Patricia A. McKillip 
Od Magic [EPUB ebook] 

Brenden Vetch has a gift. With an innate sense he cannot explain to himself or describe to others, he connects to the agricultural world, nurturing gardens to flourish and instinctively knowing the healing properties each plant and herb has to offer. But Brenden’s gift isolates him from people, and from becoming part of a community – until the day he receives a personal invitation from the wizard Od, to serve as a gardener in her great school of wizards. But even Brenden isn’t fully aware of the full extent of his power – and the true reason Od has recruited him …