To create the highest and mightiest tall phlox plants, you'll need to plant them in the spring. These plants like plenty of water, so choose a relatively open area where they will be exposed to summer rains. Tall garden phlox thrive in full sunlight or in partially shady areas. These stunning Dutch-sourced perennials are hardy in zones 3 through 9 and thrive in well-drained clay, loamy or sandy soil. When burying your tall phlox plants, certain strains need to be arranged so the top of the plant is even with the level of the soil, while others can be buried about 1 to 2 inches deep.