Plants are useful material that is required for your production. Luckily, you don;t have to plant them on your own. In order to plant trees, you hire planters that do all the job.  To hire planters or spped them up, tap on the trees and in the opened menu tap "Hire" or"Train".  

Also you can chhose which threes you want to plant in the forest menu. The available types of trees depends on your collection of tree cards.