Answer: Annuals around the base of a young tree won't have any effect on the tree, but perhaps you'd be more inclined to apply water because of the annuals, which would also benefit the tree. All you need for success with your young trees is to water deeply once each week during the growing season. If you like having colorful annuals around the trees, go ahead and plant them. Otherwise, it isn't necessary.
Best wishes with your landscape!
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