What is the ideal nutrient ppm concentration for watermelons, and what special care is needed to grow square watermelons?

Q:

I’ve been following the Ask the Experts pages for advice on hydroponic cultivation, especially Dr. Morgan’s advice. I’m experimenting with watermelons in my hydroponic system and I wanted to know the ideal feed ppm concentration for watermelons. Also, what special care is needed to grow square watermelons? Does the square enclosure need to be transparent, or can opaque boxes work as well?

A:

Watermelons grow extremely well in hydroponic systems. Provided conditions are warm enough, plants will produce large, sweet fruit. The elemental levels for growing watermelons, as with most fruiting crops, changes as they go through the vegetative and into the fruiting stages of production. Below are some nutrient recommendations for each stage of growth in ppm:

Watermelon ppm values for hydroponic production:

Vegetative (EC = 2.0–2.4 during this phase): N=243 ppm, P=67 ppm, K=208 ppm, Mg=85 ppm, Ca=258 ppm, S=113 ppm, Fe=5 ppm, Mn=2.5 ppm, Zn=0.25 ppm, B=0.70 ppm, Cu=0.07 ppm, Mo=0.05 ppm

Fruiting (EC = 2.0–2.2 during this phase): N=202 ppm, P=91 ppm, K=322 ppm, Mg=101 ppm, Ca=169 ppm, S=134 ppm, Fe=5 ppm, Mn=2.5 ppm, Zn=0.25 ppm, B=0.70 ppm, Cu=0.07 ppm, Mo=0.05ppm

Growing square watermelons involves placing the young fruitlet inside a square box or mold while it is still attached to the vine. As the fruit grows and fills the mold, it takes on the shape of the cube. Japanese melon growers also produce heart-shaped melons using a different type of fruit mold. The main points to consider are the size of the square enclosure—it needs to be just slightly smaller than the mature size of the melon—and growers need to be continually checking the watermelon towards the end of growth so that it can be harvested before cracks or splits start to occur. In Japan, the square watermelons grown in this way are typically not edible, they are ornamental fruits only for display, as the melon has to be harvested before it is fully mature. Japanese growers traditionally used tempered glass cubes for these melons, although nowadays commercial melon molds can be purchased that are made from acrylic or clear polycarbonate that bolts together in the middle, making removal at harvest much easier. The square molds are usually transparent, as this allows the growers to check that the melon is in the correct position and it also promotes better coloration. Square molds for melons also need some drainage holes/ventilation to help prevent moisture buildup inside the cube and the fruit rot that this would promote. Good luck with your watermelon experiment.

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Written by Lynette Morgan
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Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. Lynette is a partner with Suntec International Hydroponic Consultants and has authored several hydroponic technical books. Visit suntec.co.nz for more information.  Full Bio