Help Your Plants Make a Gradual Transition to Summer

For most of our woody landscape friends, the Seattle-like June of 2003 was terrific. The cool weather and timely rainfall were a welcome change from the norm for this time of year, and growth and flower display were outstanding for most species. However pleasant the past spring has been, it may create problems when the conditions turn more "normal." Keep in mind that July and August are typically months during which the amount of moisture lost from the soil to the atmosphere through evaporation and transpiration from plant leaves is 4 to 6 inches greater than the amount returning to the soil through precipitation. Landscape irrigation is nearly always necessary to keep trees and shrubs in good condition during the heat of summer in Missouri. Careful watering will be especially important this year.

Plants adapt gradually to changes in environmental conditions. This spring, when temperatures were cooler than normal and soil moisture was ample, many trees and shrubs responded by putting on greater than normal shoot growth. Leaves that develop under such conditions are succulent, with a thin cuticle on the leaf surface and a larger than typical surface area. The stomata on the leaves of a non-stressed plant tend to be less effective at regulating moisture loss than those that develop on a drought stressed plant. Root growth is strongly affected by soil moisture conditions. During a rainy spring, root systems of woody plants become shallower than normal. Roots proliferate near the surface, and some deep roots may suffocate due to excessive soil moisture. Thus, after a spring like that of 2003, plants tend to have succulent, water-inefficient leaves and shallow root systems. When the rain stops suddenly and the air temperature rises into the 90’s, such plants will need human assistance to keep them from suffering undue stress. It is common for leaves of trees and shrubs to wilt or scorch during the first blast of hot weather following a mild spring, especially those on newly established plants.

The best strategy for getting plants through the summer after a mild spring is to control soil moisture carefu lly. A 2- to 3-inch-thick layer of mulch is very effective at reducing evaporative water loss from the soil surface around ornamental plants. It also reduces competition for water from weeds. As previously noted, irrigation is critical to keeping landscape plants in good condition following a mild, rainy spring. However, it is important to put some thought into how much and how often to irrigate. Don’t kill your plants with kindness. Prolonged periods of soil saturation when air temperatures are in the 90’s can be just as deadly as drought stress. Every gardener should learn about the percolation and water holding characteristics of his or her soil. In general, the old rule, "supplement rainfall to make 1" of water per week," works well. Some form of drip or low volume irrigation is generally more effective at controlling soil moisture than an overhead sprinkler. See June, 26, 2000, MEG ( http://ppp.missouri.edu/newsletters/meg/archives/v6n7/meg2.htm) for more information on drip irrigation. Regardless of the method used, the application rate should match the soil’s infiltration rate. Most soils in Missouri can only absorb 1/4 to 1/3 of and inch per hour. When unsure about how deeply irrigation water has penetrated the soil, dig down with a shovel or push a metal rod into the soil until it stops. The soil should be moist to at least 10 inches.

Whether you use a sophisticated automatic sprinkler system or a leaky 5-gallon bucket for irrigation, it is important to follow an irrigation regime that will help ease the stress associated with the sudden transition from Seattle to San Antonio that we are currently experiencing. Imagine how quickly you would call the repairman if your air conditioning unit suddenly stopped working, and perhaps you will have a little sympathy for your plants.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630