Answer: Your cotoneasters might be suffering from over or underwatering, oddly enough dropping foliage can be a sign of either one. Your goal in watering is to supplement natural rainfall enough to keep the soil evenly moist but not sopping wet or totally saturated. It is better to water deeply less often rather than do a daily or every other day sprinkling. Before watering, dig down into the soil with your finger to see if you need to water. Check both the potting mix and the surrounding soil, because they can dry out at different rates. Water deeply, wait a few hours, then dig down and see how effective your watering was. It can be surprising. You will need to water less often in cooler weather, more often in hot or windy weather. Finally, a layer of several inches of organic mulch can help keep the soil more evenly moist longer and it will help keep down weeds.
Since defoliation can sometimes be a sign of other problems, you might also want to inspect the leaves for signs of insects, such as fine webbing along the stems and leaves or speckling on the undersides of the leaves. If you find anything unusual, your county extension or professional nursery staff should be able to help you identify the pest and suggest what steps to take to control it.
Good luck with your shrubs!
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