Watermelon

Watermelon is not a garden plant — it is a field plant that gardeners try to grow in their gardens. Its native habitat is the arid savannas of northeast Africa, and everything about the vine reflects that origin: it sprawls to catch every bit of sun, it stores water in its fruit against dry spells, and it produces best in heat that would wilt a lettuce. In a small garden, it demands more real estate than almost anything else you could plant. That is the honest opening fact.

Start seeds indoors about three weeks before your last frost date — not much earlier, because watermelon doesn't like being root-bound, and the seedlings grow fast. Transplant two weeks after your last frost date into soil that has warmed above 65°F. Cold soil stalls growth at exactly the point when the season is shortest. Black plastic mulch over the planting area a week before transplanting can add several degrees of soil warmth and dramatically improve early-season growth.

Space plants at least 48 inches apart, and allow the vines room to run. In a small garden this means letting them wander into unused space, over a path, or along a fence. Attempting to confine watermelon vines by pruning tends to reduce yield without saving as much space as gardeners hope. Water deeply and consistently until the fruit starts to ripen; once the vines have set fruit and the melons are sizing up, reduce irrigation slightly to concentrate sweetness.

Vine borers can attack watermelon, though this crop is less vulnerable than butternut or zucchini. The more common problems are powdery mildew in late summer and the persistent uncertainty of knowing when a melon is ready. Picking too early is the most common disappointment — an underripe watermelon has pale pink flesh and almost no sweetness, and it will not ripen further off the vine. Unlike a tomato, a watermelon is finished the moment you pick it.

The reliable ripeness signals: the curly tendril nearest the fruit should have turned brown and dry; the spot where the melon rests on the ground should have shifted from white to creamy yellow; the melon should produce a dull, hollow thud when thumped, rather than a sharp ping. None of these is perfectly reliable on its own, but together they build a case. When all three line up, pick it. Don't wait another week hoping it gets sweeter.