June 12, 2020
Every growing season seems to be different. Already, we have had record-setting rainfall in late spring. Who knows what the rest of the summer and fall will bring? While we may receive lots of rain in spring and early summer, conditions can change quickly, becoming hot and dry the latter half of the summer and even into early fall. That’s why watering trees and other woody landscape plants during dry periods is very important.
Remember that the root system is the belowground infrastructure for the plant and is the means by which it takes up water and nutrients for the aboveground portions. Most plant problems originate below ground due to damaged root systems. Water moves from the soil into and throughout the plant via transpiration, which acts as a conveyor system for moving water and nutrients to the leaves for food making. If the root system has been damaged for any reason, the plant is severely handicapped.
All living organisms require water to live, including humans. Without water, transpiration and photosynthesis do not occur. You may remember from your science class that carbon dioxide plus water yields carbohydrates plus oxygen (i.e., photosynthesis). Additionally, plants need water to maintain “turgor pressure,” or the ability to keep leaves fully expanded to maximize sunlight needed for growth.
For roots, if soils get too hot and dry, then roots dry out and die, creating major problems. It may require months or years for roots to be regenerated. This was evident following the 2012 drought, when many trees were stressed, went into early fall color, and showed evidence of dieback and decline. It was not until 2015 — three years later — that conditions began to improve and trees began to recover.
Help plants thrive in dry conditions
A good rule of thumb is that for most woody plants, it takes about an inch of precipitation or irrigation per week during hot, dry periods. If you have not had rain for several weeks, along with warm to hot temperatures and windy conditions, you should consider starting a watering program.
Also, look at your trees — particularly species that have large leaves, such as sycamores and maples. Typically, during hot, dry spells, the leaves of these tree species will begin to droop or even look wilted at the end of the day. This is because the tree cannot keep up with water loss. If there is adequate soil moisture, the trees will catch up overnight, and the leaves will look normal the next day. However, if this does not occur, it means the tree is losing more water than it can replace. This is when things start to go downhill.
Be sure not to overwater! If the soil appears too moist to the touch, then you may not need to water right away. Also, examine the soil texture. Sandy and loam soils will drain and dry out quicker than clay soils, requiring more frequent watering. Clay soils take longer to drain, so be sure not to overwater, as the roots will “drown.”
How much is enough?
Using 1 inch of irrigation per week as a guide, place your sprinkler within the drip line and position water-collecting containers under the canopy. Turn the water on, set the clock, and come back in an hour and measure how much water you have collected. Record it for future reference so you know when you hit 1 inch. This may take all day. If your irrigation rate exceeds infiltration, then runoff will occur, and that is wasteful and inefficient.
Soaker hoses can work just as well, but you will probably need to run them for several days, as they deliver water more slowly.
Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at [email protected]. The opinions of this writer are not necessarily those of Farm Progress/Informa.
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