Fertilizing gives your plants much-needed nutrients that cannot always be found in the soil. Fertilize flowering annuals every few weeks with Espoma FT4 4-Pound Flower-tone 3-4-5 blossom booster Plant Food. Fertilize perennials and shrubs in the spring and fall with Espoma PT18 Plant Tone, 18-Pound. Fertilize trees in the fall with Espoma TR20 18-Pound Tree-Tone Plant Food.