Answer: How often you need to water really depends upon your soil type, and how much (and how often) it rains. The general rule of thumb is to appy one inch of water per week. While that might be difficult to measure, you should be able to judge the moisture level of your soil by watering deeply as usual and then waiting 3-4 days. Stick your finger into the soil (or dig a small hole with a trowel) and check to see if the soil is moist 3" beneath the surface. If so, you don't need to water; if it is dry, it's time to water. Hope this helps you determine just how often to water your ficus tree.
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