Answer: There are a number of possible reasons for dogwood (Cornus florida) leaves to turn brown. Among the most common are both over and under-watering as well as a number of diseases. Unfortunately, based on your description it is impossible to give you a definitive diagnosis. You might wish to take a sample to your County Extension agent for a positive identification of the problem and recommended control, if any. (Their telephone number is 454-0900.)
In the meantime, watering a dogwood depends to some extent on its location, on the weather and on your soil type. For example, any tree planted on a windy slope in hot sun will need more frequent watering than a tree planted in a protected spot in semi-shade. Dogwoods need a moist but very well-drained (not soggy) soil, so the best way to tell if it needs water is to check the soil under the tree to see if it is dry or not. If it has been raining you may not need to water. Another thing to keep in mind is that trees do better if watered deeply about once a week rather than given frequent light sprinklings.
In nature, dogwoods are generally found growing in dappled shade as an understory tree and this is the type of light they prefer, although they will tolerate full sun. A two or three inch deep layer of organic mulch around (but not touching) the tree is also helpful.
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