Balcony Garden in Summer: Heat, Sun and Thirsty Pots

By midsummer a balcony can feel less like a garden and more like an oven. Pots that stayed moist for days in spring are dry by lunchtime, leaves droop in the afternoon, and flowers that were covered in buds seem to stall.

Looking after a balcony garden in summer is mostly about two things: getting water to the roots when they need it, and keeping those roots cooler. A few small changes to where and how your pots sit make a real difference.

Why balconies run hotter than gardens

A balcony is surrounded by hard surfaces. Walls, glass doors and paving absorb sun during the day and reflect or release that heat onto your plants, sometimes well into the evening.

Containers add to the effect. A pot in direct sun heats up from the sides, and dark-colored containers warm faster than pale ones. Roots near the pot wall can be damaged by that heat even when the top of the plant looks fine.

Wind is the third factor. Moving air pulls moisture from leaves and from the potting mix, so a breezy balcony dries out faster than a still one at the same temperature. If wind is a constant feature of your space, see the guide to a windy balcony garden for plant choices and shelter.

Water: check more often, water deeply

In hot spells, check your pots at least once a day, and look at small pots and baskets in both the morning and the evening. Checking is not the same as watering: push a finger into the mix and water only the pots that are dry at that depth.

When a pot does need water, water it thoroughly, until it begins to run from the drainage holes. Morning is usually the best time, because the plant enters the hottest hours with a full supply. The full method, including what to do when dry mix repels water, is in the guide to watering balcony plants.

Keep the roots cooler

Lowering the temperature of the pot is often more effective than adding more water. Try several of these together:

• Group pots closely. They shade each other’s sides and create a slightly more humid pocket of air.
• Move the most stressed plants. A spot with morning sun and afternoon shade is easier on most plants than one that bakes from noon onward.
• Use larger containers. More potting mix holds more water and heats up more slowly.
• Choose lighter colors. Pale containers absorb less heat than dark ones.
• Double-pot. Slip a plastic pot inside a slightly larger decorative one so the outer wall takes the sun. Make sure water can still drain from both.
• Mulch the surface. A layer of fine bark, straw or gravel about 1 in (2.5 cm) deep shades the mix and slows evaporation.

Adding shade without adding risk

An umbrella or a length of shade cloth can take the edge off the afternoon sun. Use one only if your building allows it, fix it as its maker instructs, and take it down or close it when wind picks up, because a shade structure catches wind like a sail.

The same care applies to anything raised off the floor. Hanging baskets and railing planters dry out fastest in summer and are at their fullest when watered and in full growth.

What heat does to plants

Several summer symptoms look alarming but are common responses to heat. They overlap with other causes, so check the mix and look for pests before deciding what is going on.

• Midday wilting that recovers by evening is often the plant reducing water loss, not a sign that it is dry.
• Sunscald shows as pale, bleached or papery patches on leaves and on the exposed side of tomatoes and peppers. It is commonly associated with sudden exposure to strong sun.
• Blossom drop. Tomatoes, peppers and some flowering annuals may shed flowers without setting fruit during very hot spells, and often resume when temperatures ease.
• Bolting. Lettuce, cilantro and other cool-season plants tend to go to seed in heat and long days. Leaves usually turn bitter at that point. Replace them with heat-tolerant plants and sow again when the weather cools.

Feeding, deadheading and trimming

Frequent summer watering washes nutrients out of the mix, so feeding continues through the season. Follow the product label for rate and timing, and never use more. Water first if the mix is dry, and skip feeding a plant that is badly heat-stressed until it has recovered.

Deadheading, which means removing spent flowers, keeps many annuals blooming instead of setting seed. If petunias and similar plants have become long and bare at the base, cut the stems back by about a third. They usually respond with bushier growth and a fresh round of flowers within a few weeks.

Going away in summer

A few days away in hot weather can undo a season of care. Before you leave:

1. Ask a neighbor or friend to check the pots, and show them how to test the mix with a finger.
2. Move containers together into the shadiest part of the balcony, away from hot walls.
3. Water everything thoroughly and top up the mulch.
4. Take hanging and railing planters down and set them on the floor in the shade.

If you travel regularly, self-watering planters give you a reservoir to draw on, though the hungriest plants can still empty one quickly in a heat wave.

Before a summer storm

Summer heat often ends with a thunderstorm and sudden gusts. When one is forecast, bring in or lay down anything that can blow over or fall: light pots, tall top-heavy plants, baskets, umbrellas, cushions and empty containers.

Set what you cannot bring indoors on the floor against the building wall. The National Weather Service has general advice on wind safety.