I don't recommend the use of herbicide around newly planted trees and shrubs. They often have a compromised root system, which can't withstand any chemical herbicide.
Instead, it's better to simply lay several sheets of newspaper around the base of the tree then weigh it down with some kind of organic mulch, like shredded bark or wood chips.
Don't use pebbles for this, because once the newspaper dissolves, the pebbles are hard to remove, and even worse, they end up in the lawn, which can cause issues with lawnmowing equipment.