Answer: You've chosen well! The Dappled Willow, according to the description on the Monrovia Web site, says it's "scaled perfectly for the small garden...moderate-growing to 4 to 6 feet tall and wide." In which case, you can expect the roots to extend 1.5 times the reach of the canopy of the tree. However, willows have very extensive roots, so I would count on more like twice the reach of the canopy -- 12 feet.
If you'd like more specific info on your tree, visit the Monrovia Plant Catalog Search at http://www.monrovia.com.
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