By Tim Smith, WSU Extension, North Central Washington, USA
The two most important horticultural practices that affect fruit quality in producing orchards are pruning and irrigation. If both of those practices are well done, you are on your way to high quality fruit production. I believe we are doing a good job pruning, and have many experts ready and willing to help growers improve their tree training skills. On the other hand, we could improve our irrigation management and further improve the already excellent quality of our fruit.
Most growers use about the correct amount of water per acre each year. Unfortunately, the water may not be applied at the time that the trees need it most. The most common irrigation pattern I have found is over- irrigation in the Spring and Fall, with too infrequent irrigation from mid-June through late August.
Trees re-grow their fine “feeder” roots each spring. Over-irrigation in the spring:
- Slows tree root development.
- Increases the possibility of root rot and of collar rot of susceptible rootstocks.
- Leaches the nitrogen, sulfur and boron from the root zone.
- Reduces uptake of calcium and phosphorus.
- Induces excessive vegetative growth.
Allowing the development of an active root system is often more beneficial to the tree than attempting to replace lost nutrients during the summer.
In North Central Washington lower elevation orchards, trees will use about 1.6 to 2.1 inches of water from the soil by the end of April. Most irrigation systems will apply this much water in an 8 to 12 hour set.
The soil surface, grass cover crop and weeds will often begin to look water stressed when the surface 6 inches of soil dries. These plants develop foliage early in the spring and have shallow roots. They will dry out the surface soil long before the orchard trees deeper roots dry the soil in the second and third foot. Weed and cover crop drying is a poor way to judge the need for the first early season irrigation.
Before you irrigate in the Spring, dig holes and inspect the second foot of soil. A soil probe will make it easier to inspect the second and third foot of soil. This soil inspection will often allow you to skip two or three sets in April and May. Trees are under very little stress this time of year and do not need a high soil moisture level to keep them growing and productive.
Under-irrigation during the summer results in the soil becoming ever drier in the third and fourth foot, as applied water penetrates to less depth each irrigation. The dry deeper soils will not reduce fruit quality and yield during the less stressful months of April, May, early June, September and October. As soon as daily water demand becomes consistently high (usually in early to mid-June), the soil profile needs to be filled and maintained at about 50 percent of its holding capacity or higher.
- The most obvious symptoms of under- irrigation during the hotter part of the summer are: smaller than industry average fruit,
- Poor fruit shape. Red Delicious fruit shape often looks great until June, but the fruit becomes less elongated (typy) as the summer progresses.
- Bitter pit in apple, or increased cork spot and “hard-end” or “pink end” in pears.
- Over-stressed fruit often does not store well, losing its pressure more rapidly than the industry average.
As you can see, there are compelling economic reasons to carefully manage your irrigation that go far beyond fish, power and water conservation.
To keep the proper amount of water in the orchard soil during the growing season, you must balance three irrigation basics (click on the topic for more information): the water holding ability of your soil, the amount of water applied by your system, and the rate the trees use water. Each of these factors is relatively simple, but there are many potential combinations. Each orchard block must be evaluated and managed individually.
There is no standard, set daily interval irrigation cycle that you can use during the growing season.
Unless your orchard is gaining water from some uphill source, the amount of water available to your trees between irrigations depends on the soil texture and the depth of your trees roots. Coarse textured, rocky and shallow soils hold far less water than fine textured deep loams. The bulk of orchard tree roots are in the top two or three feet of soil. Many irrigation systems have been designed with the assumption that tree roots are five or six feet deep. If your system was designed years ago with these assumptions, your irrigation cyclde may have too-long intervals between sets, and your fruit may be affected. We have found that you can not depend on those deeper roots if you wish to limit tree stress. Since the soil in North Central Washington is quite variable both in texture, layers and depth, usable water storage can range from .8 inch on shallow sands, to 3.5 inches on the best soils.
The amount of water applied by your system should not greatly exceed the amount of water the soil can hold. If your sprinkler system is properly balanced to your soil, it should replace the water that the trees have used since the last set. The amount of water used between sets should be no more than 50 percent of the soils water holding capacity. Many systems were not well designed, and may apply more water than your soil can hold. Growers often reduce set length from 24 to 12 hours to improve their systems match to their soil.
Understanding the rate that your trees are using water will help you reduce the stress on your fruit, both from over and under-irrigation. The trees daily water use depends on leaf growth, heat, humidity, day length, and wind. In the Spring, with a light leaf load, shorter days, and cool, humid weather, the trees use water slowly. An acre inch of soil moisture will last about two weeks in April. As the trees develop more foliage, and the weather warms, water use rate increases. That same acre inch of soil water will last about 6 days in May, 4 days in June and 3 days in July. The use rate drops to 4 days per inch in August, 7 days per inch in September, and 10 or more days in October.
If you want to keep an even amount of water in your orchard soil, you must vary the number of days between sets, or vary the number of hours per set. Longer intervals in the Spring and Fall, shorter intervals during the Summer heat. This is not always easy, but it is what the trees require.
Many growers have made minor set length and timing adjustments in the past, with major tree and fruit response. If you suspect that your tree and fruit growth is not up to industry standards, look carefully at your irrigation management. Correcting irrigation problems often leads to a rapid improvement in both fruit quality and yield.