Protecting garden plants during summer heat

Plants tend to suffer most when heat arrives on top of dry soil. A well-watered plant can handle a hot afternoon, but a heat wave combined with shallow, irregular watering quickly leads to scorched leaves, wilting and slowed growth. Protecting garden plants in summer means keeping the root zone moist, reducing the stress of the hottest hours and paying closer attention to containers, which dry out first.

The steps are mostly simple. Water deeply and at a sensible time of day, use shade where the sun is most intense, keep mulch on the soil and avoid adding stress through heavy feeding or large cuts. Each one works on its own, and together they help plants get through hot weeks without losing vigor.

Recognizing heat stress and midday wilting

Wilting during the hottest part of a summer afternoon is common. Leaves lose water faster than roots can supply it, so they droop to reduce loss. Many plants recover by evening when temperatures drop. If plants look fine in the morning, droop at midday and firm up again by evening, the soil may still be adequate. If they remain limp into the evening or look dry and crisp, they need water.

Other signs of heat stress include scorched or crisp leaf edges, lower leaves that yellow and drop, flowers that fade early and dry soil that pulls away from the edges of containers. Check the soil itself before deciding. Wilting alone is not a reliable guide, because the same droop can come from heat, from too little water or from roots damaged by a poorly drained spot.

Watering deeply and at the right time

Water thoroughly so moisture reaches the root zone. For established beds, this usually means a long, deep soak rather than a brief daily sprinkle, which keeps roots near the surface where they dry out fastest. Most fine roots of established garden plants grow in the top 6 to 12 inches (15 to 30 centimeters) of soil. Check moisture at that depth with a trowel or a finger after watering, and water again when it begins to dry.

Morning is the best time to water. Less water evaporates in the cool hours, and foliage has time to dry during the day, which lowers the risk of fungal disease. Watering at the base of plants rather than over the foliage keeps the leaves drier. Avoid watering in the heat of midday, when much of the water is lost before it soaks in, and avoid evening overhead watering that leaves foliage wet overnight.

Using shade and easing stress on plants

Shade cloth is one of the most effective tools for a heat spell. Cloth with 30 to 50 percent shade density can cut the intensity of direct midday sun on heat-sensitive plants while letting light and warmth through the rest of the day. Temporary shade from a light fabric or a taller neighboring plant can help during the hottest hours. Shade is not a substitute for watering, and sun-loving plants that get too much of it will grow weak and leggy over time.

Reduce other stresses during hot weather. Avoid heavy feeding while temperatures are high, because fertilizer pushes soft growth that struggles to cope with heat and water loss. Avoid large pruning cuts during a heat wave; remove damaged or dead growth as needed, but save major cutting for cooler weather. Plants already stressed by drought or pests recover more slowly, so checking them regularly through the hot weeks pays off.

Using mulch and soaking dry soil

Mulch shades the soil and slows evaporation, which keeps the root zone moist for longer between waterings. Keep a layer of loose organic material a few inches away from stems, and check the moisture under it, since mulch can hide dry soil from view. Lifting the mulch and feeling the soil beneath shows whether water is reaching the roots.

Soil that has dried into a hard crust or shrunk away from the roots may need a slow soak. Water poured onto dry, hard soil runs off the surface instead of soaking in. Water in several slow passes, letting each one soak in before the next, until the moisture spreads into the root zone. Once the soil is wet again, return to the regular deep-watering routine rather than letting the surface dry out between sessions.

Protecting container plants in heat

Containers dry faster than plants in the ground, because the soil volume is small and exposed to air and sun on several sides. Terracotta dries especially quickly. During a hot spell, potted plants may need water every day, and sometimes twice in the hottest part of the season. Check each pot with a finger, and water until it drains from the bottom holes, then empty the saucer so the roots are not sitting in water.

Placement helps. Move pots out of hot wind and away from reflected heat off walls and paving, and group them together so they shade one another and lose water a little more slowly. Dark pots absorb heat and can overheat the roots in full sun, so light-colored or insulated containers are a better choice where the sun is strong. A pot moved into light afternoon shade can often carry its plant through a heat wave without other changes.