Fortunately there are some simple measures gardeners can take to counter the heat and keep a garden growing.
Most common vegetable crops and native shrubs can withstand periodic heat waves without losing vigor. However, the shallow surface roots cannot withstand the stress of extreme heat which dries and cakes the soil in the top few inches. By paying attention to the condition of the soil, a gardener can offset the effects of a heat wave on growing plants.
Apply mulch, preferably a reflective mulch such as dry grass clippings
The first line of defense against hot weather, and against windy conditions which can dry surface soil, is to apply a liberal layer of mulch around the plants. This protects the soil from direct sun exposure, keeping it moist at the surface. Mulch also reduces evaporation of water from the soil which reduces the need for watering.
There are many different mulch materials which gardeners can use. During a heat wave, light colored mulches will reflect the sunlight and help maintain cooler surface soil conditions. In our yard, we keep a patch of grass set aside to grow tall for a ready source of mulch.
Freshly cut grass clippings are best left a few days on the lawn before raking if they are to be used for mulch. In a few sunny days the fresh green clippings will turn a light brown and are then ideally suited for use as mulch. In past years when we used fresh green grass clipping as a mulch for our tomatoes, which were just beginning to flower, the high level of nitrogen in the green grass stimulated vegetative growth and suppressed flowering. We ended up with giant tomato plants which produced only lightly compared to previous years.
Bark mulch is commonly used for shrub beds, as it prevents weed growth while also shading the soil and helping to conserve water. But gardeners should be aware that bark mulch can contain weed seeds which may introduce an invasive species to the shrub bed. A local landscaper recently remarked that about 50% of his clients have had their shrubs inundated with horsetail, a persistent weed which is difficult to eradicate. If you are buying bark mulch, be sure to ask about its source and if there have been complaints about weed seeds in the mulch.
Water your garden and shrubs early in the morning
A heat wave can dry surface soil quickly which dehydrates shallow roots, and water is also lost through leaves in hot weather, so your plants will need a thorough watering. This should be done early in the morning, especially if you use a sprinkler to water, since most water from a sprinkler is lost to wind and evaporation during the hot times of the day. Watering in the morning also prevents heat scald which can damage leaves which are watered while the sun is directly overhead. In extreme hot weather, seed and nursery beds may need a second watering later in the day.
During a heat wave, the need for water conservation is heightened. Hand watering has the advantage of delivering just the right amount of water for each crop. It is also a more efficient method of watering compared to sprinklers, since only the targeted crops are watered. Soaker hoses are ideal since they can be used anytime during the day, because plant leaves are not wetted. Soaker hoses can be placed beneath the mulch to access the soil directly while hidden from view.
Watering in the morning is also a great defence against slugs, since conditions are drier overnight. Fungal diseases are also discouraged. It’s also more pleasant for you to water in the early morning while it’s still cool in the garden.
Use shade cloth or protective row covers
Shade cloth offers partial and temporary protection from the sun for garden plants, and is available at garden centers in a variety of sizes, shade factors and configurations. ‘Shade factors’ refer to the degree of blocked sunlight, and can range from 25% – 90%. Sensitive plants like salad greens may require a 50 – 60% shade factor while more heat tolerant plants like squash and beans may benefit from a 30% shade cloth.
A shade cloth is temporary but care should be taken to position it to block sunlight while not reducing aeration. Plants do not need to be enclosed in shade cloth; usually positioning it on only one side or above the plant will suffice. Care should also be taken to ensure that the cloth structure can withstand wind, and won’t harm the plants by falling on them.
Homemade shade cloth can be made using fish net with strips of cloth woven through, and strung up temporarily over vulnerable crops. Row covers provide similar benefits of shade cloth provided there is plenty of ventilation, but are not available in shade factors.
Locate new transplants within the cover of taller neighbors
A heat wave is hardest on transplants. The root systems of young starter plants are shallow and therefore more susceptible to drying conditions in the top few inches of the soil. Ideally, you could wait until the heat wave has passed before setting out transplants. But if you have transplants which need to be set out, look for partial shading opportunities provided by taller more mature plants. You don’t want to locate the transplants in an area of permanent shade, so look for bare spots in the garden alongside plants which are nearing maturity and will soon be pulled. This way, the sun will be available to the transplants when they are better established.
In the picture above, broccoli transplants have been set out alongside the tall fern-like stalks of mature asparagus plants. The ferns provide partial shade for the young seedlings, but not enough shade to interfere with plant growth once the starters become established. The fern stalks also provide a wind screen for the fragile transplants as an added bonus.
Keep lawns at least 3” tall
It stands to reason that taller grass casts longer shadows. And the added shading from leaving your grass taller than usual will benefit the soil by helping to retain moisture. A minimum depth for getting a shade benefit is 3”, and some groundskeepers set mowers as high as 6” during heat waves or drought conditions.
Avoid applying fertilizers to your lawn or garden during a heat wave, since the roots ability to absorb nutrients is diminished during a heat wave. Wait until the weather cools down before adding fertilizers to garden crops and the lawn.
And while you’re thinking of ways to protect your garden during a heat wave, don’t forget to set out some water for the birds!
Heat waves usually are of short enough duration that gardeners can manage to produce successful crops. Prolonged heat waves, of course, are more challenging and crops may be stunted or crop yields reduced. Unfortunately, the long-term outlook for our climate indicates that in upcoming years we gardeners will need to hone our hot weather gardening skills. The measures described above will likely be common knowledge in the years to come.