-cotyledon method. Also, Rob feels the graft attachment is stronger with hole insertion, which he desires since his plants are grown on mounded raised beds.  On average, Rob will graft 325 watermelon plants in 1-2 days, and healing takes 9 days, and on day 10 plants are moved back to the grow room for 2-3 weeks prior to transplanting.

Healing is in a grow tent (50” X 50” X 79”; Milliard, New Jersey ) inside the house (Fig. 3). In the grow tent, humidity is provided by an ultrasonic humidifier (Vicks Ultrasonic Cool Mist, Procter & Gamble, Cincinatti, Ohio) set at 90% (+/- 5%) with a humidistat (Model RHC-R, Greenair Products, Boring, Oregon). Rob would like a fogger that provides more even humidity throughout the chamber, so that water does not condense and drip on plants. Temperature is set at 74-75 ^oF. Lighting is T5 single fluorescent tube, 4 ft that provides 350-400 foot candles (Sunblaster F54T5-HO). Plants are placed 2 shelves below the lights then as the plants are acclimated they are moved closer to the light source. Each day, the plants are moved backward and forwards and side to side under the light to try and attain equal coverage. Rob would like lights that are better suited for a wet environment, that provide better coverage over a wider area with less of a ‘spot light effect’.

Transplanting into the high tunnel on Rob’s farm was 29 June. The target date was the first week of June, but the first 2 graftings were not successful. A randomized complete block design was used for the experiment (Fig. 4). Rob had been measuring soil temperature at a 6 inch depth and tries to plant when soil temperature reaches 65F, but has since learned that a 2 inch depth is typically used and anticipates changing to this method in 2018.  This could enable even earlier planting.  Treatments include grafted and non-grafted plants of two cultivars. IRT Olive mulch (1 mil) covered the raised beds. Finished, on-farm-produced compost was added at 5 cubic yards per 1000 square feet.  Alaska Sea-Ag fertilizer (Analysis 5.4-9-.5 = ground bones from Alaskan fisheries, Palmer, Alaska) was added at 127.62 pounds per acre (1.875 pounds per 32 row feet).  Micronized Azomite trace minerals (Volcanic Ash deposit, Sanpete County, Utah) were added at 170.16 pounds per acre (2.5 lbs per 32 row feet).  Liquid fish fertilizer (Analysis 3-1-0, Foxfarm “Wholly Mackerel”, Arcata, California) was supplemented once when plants averaged 12 inches long and vines were running (tip of vine is pointed up 3” off ground) at a rate of 1 Tablespoon per gallon and each plant was given 1 quart. 

Plants were hand pollinated as it is difficult to attain consistent and reliable pollination by wild pollinators (Fig 5). Though Rob keeps European honeybees right next to his crops, he rarely observes them visiting flowers of the crops he grows and has not found them to be a reliable pollinator of anything but raspberries until the end of summer when the wildflowers in the mountains around his farm have finished blooming.  Pollination was done within a cultivar (there was no cross-pollination of the two cultivars in the experiment) before 10:30 am as Rob read this was the most successful time for pollination to occur.  With near 24 hour sunlight during the crucial pollination period, he is interested in exploring whether the flowers may be ready at other times, though he knows of no metric to do so other than trying it, which he feels is too risky within the scope of this project.  Hand pollination of watermelon is difficult and Rob appears to have had good success with his method. He says a critical quality of any watermelon variety to be grown in Alaska is that it produce female flowers early on.  Rob is working with Xerces Society to establish native pollinators (various Bombus species) around the high tunnel (Fig. 6). But even so, the tunnel is constructed and kept closed except during periods when his automatic exhaust fans cannot keep up to maximize heat retention as temperatures are generally low and unpredictable during the summer growing season (Fig. 7). The 2017 growing season was much cooler during the day than the previous two years, though night temperatures were slightly warmer due to cloud cover.  On 24 Aug. soil temperature in the watermelon plots was 60 ^oF (Fig. 8). The target for first pollination is the last week of June and for final pollination is the third week of July as fruit needs 50-60 days from fruit set to harvest in this climate. In addition to planting grafted watermelon on his farm, Rob also planted grafted watermelon plants in low tunnels at the Alaska Botanical Gardens (Fig. 9).   That location is used as a trial ground for varieties that are new to him.

In 2018, 6 plots of grafted and 2 plots of non-grafted Marmaladnyi watermelon plants were randomly installed in raised beds in the same unheated high tunnel as 2017.  The same parameters as 2017 were recorded.

Expected yield is 2 fruit per plant (Fig. 10). Number of all fruit that are greater than 8 inch diameter at time of harvest will be recorded. Only fruit that are close to full maturity will be harvested (tendril and leaflet at fruit connection to the vine are brown). Number and weight of harvested fruit per plot will be recorded. Brix will be measured for 2 fruit selected randomly per plot at time of harvest.