Answer: The dappled willow can defoliate for a number of different reasons including lack of water (it prfers evenly moist soil) or a pest or disease problem. Based on your description I am not certain if your plant has lost leaves or if it has died back somewhat. (The new growth is a good sign.)
You should trim off any dead branches, since dead wood will not leaf out again. Begin at the tips of the branches and working your way down to the plant until you reach live wood. Live wood is supple and has green just inside the bark, dead wood is brittle and off-color brown or gray. Also clean up any fallen leaves and dispose of them in the trash to limit reinfection if there was a pest or disease issue.
Be sure to water as needed to supplement rain so the soil stays wet enough. This is especially important with a new plant during the first year or two. Over watering and under watering can both cause drooping or wilting and eventually leaf loss due to root damage -- but with a willow it would be difficult to ever overwater.
Your goal in watering is to keep the soil evenly moist like a wrung out sponge, not sopping wet and not dried out. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
There is no set schedule for watering, it depends on your soil type and on the weather. Using an organic mulch several inches thick over the root area will help reduce watering needs as well as feed the soil gradually as it breaks down over time.
I hope this helps.
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