How to tell what went wrong
Start by cutting one open and looking closely. A melon that is pale pink or white inside, with white seeds in a seeded type, was picked before it ripened, and unlike muskmelon, watermelon doesn't keep ripening once it's off the vine. A melon that is fully red but small, lopsided, narrow at the stem end, or split open inside (hollow heart) points to pollination. Fruit that is ripe-colored but watery and flat-tasting usually means the vine couldn't feed it enough sugar in the last few weeks. Each of these has a different fix.
Why melons stay small
Watermelons need bees to carry pollen from male flowers to female flowers, the ones with a tiny melon at the base. University of Delaware researchers note that a fruit needs roughly 1,000 pollen grains spread across the flower to grow evenly; too few bee visits give small, misshapen or hollow fruit. Poor set is common in rainy weather, when bees stay home, and sprays on open flowers can drive them off. Seedless varieties need a seeded variety planted nearby as a pollen source. A vine also has limits: University of Maryland Extension expects about 2–4 fruit per plant, so a vine that sets many melons often can't fill them all.
Season and variety matter as much. Watermelons are very tender plants that need about 80–100 days from . Planted into cold soil, or grown where summers are short or cool, vines can run out of warm weather before the fruit sizes up and sweetens. Icebox and personal-size varieties are bred to stay small, often well under 15 pounds, so a small melon from one of those may be exactly right. Check the seed packet for the expected size and before deciding something went wrong.
Why melons taste bland
Sugar is made in the leaves and moved into the fruit as it ripens. University of Delaware notes that losing leaves or stems to disease such as gummy stem blight, or to insect and spider mite feeding, cuts that supply and can leave fruit ripening unevenly with less sugar; low potassium and weak vines add to it. Heavy watering or rain while fruit ripens can also water down flavor and split rinds, which is why Maryland Extension suggests easing off water once fruit begins to ripen, unless a long dry spell sets in. Heavy nitrogen can lower both yield and quality.
What to do now
For melons still on the vine, wait for the ripeness signs together: a creamy-yellow ground spot, a dry brown tendril nearest the fruit, and a dull thump instead of a high ring. Keep the soil evenly moist while fruit is swelling, then water less as it ripens. Watch for spider mites in hot, dry weather, which show as fine pale speckling and webbing under the leaves. If bees are scarce, you can hand-pollinate in the morning with a small brush, moving pollen from a male flower to a female flower the day it opens.
Next season
Next year, wait to plant until frost danger is past and the soil is warm, or set out transplants started indoors about 3 weeks earlier for a head start. Maryland Extension notes that melons grown on black plastic or landscape fabric tend to give larger, earlier harvests, and a drip line under it keeps moisture even. Work in before planting rather than feeding heavily later, plant flowers nearby to bring in bees, and keep insecticides off blooms. In short-season areas, choose a smaller, earlier variety.
Before using any product, read the whole label. It must list the crop you’re treating, and its directions on amount, timing, protective gear, bees and days before harvest are legal requirements. If this page and the label disagree, follow the label.
Growing it again? When to plant watermelon for your ZIP code
- How do I know when a watermelon is ripe?Check the ground spot (it should be creamy yellow, not white), the tendril nearest the fruit (dry and brown means ripe), and the sound when you thump it (a hollow, low thud rather than a high-pitched ping).
- Why are my zucchini and squash flowers falling off?The first zucchini and squash flowers are often all male, and male flowers fall off a day after opening — that's normal. If female flowers (tiny fruit at the base) drop, look at pollination and heat.
- Why are the edges of my older leaves turning brown and scorched-looking while the centers stay green?Marginal leaf scorch on lower, older leaves — a brown or yellowing burn along the leaf edge — is a characteristic sign of potassium deficiency, which also tends to produce weak stems and poor fruit quality.
- Can you give a plant too much nitrogen, and what does it look like?Yes — excess nitrogen produces very dark green, lush leafy growth at the expense of flowers and fruit, and in high concentrations can burn roots and cause wilting.