Watermelon Growing Calendar: From Sowing to Cutting the First Fruit

The shape of a watermelon season

A watermelon needs roughly three to four months from sowing to the first cut fruit, and nearly all of it has to be warm. The season divides into four unequal parts: several weeks building the plant, a short intense fortnight of flowering and pollination, six or seven weeks of fruit filling during which you mostly stay out of the way, and a final ten days of ripening where what you do about water decides how sweet it tastes.

The two decisions that shape the whole season are made early. When you plant, which is governed by soil temperature rather than the calendar, and how many fruit you allow the plant to keep. Almost everything else is maintenance.

Raising the plants and getting them out

Sow three to four weeks before you expect to plant out, and not earlier. Watermelon grows fast in warmth and an overgrown transplant with a coiled rootball never establishes properly, so a smaller plant set out into warm soil beats a larger one every time. Sow into individual pots rather than a tray, because the roots resent being separated.

Germination needs real warmth, around 80 to 85F (27 to 29C), which usually means a heat mat or the warmest spot in the house rather than a windowsill. Below 70F (21C) germination is slow and patchy and the seed often rots first. Harden the plants off over a week before they go out.

1. Sow warm, in individual pots One or two seeds per pot, half an inch (1 cm) deep, at 80 to 85F (27 to 29C). Thin to the stronger seedling once the first true leaf shows.
2. Warm the bed while they grow Lay black plastic over the planting bed two to three weeks before planting so the soil is genuinely warm on the day. This is where a cool climate wins or loses the season.
3. Harden off over a week Move the plants out for progressively longer each day for about a week before planting. A plant taken straight from indoors to full sun scorches and checks.
4. Plant at two or three true leaves Set them out at two or three true leaves into soil above 65F (18C), on a raised hill, with a cover for the first fortnight.

Building the vine before you build the fruit

For the first month after planting the entire job is growing a big healthy plant. Male flowers will appear during this period and that is fine; if the occasional early female sets, remove it. A fruit set on a small plant either aborts or fills poorly, and either way it costs the plant growth it needs.

This is also the window for the training work: laying the runners out, pinching the leading runner at five or six feet, and selecting the three or four side runners you intend to keep. Get the first feed on about three weeks after planting, and keep the nitrogen moderate, because a plant fed hard now makes leaves instead of fruit.

1. Feed at three weeks About 2 oz (60 g) of a balanced fertilizer per plant, scattered around it and watered in. Do not repeat until fruit have set.
2. Pinch the leader at five or six feet Stopping the main runner pushes the plant into side shoots, and side shoots are where the female flowers come. Do it on a dry day.
3. Remove very early fruit Pinch off any female flower that sets while the plant is still small. Building the vine now buys larger fruit later.
4. Water steadily but not heavily Keep the soil evenly moist while the roots extend. Deep watering twice a week is better than a little every day, which keeps the roots shallow.

Flowering, setting and the labels

The flowering fortnight is the busiest part of the season and it is short. Male flowers open first, females follow a week or two later on the side shoots, and each flower is open for a single morning. Hand pollinating before nine in the morning during this window is what turns an uncertain crop into a certain one.

Label every fruit you pollinate with the date, on a plant label pushed into the soil beside it. Six weeks later that number is the backbone of the harvest decision, and nothing else will tell you. Once you have two or three good fruit set per plant, remove the rest so the plant can fill what it has.

1. Take covers off at first flower Any row cover has to come off the day the first flowers open, or nothing gets pollinated at all. Rely on inspection for pest control from then on.
2. Pollinate before nine in the morning Use two or three fresh male flowers per female and cover the whole stigma. This is the highest value half hour in the entire watermelon season.
3. Write the date on a label Push a labeled marker into the ground beside each pollinated fruit. Do it the same morning; you will not reconstruct it later.
4. Thin to two or three fruit When the leaders reach tennis ball size, remove the rest. Fewer fruit means larger fruit and noticeably sweeter flesh.

Filling, then the last ten days

Through the filling weeks the plant mostly wants to be left alone with steady water and a potassium rich feed once the fruit reach fist size. Erratic watering during this period is what causes split fruit, hollow heart and blossom end rot, all three of which are avoidable and none of which can be fixed once they appear. Get a board or a tile under each ground fruit, or a sling under each trellised one.

The last ten days before harvest are different, and this is where a good watermelon is separated from a merely large one. Cut the water back sharply as the fruit approaches its day count. Water taken up in the final stretch dilutes the sugars, and a fruit finished dry is markedly sweeter than the same fruit watered to the end.

Stop feeding nitrogen entirely once fruit are filling. Late nitrogen puts the plant back into leaf growth and the fruit finish pale and bland.

Closing the season and what to carry forward

Once the last fruit is cut, clear the bed completely rather than leaving the vines to die back. Cucurbit diseases overwinter in the debris more reliably than anywhere else, and a bed cleared in autumn is a bed that starts clean. Do not compost anything from a plant that wilted.

Then write the season down while it is still fresh. Four numbers are worth more than any amount of general impression: the date you planted out, the soil temperature that day, the pollination and cutting dates for each fruit, and whether the flesh was underripe, right or mealy when you cut it. Two seasons of those four numbers gives you a harvest date for your own garden that no general guide can supply.

1. Clear every scrap Take vines, leaves and unpicked fruit off the plot. Anything left is next year's source of anthracnose and beetle overwintering habitat.
2. Record your real day count Write down the days from pollination to cutting for each fruit, and what the flesh was actually like. That number, from your own garden, beats any published range.
3. Note the bed for the rotation Record which bed grew watermelon so no cucurbit goes back there for at least three years. Fusarium makes this rule expensive to break.
4. Note the position of the best fruit Write down which hill and which end of the row produced the best fruit. Watermelon responds so strongly to heat and drainage that the position often explains the whole difference.