Growing Bitter Melon as a Living Shade Screen on a Sunny Wall

What a living screen actually does

A wall of leaves in front of a sun facing window works in two ways at once. It blocks direct sun before it reaches the glass, which a blind inside the room cannot do because by then the heat is already indoors. And the leaves themselves are transpiring constantly, so the air passing through the screen arrives cooler than the air outside it. On a still hot afternoon the surface of a shaded wall runs many degrees cooler than the same wall in full sun, and the room behind it follows.

Bitter melon is one of the better plants for this because its leaves are deeply lobed and held on long stalks, so it builds a layer that is dense enough to block sun without becoming the solid sheet that a squash makes. Air moves through it. And unlike an ornamental climber, it is producing food the whole time it is doing the job.

The screen only works while the leaves are alive and turgid. A plant that has been allowed to dry out stops transpiring, and a wilted screen is close to useless for cooling even though it still blocks light.

Setting the frame up

The structure has to be up before the plants go in and it has to be stronger than you think. A mature screen eight feet (2.4 m) high and six feet (1.8 m) wide, wet after rain and carrying fruit, is heavy, and it catches wind like a sail. Netting sagging away from the wall in the first summer storm is the commonest failure.

Use garden netting with a four inch (10 cm) mesh, which the tendrils grip readily, stretched between a top rail anchored to the wall or a fascia and a bottom rail pegged or weighted at ground level. Keep the netting four to six inches (10 to 15 cm) off the wall itself. That gap lets air rise behind the screen, which is where a good part of the cooling comes from, and it stops the leaves scorching against hot masonry.

1. Anchor the top first Fix the top rail before anything else and take the whole load there. Screw eyes into a fascia board, or a rail between two uprights set in the ground, both work. A hook stuck to glass with suction will not last the season.
2. Tension the bottom Peg the lower edge into the ground or tie it to a weighted rail. Netting that swings in the wind tears the tendrils off and the plant has to start gripping again.
3. Leave an air gap Hold the netting four to six inches (10 to 15 cm) clear of the wall using spacers. Air rising in that gap carries heat away instead of pushing it into the wall.
4. Plan the sweep Angle the bottom of the net out from the wall so the plants have room at the base. Vertical netting flat against a wall makes watering and picking awkward all summer.

Planting for coverage, not for yield

A screen needs plants closer together than a plot grown for fruit. Set them eighteen inches to two feet (45 to 60 cm) apart along the base of the netting rather than the two to three feet (60 to 90 cm) you would use in a row. You are trading some fruit size for the density that makes the screen work, and that is the right trade.

If you are growing in containers against a paved terrace or a balcony, go bigger than feels necessary. Allow at least ten gallons (40 liters) of compost per plant, and more if you can. A screen in an undersized pot needs watering twice a day in high summer and will fail on the first day you are away, which is exactly the weather you built it for.

Filling the gaps

The screen will not cover evenly on its own. Vines run for the top of the net and leave the lower third thin, so the sun comes under the screen in the late afternoon exactly when you least want it. The fix is to train horizontally in the early weeks: as each lateral extends, tie it sideways along the netting rather than letting it climb, and only release it upwards once it has laid down a foot or two (30 to 60 cm) of horizontal growth.

Weaving the growing tips through the mesh at a shallow angle achieves the same thing with less tying. Check the screen every few days in the first month, because a shoot that has gripped the wrong way for a week cannot be moved without breaking it.

Bitter melon leaves let noticeably more light through than gourd or squash leaves. If you want deep shade rather than dappled shade, plant one more plant than you think you need rather than trying to make each one denser.

Water, feed and what to record

A screen in full sun uses a startling amount of water. In high summer expect to water a container grown plant every day, and twice on the hottest days, watering in the early morning and again in the evening rather than at midday. Ground grown plants against a wall are also in the rain shadow of the building, so they dry out even after rain that soaked the rest of the garden. Feed every ten days once the fruit starts, because a plant carrying a full screen of leaves plus fruit is working hard.

For the record, the useful things to note are the date the screen reached the top of the netting, and how much of the netting was covered by mid summer. Note the aspect the screen faces and the width of the air gap you left, and, if you can be bothered, the indoor temperature on a hot afternoon in the room behind it compared with a room on the same side without a screen. That last figure is what tells you next spring whether to build the screen wider.

1. Record coverage dates Note when the vines reached the halfway point and the top. If the top comes late in the summer, start the plants earlier or add one more.
2. Record the watering pattern Note the point in the season when once a day stopped being enough. That date tells you what size container to use next year.
3. Record where the sun gets past Walk the room in the late afternoon and note which part of the screen is thin. That is where to train horizontally next season.