You're more likely to have success with root cuttings on increasing photo-period, that is to say when days are getting longer instead of shorter. Increasing night length, which starts around Father's Day (actually at the summer solstice) causes growth regulators to signal the plant to stop extending (branches) and start storing energy i cambial tissues, so the cutting would be reluctant to push growth.

You're also more apt to have success if part of the root you want to grow into a plant was growing exposed to sun and air movement. The reason is there may then already be differentiated buds waiting to emerge; whereas if you simply expose a large root, tissues may need to dedifferentiate into meristematic regions that can then redifferentiate into apices that will produce buds that develop into branches.

If the roots are fine, smaller than 3/16" diameter, chances of success diminish.

A short while back, I pruned a heavy root (1"+) off a F. salicifolia (narrow/willow-leaf ficus) that had interesting movement. It had laid on the cement in the sun for an hour while I worked on the tree before I decided it was interesting enough to start something from for a friend. That was in July. Nothing happened with the cutting, though it kept passing the 'nick the bark & look for green' test repeatedly, so I knew it was still viable. It wasn't until Jun of this year that it finally started pushing growth.

Al