(On a throwaway) The fastest way that I've done it is to get a large area of land (I did about 8x11) and put a lot of large dirt piles in each square to be able to have a variety of plant sizes. I planted the most expensive plant I could and waited (while leveling my gardening skill I did athletics and intelligence at the same time by running on a treadmill and reading a book). It's a huge investment if you don't have a lot of money since every plant you plant adds onto your house value and the method I did brought my house worth up by about 100k, The most expensive plant is the Noobish Hedge Sculpture at $600 per plant.