Training Watermelon Vines: Pruning Runners and Growing Up a Trellis

What training is actually for

Training a watermelon is not tidiness. It has two purposes: putting the leaves where light reaches them, and limiting the number of fruit so the plant can fill the ones it keeps. A vine sprawling over itself has three layers of leaves, only the top one working, and the fruit underneath sit in permanent damp. Laying the runners out in one plane roughly doubles the working leaf area for the same footprint.

The second purpose matters more. A standard watermelon plant has enough leaf to fill two or maybe three fruit properly. Left to itself it will set eight, fill none of them well, and give you a row of bland fruit half the size they should be. Choosing which fruit to keep is the single decision that most changes what your watermelon tastes like.

Laying the runners out on the ground

Once the main runner reaches about three feet (90 cm) it starts to wander. Every week or so, walk the bed and lay each runner out in the direction you want it to go, spreading them like the spokes of a wheel from the crown so that no runner lies on top of another. Do it in the afternoon when the vines are slightly limp and less brittle: a runner moved on a cold morning snaps.

Move them, do not lift them. Watermelon runners root at the nodes where they touch damp soil, and those roots feed the plant. Once a runner has rooted, leave it where it is; the time to be directing traffic is the first month, before the roots form. After that, work around what the plant has already done.

1. Pinch the main runner at five or six feet Stopping the leading runner once it reaches five or six feet (1.5 to 1.8 m) pushes the plant into side shoots, and side shoots carry most of the female flowers.
2. Keep three or four side runners Select three or four strong side runners spread evenly around the plant and remove the rest while they are small. Removing a runner later wastes everything the plant put into it.
3. Never walk on the vines Leave a clear path and stick to it. A trodden runner loses its node roots and the fruit beyond that point stops filling within days.
4. Stop pruning once fruit are set Once you have the fruit you intend to keep, stop cutting. Every leaf on the plant from that point is feeding those fruit and removing any of it costs you sugar.

Do not prune watermelon in wet weather. Cut ends on cucurbit vines take in bacterial and fungal infections readily, so work on a dry morning when the wounds can seal by evening.

Choosing which fruit to keep

Let the plant set fruit freely at first and then choose. The fruit to keep are the ones that set on a side runner between roughly the tenth and the twentieth node, and that are already growing faster and rounder than their neighbors. Fruit set very close to the crown tend to be small and misshapen, and fruit set right at the far tip of a runner rarely have the leaf area behind them to fill.

Do the thinning when the best fruit are around the size of a tennis ball, which is early enough that nothing is wasted and late enough that you can tell them apart. Remove the rest with pruners, cutting rather than pulling, and take them off the bed.

1. Wait for a clear leader Within a few days of setting, one fruit on a runner is visibly ahead and rounder than the others. That difference only grows, so back the leader.
2. Reject anything lopsided A fruit already uneven at tennis ball size was partly pollinated and will stay uneven. Remove it however promising its size.
3. Space the keepers along the vine Keep fruit on different runners rather than two on one. Two fruit on one runner compete for the same leaves and both come out small.
4. Note the date on each keeper Write the pollination date on a label at each fruit you keep. Six weeks later this is the only way to know which fruit is which age.

Growing watermelon up a trellis

Vertical growing is what makes watermelon possible in a small garden. Trained up a sturdy trellis a plant occupies two or three feet of bed instead of thirty square feet, the leaves get even light, and the fruit hang clear of the soil so ground rot and slug damage disappear. The fruit are also in plain sight, which makes reading the ground spot and the tendril much easier.

The catch is weight. A trellis that will hold a tomato will not hold a watermelon: three fruit at twenty pounds each plus wet foliage is a serious load, and trellises collapse in the last month when the fruit are nearly ready and there is nothing to be done about it. Build it for a hundred pounds and sink the posts properly.

Face the trellis so it does not shade the rest of the bed and so the prevailing wind hits it end on. A trellis broadside to a summer gale acts as a sail and takes its posts with it.

Slinging the fruit and lifting it off the ground

On a trellis every fruit needs support from the moment it is the size of a large orange. Without a sling the stalk tears under the weight, usually a fortnight before the fruit is ready, and the fruit is lost. Make the sling from something breathable and stretchy, tie it to the frame rather than to the vine, and check it every week as the fruit grows.

On the ground, the equivalent job is getting the fruit off wet soil. Slide a tile, a flat stone, a piece of board or a thick layer of straw underneath each fruit once it is the size of a fist. It keeps the underside dry, deters slugs, warms the fruit slightly and gives the ground spot a clean surface to develop against.

1. Sling at orange size Put the sling on when the fruit is the size of a large orange, well before the weight becomes a problem. Retrofitting a sling to a heavy fruit usually damages the stalk.
2. Take the load off the stalk Tie both ends of the sling to the trellis frame, not to the vine, and leave the stalk with no weight on it. The vine cannot carry what the frame should be carrying.
3. Slide a board under ground fruit A tile or board under each fruit keeps it out of the mud and stops the underside rotting after heavy rain. Straw works but needs topping up.
4. Do not roll a fruit repeatedly Turning a fruit over to check it is fine; turning it every few days is not. It stresses the stalk and it resets the ground spot you are trying to read.