Irrigation. This is a large and tricky subject; sometimes it works, many times the trees suffer. I have seen a lot of irrigated-only gardens with unhappy trees.
If your garden and turf seem OK but the trees are suffering—the first sign is little annual growth—then keep doing what you’re doing. But either reprogram for an occasional deeper watering or water the trees once a month or every three weeks by hand, especially during periods of prolonged drought.
Irrigation systems are the children of golf courses, where turf reigns supreme. Lots of regular brief waterings during the week will always keep the surface plants and the turf happy. But frequent, short duration watering has its own problems. Evaporation and lack of deep penetration being the two most important. As described in detail above, trees need a steady available source of water; wet, damp, moist soil to stay strong and trouble-free. Turf is a major competitor with trees for available water; being surface rooted, it will get the lion’s share of frequent but shallow waterings.
Like anything else important in life, test it, test your irrigation system. It should also be occasionally maintained; anything this important needs to be in tip-top shape. Have your tech inspect and do test runs on each zone to see that they are as they should be. Or test the system yourself by measuring your weekly output: place small, straight sided-containers—a yogurt container will work—randomly in the watering zone. Either after each cycle or at the end of the week, check your water levels. A minimum of an inch a week is a good start. During really hot weather your test will lose accuracy due to evaporation, so you might be better checking after each watering cycle.
Another important aspect of irrigation watering is dry spots—dry areas no longer receiving the water they once did. This is caused by plant material that has grown much larger than when it was initially planted and now creates “rain shadows” areas behind the taller plants that are blocked from receiving the water they need. Either prune and/or add “towers” to the irrigation heads.
