Watermelon

Direct sow once soil hits 65F

Wait until frost danger has passed and soil has warmed to 65F, then sow seed 1 to 2 inches deep in mounds spaced 4 feet apart, several seeds per mound. After the seedlings are up, thin to the best 2 plants per mound. Watermelon seed rots rather than germinates in cold wet soil, and the crop makes no useful growth until soil is genuinely warm, so sowing early buys nothing and often costs the stand.

Short season: set out 6-week transplants instead

Where the season is too short to ripen a melon from seed, start plants indoors in peat pots or cell trays and set out 6-week-old transplants carrying 2 to 3 mature leaves and a well-developed root system, spaced 2 feet apart in rows 4 to 6 feet apart. Set plants out once soil is 65F and all frost danger has passed; with plastic mulch and row cover you can go 2 to 3 weeks before the last frost. Handle the rootball gently and set it at the depth it grew in the pot. Transplants mature about two weeks earlier than direct-seeded plants, which is often the whole margin between a ripe melon and a frost-killed vine in a cool zone. Watermelons resent root disturbance, so they must be grown in containers that go into the ground intact.

Water deeply all season, then ease off as melons ripen

Give 1 to 2 inches per week, applied deeply and infrequently, ideally by drip. Once the fruit has reached full size and is ripening, cut the water back. Water early in the day if you must wet the leaves. Sugar is concentration as much as production, so water taken up in the final ripening stretch dilutes the flesh and gives you a big bland melon. Watering early and low also lets leaves dry fast, which starves the splash-borne leaf diseases of the free water they need to infect.

Sidedress nitrogen once the runners take off

After the vines put out runners, sidedress 3 to 4 tablespoons of 21-0-0 per plant and water it in. On long-season plantings, split the nitrogen: roughly half before planting and the rest 4 to 6 weeks after seeding, with a further feeding as fruit begins to size. Vine run is when the plant builds the leaf canopy that will later fill the fruit, so nitrogen pays there. Split applications match nitrogen to that demand curve instead of dumping it where rain can leach it away.

⚠ Do not overdo the nitrogen. Excess nitrogen pushes vine at the expense of fruit and lowers both yield and eating quality.

Cucumber beetles: watch the seedlings from the day they emerge

Check seedlings for striped and spotted cucumber beetles as soon as they break ground and keep checking until vines are running. Beetles feed on plants as soon as they emerge and will kill them outright or badly stunt them, and they will even enter soil cracks to feed on seedlings below the surface. Handpick, keep weeds down, and use row cover as the first line. If beetles are numerous on unflowered seedlings, bifenthrin, cyfluthrin, carbaryl, permethrin, or spinosad are labeled for them. The bacterial wilt pathogen overwinters inside the beetles' guts, so a beetle feeding on a two-leaf seedling is not just chewing, it is injecting a disease that kills the vine outright. Beetles also stunt or kill small plants by feeding alone.

⚠ Do not apply insecticide once the vines are flowering: watermelon is entirely dependent on insect pollination, and a female flower needs about eight bee visits to set a well-shaped, fully expanded melon - fruit set falls off sharply at only two to four visits. Confine treatment to the pre-bloom seedling stage. Pre-harvest intervals on cucurbits: bifenthrin 3 days, carbaryl 3 days, spinosad 3 days, cyfluthrin 0 days, permethrin 0 days. Always follow the product label, which is the legal authority.

Gummy stem blight: protectant sprays once vines run

From the time vines run, apply chlorothalonil or mancozeb as a preventive protectant on a 7 to 10 day schedule. Alternate chemistries rather than spraying the same mode of action all season - rotate in a different mode of action such as tebuconazole or a premix. Gummy stem blight needs free water to infect and is splashed onto leaves and stems by rain and overhead irrigation, so it establishes during wet weather. Protectants only work ahead of infection, not after lesions are widespread. The gummy stem blight fungus has documented fungicide resistance, which is why rotating chemistries is not optional.

⚠ Do not apply chlorothalonil within 21 days of harvest: it causes rind burn on maturing watermelons. Late-season pre-harvest intervals can run as high as 14 days, so plan the last sprays around your picking date. Always follow the product label, which is the legal authority.

Row cover the young vines, then pull it off at first flower

Cover new seedlings or transplants with spunbonded fabric, hotcaps, or a low tunnel to hold warmth and keep cucumber beetles off. Ventilate whenever air under the cover passes 80F. Remove the cover entirely as soon as the vines start flowering, or when temperatures top 90F. The cover buys early warmth and is the single most effective barrier against the beetles that vector bacterial wilt into seedlings. But watermelon flowers open for one day only and are strictly insect-pollinated, so a cover left on through bloom simply starves the crop of bees and you get misshapen fruit or none.

Pick on the tendril, the belly spot, and the dull rind

Start checking about 35 to 45 days after the flower opened, sooner in hot weather. Harvest when the curly tendril nearest the fruit stem has dried brown, the patch where the melon rests on the ground has turned from white to a creamy yellow, and the rind has lost its gloss and gone dull. A hollow thump backs up the other three but is not reliable on its own. A watermelon does not ripen further once cut. Unlike a muskmelon it never slips from the vine to tell you it is ready, so these external signs are the only honest read on a fruit you cannot see inside.