It is summer in Memphis, and this winter and spring have been one of the wettest on record. With the increased heat, plants now face increased water demand. Plants move a great deal of water through their systems. A mature oak can absorb as much as 400 gallons of water a day when the air temperature nears 100°.
Remember also, the root zone of that oak is approximately 2½ times the canopy width, so all your grass, shrubs, perennials, and annuals nearby are competing for the same water. The oak has thousands of miles of roots and feeder roots, so is there any doubt about who wins that competition?
How much do I water?
We should be applying approximately 1 to 2 inches of water per week to keep plants healthy. Watering should be infrequent but thorough. To accurately set up your sprinkler system, place a couple of empty soup cans at different distances in the pattern and let it run for an hour. This will show you how much water is applied in an hour and whether you are getting even coverage.
Try to apply between ½ and 1 inch of water each time you irrigate. Some gardens will require more water because of the soil type.
When is the best time to water?
Watering early in the morning is best. It allows for maximum absorption and less evaporation. Watering anytime before 4 PM is generally fine, as it allows enough time for the foliage to dry. That is important in Memphis because we grow so many plants prone to leaf diseases.
If you work long hours and evening is your only option, go ahead and water. Mother Nature waters 24/7, so if a plant needs water, water it. There is no evidence backing the claim that watering in sunshine causes burning.
Some trees and shrubs begin to slow down when soil temperatures rise in summer. Heat and wind can dry foliage faster than the root system can resupply due to slower metabolism, which produces some evidence of leaf burn. This is seen most often in Japanese Maples, especially lace leaf varieties.
How do you feel about irrigation systems?
Irrigation systems are a testament to the fortitude of plants. Unless you adjust it, an irrigation system puts out the same amount of water regardless of the temperature, whether it is raining or not, and it totally disregards the needs of different types of plants. It also does not compensate for new plantings.
The average root zone on a plant is 2½ times the canopy. An established 3-foot-wide plant has a root zone of about 44 sq. ft. A new plant from a 5-gallon container has a root zone of less than 1 sq. ft. The established plant is receiving 44 times the water the new plant is getting, even though they probably need the same amount. Quite often customers cannot understand why a new plant under irrigation dies. It is getting watered, yes, but not nearly enough.
That said, a properly designed irrigation system does an excellent job if it is programmed to water deeply and the correct spray heads are used based on the needs of the plants. Frequent shallow watering requires more water because of evaporation and causes roots to stay near the surface. This makes them more prone to drought stress and winterkill.
So how can I tell if I need to water?
Nothing beats the finger test. Pull the mulch back and check the soil about knuckle deep. If it feels moist, do not water. If it is dry, water.
Some plants will signal you by wilting, but that can be misleading. Young hydrangeas will wilt in the sun even with moist soil. They have large leaves and the transpiration rate in hot, windy weather can exceed the root absorption rate. Once they are in the shade they pump back up. Once established, which can take several years, that should stop.
Some plants recover quickly from wilting, but others do not, so avoid letting them wilt repeatedly. Some plants shed leaves instead of wilting. Drought-stressed plants often shed interior foliage to decrease water loss. That is simply the plant protecting itself.
Some plants show no signs of drought stress until they turn brown, so waiting for a plant to show stress is not always the best approach.
Can I over-water in this heat?
Yes, you can. Quite often we see plants stressed or die in spring from excessive rain during the winter. In spring the air temperature rises quickly and plants flush with new growth, but the soil is still cool and wet, so root development is minimal. As summer progresses, top growth slows and root growth accelerates in the warm soil.
Many plants will not survive warm soil being overly saturated in summer, especially plants not native to our area that require better drainage. Plants take in water through microscopic roots. If those roots rot off, the plant cannot take in water and dries up, even in wet soil. If those roots dry up from lack of water, the same thing happens. The net result is the same: a dead plant.
A wilted plant needs water, but it may not need watering.
Because of our extremely wet winter and spring, pay close attention to any trees and shrubs planted in the last two years. These are still becoming established and are highly susceptible to root problems. Using root stimulators will help plants recover from root loss due to sitting in wet soil. Feeding with a good organic fertilizer like Milorganite, or plant-specific Espoma products, will improve soil health and drainage.
Even though most plants appear healthy right now, the stress of summer heat and drought could affect what seems like a healthy plant. A little preventive care can go a long way to keep your landscape looking its best.
As always, if you need advice, just pick up the phone. We are here for you.
Happy Gardening