Heat Wave Gardening: How to Protect Your Plants in Extreme Heat

Heat waves are one of a gardener’s greatest challenges, but with these tips your plants will continue to thrive

Adding mulch around your plants and checking for signs of stress are several ways to make sure plants stay hydrated and healthy during heat waves. (zlikovec/Shutterstock)

As the mercury rises, keeping plants properly hydrated and thriving becomes a battle that requires a significant change in watering practices, protecting plants from the sun’s scorching rays, and more.

First things first: Make sure you’re staying safe, cool, and hydrated yourself. Schedule yard work during the cooler times of day, such as early morning. Drink before, during, and after gardening, rather than wait until you’re thirsty and risk dehydration. Apply sunblock 30 minutes before going outside. Wear a large hat, garden gloves, and loose-fitting, lightweight, long-sleeved clothing.

Hydration, Hydration, Hydration

In the garden, start with checking soil moisture levels daily, and possibly twice a day for container plants. If it’s dry to a finger’s depth, or worse yet, dry even deeper, water immediately. Also, switch to watering in the early morning, as early as possible, to allow the plants to get a good drink before moisture begins to evaporate from the soil’s surface and to properly hydrate the leaves to mitigate another source of water loss: transpiration.

The exception to this rule is container plants that need watering twice daily, which should be done in the morning and afternoon. Don’t wait for the plant to signal its distress via wilted leaves.

High temperatures come with the concern that water will evaporate even before it reaches the roots. Avoid sprinklers, and water directly to the base of the plant. Hand water with a hose or watering can, or run soaker hoses or drip irrigation for maximized efficiency.

Pay particular attention to recent plantings, including shrubs and trees that have been planted in the past two to three years that still may not yet have fully rooted. Check both the root ball area and the surrounding soil. If either is dry, water.

Never assume, though, as overwatering can be just as bad as underwatering. Many plants slow their metabolic process during a heatwave, and may—surprisingly—use less water at 95 degrees F than they would for temps in the 80s.

Of course, this doesn’t take into account soil evaporation, so get out that test finger. Or, for a large garden, buy a moisture meter for fast accurate checking of multiple spots or containers. If the budget allows, consider an automated irrigation system that uses moisture sensors to water only when needed.

Mulch, Fertilizer, and Plant Stress

Mulch is good. Fertilizer ... probably not. Lay down a two- to three-inch layer of woodchips, cocoa hulls, compost, leaf mold, or your mulch of choice. It does double duty, helping to lessen soil evaporation while shading the roots to keep them cooler.

While there’s a great temptation to give flagging plants a “vitamin boost” in the form of fertilizer, this is the wrong thing to do. Fertilizer promotes growth, straining the plant’s resources and adding additional stress.

Speaking of plant stress, refrain from planting or transplanting plants during a heatwave; the planting process is stressful under any conditions, but it can be a killer during extreme temps. Move seedlings that haven’t been transplanted yet to a shadier spot—but not so shady that they become leggy (growing fast and thin trying to reach the light). Avoid pruning or taking cuttings for propagation as well (unless you’re trying to save a particular plant type).

Keep the garden well-weeded, because weeds compete by stealing critical water and soil nutrients.

Provide Shade

While container plants do dry out faster than in-ground plantings, they offer the wonderful option of moving them into the shade—or an area that still gets full sun, but less of it during the heat of the day, such as an east wall of a house.

For most gardens, the shade has to be brought to the plants. This can be in the form of row covers on wire hoop frames that hold the shade cloth several inches over the plant to allow for air and growth.

You can also use a large shade cloth secured at the corners to poles, a do-it-yourself PVC frame, or otherwise stretched over the entire garden or a section of it. It is essential that the shade cloth never come into contact with the plant because this could make the plant even hotter, resulting in leaf burn and other damage.

While some people use tarps, screening, or even old sheets, the ideal is a commercial garden shade cloth. These are designed for outdoor exposure, including strong sun, allow rain to get through, and are available in different densities.

Heat-tolerant plants such as pepper, squash, and tomatoes will have an easier summer if a 30 percent shade cloth is deployed during a heatwave. For less heat-tolerant plants, consider a 40 to 50 percent density. Sensitive plants such as lettuce and spinach, which want to bolt (go to seed) in high temps, need 60 percent coverage. Vegetable gardens don’t generally use 70 to 90 percent, but it will help if you raise ornamental plants including ferns and philodendron that normally reside on a forest floor.

Color plays a role. White reflects sunlight without interfering with wavelength, meaning plants will grow quickly with little risk of sunburn. Green is the middle of the road, allowing light to pass through while absorbing some wavelengths and heat energy. Black cloth provides the most heat and sunlight absorption, which can negatively affect some plants. Whatever shade cloth is chosen, a good rule of thumb is to take the shade cloth down when temperatures drop consistently below 90 degrees F.

Talking Plants

A plant’s natural defense mechanisms include cupped leaves, dropping flowers, wilted plants, stunted, and not growing fruits or vegetables. Here’s the scoop.

Leaf Language

Cupped and rolled leaves of tomatoes and corn are the plant minimizing the surface area to reduce water loss. Dry edges on large leaf plants like squash still leave them enough usable surface for growth. Both are survivable with a little TLC.

Wilting

Wilt occurs when a plant’s moisture level is too low to maintain leaf and stem structure. It is a ticking clock with the potential for damage increasing the longer it is wilted. Some plants wilt regularly during the heat of the day and revive each night.

Blossom and Fruit Drop

Plants such as tomatoes, cucumbers, peppers, and squash will drop their blossoms, and even fruit, during a heatwave. Don’t give up on them, as they’re simply conserving resources and should bounce back once the extreme heat is over.