Answer: How long to water is based on the plant's root system. Water should run long enough to reach 1 foot deep for small plants, 2 feet for shrubs, and 3 feet for trees. Frequency is based on many factors including your soil type (sandy soil needs more frequent watering than clay soil), weather conditions (hot summer months need more frequent watering than cool winter months), your particular microclimate, the age of the tree (young plants more frequently than older, established plants), etc.
When leaves brown around the edges, the problem is often salt burn. Salts in the water and in fertilizer build up over time. Browning usually occurs on the old leaves first. This excess salt accumulates in the leaf edges, where it kills the tissue and the leaf dries out and turns brown. It's important to water deeply and slowly. At least once a month, water deeply enough to "leach" or push salts well below the root zone. Frequent, light "sprinklings" allow salts to accumulate in the top layers of soil, where the roots are, which is bad news. Similar symptoms occur when too much fertilizer has been applied. Always water plants thoroughly before and after applying fertilizer to help prevent burn.
Use a soil probe (such as a long-handled screwdriver, sharpened piece of rebar or pointed stick) and poke it into the soil. It will move easily through moist soil and stop at dry soil. This will help you determine how water moves through your soil.
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