Here’s a good article on advice on the best time to water plants in the summer by Jamie McIntosh for The Spruce.
Drive around your neighborhood on a hot summer day, and you’ll see many approaches to watering plants. Some homeowners break out the hose at the end of the work day, while others program the automatic sprinklers to come on at dawn for 15 minutes a day; still others figure on the plants getting a drink whenever the kids decide it’s time to play in the sprinkler. However, timing your irrigation will not only lead to healthier plants, but it can also save on your water bill.
Morning vs. Evening Watering
Much ado is made about watering in the morning compared to watering in the evening. What time is best? The answer is, it depends. In general, it’s wise to administer water from an overhead device like a sprinkler or rain wand in the morning. This allows foliage to dry quickly, denying fungal spores a purchase on leaves. However, not everyone has time to stand in the yard at 6 a.m. between walking the dog and getting the kids off to daycare. If evening is your gardening go-to time, it’s OK to give your plants a drink, if you take care to deliver the water right to the root zone, where plants need it most.
How to Deal With Wilted Plants
Seeing your favorite hibiscus or peony drooping on a hot day can feel like an alarm bell going off in your body. Wilting makes plants look like they are on death’s door, and in some cases they may be. However, other things can cause wilting, including pest and disease problems, and even over watering. Furthermore, plants can wilt on a hot day as a way to cope with temperature, but then rebound when the evening cools off. You can recheck your plants as dusk, and determine if the wilting was due to temporary heat stress before watering. In time, you will get to know your plants, and will recognize those that respond to heatwaves in this fashion.
Annual and Perennial Plants
Knowing whether your plants are annuals or perennials is a key factor in deciding when to water plants during a heatwave.
Annuals are plants that finish their entire lifecycle in one growing season. This includes garden favorites like marigolds, impatiens, and pelargoniums. These plants have very shallow root systems and will suffer when the top few inches of soil dry out in the summer. You must water them frequently, even daily, when the mercury rises.
Perennial plants have deeper root systems that enable them to survive periods of drought from one season to the next. Some perennials, like butterfly weed and false indigo, have tap roots that extend many inches into the soil, in effect tapping into water reserves during periods without rainfall. The best time to water perennial plants is once or twice a week, slowly and deeply so that the water does not run off before it has time to soak into the soil. A rain gauge can be helpful; many plants thrive on about an inch of water per week.
When it comes to container plants, water almost functions like fertilizer: well-irrigated plants show lush growth, while plants that are just getting by on irrigation look stunted and shabby. During the summer, rules about when to water are less important for container plants. The best time to water may be twice a day, once in the morning to give plants a boost before the sun begins to bake down, and once in the evening to replenish what the day took away. The smaller the container, the more you will be filling up the watering can, so choose pots with at least an 18-inch diameter.
There’s no such thing as a drought tolerant vegetable plant. Neglecting to irrigate your vegetable garden can lead to blossom drop, stunted vegetables, or even plant death. Vegetables plants that need extra water during heatwaves include cauliflower, celery, cucumbers, peppers, radishes, and squash. These plants demand consistently moist soil to produce the best crop. Use drip irrigation, or place a hose at the base of your plants to avoid wetting leaves. If you must rely on sprinklers, water early in the morning to allow the sun to dry the foliage.
Trees and Shrubs
Don’t be lulled into thinking that a passing rain shower gave your trees and shrubs an adequate amount of water: when it comes to long-lived plants like these, you must water deeply to encourage a healthy root system. Sometimes, the best time to water your trees and shrubs is right after a light rain, to piggyback onto this soil moisture and take it to deeper levels. The time of day isn’t as important as achieving a thorough soaking. Place a hose at the base of the shrub or tree, and let it run until the top eight inches of the soil is moist. Water newly planted specimens at least twice a week during heatwaves, as their root systems are small and still developing. Shallow-rooted specimens like hydrangeas, dogwoods, Japanese maples, and magnolias will need extra water to prevent dehydration. Keep watering through the fall to prepare plants for a healthy winter dormancy.