The best chance you have of getting a Tilia is by mutating a Baccatus, but even then you may end up changing the foliage instead of the flower, or you may get a Lilia instead of a Tilia. With each mutation there is only a 25% chance of getting what you want. The best thing to do is to plant your greenhouse full of Baccatus and mutate each plant once until you get Tilia.