Success with Growing Watermelons

Crimson Sweet Watermelon. Credit Southern Exposure Seed Exchange

 Watermelons are delicious as a snack on a hot day in the garden, helping improve your heat tolerance.  If lightly salted to balance the electrolytes, they can cure dehydration. The seeds, if well chewed to break up the indigestible seed coat, can provide amino acids, fatty acids, vitamin E, potassium and phosphorus. Watermelons are easily digested and add fiber to the diet. Second only to tomatoes as a source of lycopene (said to prevent some cancers), watermelons are also an excellent source of vitamin C, beta carotene, folic acid, biotin, potassium, magnesium and citrulline (an amino acid important for healing wounds and removing toxins from the body).

Watermelon Varieties

Amish Moon and Stars watermelon. Credit SESE

Watermelons are all Citrullus lanatus. After trying several varieties, we chose Crimson Sweet (85d from transplant, OP), a 20–25lb (9–11 kg), striped, 10″ x 12″ (25 x 30 cm) oval melon. It has tolerance to some strains of Anthracnose and Fusarium, and according to Southern Exposure Seed Exchange, it promotes beneficial soil fungi that inhibit Fusarium. We saved seed for many years, selecting for size, earliness, disease resistance and flavor. See Fruit of the Month for September for more about choosing varieties. At this point, you have probably made your choice for this year, and may have them in the ground.

Watermelon Crop Requirements and Yield

Watermelons do best in free-draining light soils that warm quickly in spring. Ensure high organic matter content, sufficient boron and a pH of 6.5. Black plastic mulch, either the removable or the biodegradable kind, will speed growth and ripening. If you want to use organic mulches, put them around the plants after the soil has warmed up, or you will delay the harvest. Drip irrigation is better than overhead, as it reduces the chance of foliar diseases. Water well during fruit development, then cut back during the harvest period for best flavor and to prevent fruit bursting. We often run our irrigation at the same time as harvesting, so we can easily check for leaks.

If drainage is an issue, make ridges or raised beds before planting. You can use straw or spoiled hay in the aisles to absorb some of the water. Watermelons easily die in waterlogged soil.

There are on average 24 seeds/g, 670/oz, 11,000/lb, 24,200/kg. Crimson Sweet seeds are about half the size of others, so need only half as much seed.

Yield of Crimson Sweet and other varieties can be 460lbs/1,000 ft2 (227 kg/m2). For our 6,600 ft2 patch, (613 m2) we can expect an average of 3,000lbs (1,400 kg), whether 150 melons at 20lbs (9 kg) or 300 at 10lbs (4.5 kg). We have got as many as 300 melons from this area, using 2′ x 5.5′ (0.6 x 1.7 m) spacing. More on spacing follows.

Dried watermelon seeds in a paper bag.
Photo Pam Dawling

Sowing Watermelon Seeds

Watermelon seeds need a soil temperature of at least 68°F (20°C) to germinate, taking 12 days at that temperature, but coming up in a mere 3 days at 95°F (35°C). If direct seeding, station-sow 4–6 seeds 1″–1.5″ (2.5–4 cm) deep at the final spacing. Later, thin the emerging seedlings to one or two at each spot. Pests are more likely to attack plants stressed by planting in cold conditions. If in doubt, wait.

Transplanting is the way to go for early melons. It allows young plants to be raised in close to ideal conditions, and it gives the soil time to warm up. We use Winstrip 50-cell ventilated plug flats for this crop, or soil blocks. Cells should be at least 1.5″ x 1.5″ (4 x 4 cm). We put two seeds in each cell and after emergence we pinch off the weaker seedling. We sow 30% more cells than we hope to take to the field, which is another 30% more than we need to plant because of their fragility. Casualties with melons are usually fatal. (We expect casualties on planting day.) We sow April 26.

They come up very fast in our hot germination chamber. Once the seedlings emerge, they need maximum light and warmth, but not too much watering. We transplant at 15–19 days old. Four weeks old is about the maximum for watermelons — they start to get stunted if held too long.

Watermelon transplants in a Winstrip tray on May 2nd. Photo Pam Dawling

Transplanting Watermelons

Don’t rush watermelon transplants into cold soils, it’s better to wait — cold conditions can permanently stunt them. Once outdoor daily mean temperatures have reached at least 60°F (15.5°C) and the first true leaf has fully opened, you can plant them out.

We have found watermelons to be amongst the crops needing the most skill at transplanting. The stems are fragile, the roots respond poorly to disturbance, and spending extra time later replacing the dead plants is frustrating and doesn’t lead to early melons. It also requires the grower to produce lots of spare plants, which all take time and care.

Pulling a roll of biodegradable mulch.
Credit Wren Vile

We roll out drip tape, test it for leaks and then unroll biodegradable plastic over the drip tape and shovel soil along all the edges of the mulch. See our method of using biodegradable plastic, setting it out by hand. The next day we turn on the irrigation while planting. This helps ensure no one stabs the drip tape, and the plants can be set by the emitters. (Yes, you can still find them, even though they are under the plastic.) Watermelon transplants can easily get leggy in the greenhouse, so make holes deep enough to bury the whole stem as well as the roots. We use pointed trowels to punch through the plastic.

We drape netting or rowcover over hoops. This prevents the cover abrading the leaves, creates a volume of warm air around the plants and keeps insect pests away. A week after transplanting, we fill any gaps with more transplants or with a few seeds station-sown at each spot where we want a plant. Sowing pre-sprouted seeds will help make up for lost time if something has gone wrong.

Watermelon Spacing

Spacing can make a difference to size and yield, but not sweetness. There are widely varying recommendations, from 9 ft2 (0.8 m2) to 80 ft2 (7.4 m2) each! The area is the important factor, so choose a row spacing that works nicely for you and adapt your in-row spacing to give the area you want for each plant.

 


Watermelons growing on (torn) plastic mulch. A tented row with rowcover is in the background. Photo Nina Gentle

We used to transplant our watermelons 2′ (60 cm) apart in rows 10′ (3 m) apart. I read about watermelons only needing 10 ft2 (0.9 m2) per plant, so we switched to a spacing of 2′ x 5.5′ (0.6 x 1.7 m), 11 ft2 (1 m2), in order to fit more plants in the space and therefore get more first and second melons. The new spacing seemed fine and the total yield was in the right range. We have tried 2.5′ (80 cm) in-row spacing, some at 3′ (90 cm) and some at 3.5′ (1.1 m). These spacings correspond to areas of almost 14 ft2 (1.3 m2), 16.5 ft2 (1.5 m2) and 19 ft2 (1.8 m2) each. We didn’t keep records and didn’t notice a difference in size. A Brazilian study on Crimson Sweet found that 13 ft2 (1.2 m2) per plant gave the highest total yield, but 15–20 ft2 (1.4–1.9 m2) gave bigger melons.

Factors in Deciding Watermelon Spacing

Total yield (by weight): reduced spacing (to a certain point) increases total yield. Reducing plant spacing 50% may increase the total yield by 37%–48%, while reducing the size of each melon only 10%-13%. Reduced spacing does not decrease the percentage of marketable fruit.

Yield/plant (by weight): decreases at close spacing, sometimes because the number of melons per plant is reduced, sometimes because the size of the fruit decreases. It is not a linear decrease.

Size: reduced plant spacing sometimes affects melon size, but not in a linear way. Other (environmental) factors affect melon size. Bigger varieties are more likely to have their size affected by closer spacing than small varieties are. Small size is an advantage in some markets.

Number of melons/plant: decreases as plant spacing is reduced, but not linearly. At close spacings, the difference is negligible.

Number of melons/area (fruit density): increases with plant density. More plants = more melons.

Early yield: variety, early transplanting, good conditions and hot weather will provide more early melons. The first melon on each plant is the early harvest. More plants means more first melons. Plastic mulch produces crops a month before organic mulch. Spacing has no influence on the ripening rate.

Sweetness: the flavor of watermelon is not related to the size of the ripe melon or the plant spacing. Healthy foliage and long hot sunny days are the biggest factors in building good flavor. August has shorter days than July, and September’s days are even shorter, so don’t expect late-season melons to be as sweet.

Plant health: overcrowding can increase foliar diseases, reducing photosynthesis and sweetness.

Labor requirement: closer spacing = more transplanting. More melons = more time harvesting.

Watermelon with healthy foliage and a flower. Photo Nina Gentle

Clarify your goals and choose your variety and spacing accordingly. If your goal is the highest weight of watermelons for a given area, plant Sugar Baby at 10–11 ft2 (0.9–1 m2) each. If your goal is the highest number of melons, try them even closer! But if you like Crimson Sweet and want fairly large melons, try 15 ft2 (1.4 m2) if 12lb (5.4 kg) melons are an acceptable size (you might still get 15lb/6.8 kg melons!). Otherwise, use 20 ft2 (1.9 m2). Go up to 30 ft2 (2.8 m2) if you want big melons and can accept a lower total yield.

Ideally, the ground will be filled with foliage by the time the first blossoms appear so that the crops can intercept and use all the available sunlight. Given that the market is for early melons, and early ones are sweeter, having many plants (one early melon each), and having them optimally cared for, is important.

Caring for the Watermelon Crop

Remove rowcover from transplants after three weeks (wait longer with direct sown crops) and remove netting once you see female flowers. Pollination is now the critical step, not warming. Pull any big weeds. (Cultivate between the rows if you have bare ground.) Water regularly — drip irrigation set out at planting is the best way to go, as there will be less chance of fungal diseases than with overhead watering.

Watermelons have separate male and female flowers on the same plant, and insect pollinators are necessary. Many species of native bees pollinate watermelons, but augmenting them with honeybees will help pollination, which means bigger, better-shaped melons as well as more of them.

Weeding is important and needs to be completed before the vines run. If big weeds get away from you and pulling them endangers the crop roots, wade in with pruners and clip off the weeds at ground level. This prevents the weeds seeding, and lets the melons get more sunlight again. Do not turn over the vines when weeding — cucurbits don’t like that! Removing damaged fruit will help the good ones grow better.

See Fruit of the Month for September for information on pests and diseases, harvesting and choosing varieties for next year. this material comes from my book, Sustainable Market Farming, where even more info can be found!

Ripe watermelons are a treasure! Photo Nina Gentle

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