In my fantasy world everyone has several (three?) magic powers that they're born with, and one of them could be control over plants. About 1/4 to 1/5 of the population will have this. They make the plants grow mostly however they like, though bigger projects will be harder.
Giant trees could be made to grow into houses (think the elven houses in the Inheritance Cycle)
Living boats (self repair!)
Food grown quicker, and out of season
Throwing the seeds on the ground, and making them sprout (perhaps they are vines that can suddenly wrap around the enemy!)
Plants keep certain characteristics; if a plant has yellow flowers, it keeps having yellow flowers.
Growing a plant too much might kill it
things can't be undone (you can't make a tree absorb a branch you just had it grow)
Significant knowledge of how plants work required
What are more practical applications of this power? What other limits? What sort of plants would be particularly suited for this? What kinds of rules of biology will I be breaking? Any cool new plant species that could be magically created?