This would be more in line with a spell or something a character could learn or have granted to them, though physical changes like plant growth on the body would go along well with it. If you wanted JUST the fertile feet then that would probably be a Simple Spell due to its passive and limited use, though if you wanted her to have more direct control of plants as well it would count as a Complex Spell, which could include both plant control and the fertile feet as a side effect.