Annuals. Sunset climate zones 7-9, 14-17: Give cool-season annuals a strong start by planting after midmonth in cooler areas and at the end of the month in warm inland locations. Keep the soil moist while plants develop and, if weather is hot, temporarily shade new seedlings. Set out calendula, forget-me-nots, Iceland and Shirley poppies, ornamental cabbage and kale, pansies, primrose, stock, sweet peas, and violas. In coastal areas, try cineraria, nemesia, and schizanthus.
Bulbs. Shop soon for the best selection of healthy bulbs; choose firm ones without soft or moldy spots. Plant anemones, crocus, daffodils, Dutch iris, freesias, homeria, hyacinths, ixia, leucojum, lycoris, oxalis, Peruvian scilla, ranunculus, sparaxis, tritonia, tulips, and watsonia. (Some, such as freesias, homeria, and watsonia, are not hardy in zones 1 and 2.) In mild climates, chill crocus, hyacinths, and tulips in the refrigerator for about six weeks before planting, keeping them away from fruits and veggies, which can thwart bulb development.
Cool-season greens. Zones 7-9, 14-17: Greens like arugula, chard, kale, lettuce, and mustard are some of the easiest vegetables to grow from seed and have much better flavor than store-bought types. For a wide selection of varieties, try a seed source such as Ornamental Edibles (408/929-7333).
Fall-blooming perennials. Zones 7-9, 14-17: Add fall color to your garden with asters, chrysanthemums, gaillardia, gloriosa daisy, Japanese anemone, lion's tail, purple coneflower, and salvia.
Permanent plants. Zones 7-9, 14-17: September marks the beginning of fall planting season -- the ideal time to get many plants into the ground. Most nurseries are well stocked now with trees, shrubs, vines, and groundcovers.
Vegetables. Zones 7-9, 14-17: Set out seedlings of broccoli, cabbage, cauliflower, lettuce, and spinach. Plant seeds of beets, carrots, leeks, onions, peas, radishes, and turnips.
Divide perennials. If perennials like agapanthus, candytuft, coreopsis, daylilies, and penstemon are overgrown or not flowering well, it's time to dig and divide them. (Zones 1 and 2: Do this early in the month.) You can also divide these plants to increase their numbers in your garden. Use a spading fork or shovel to lift clumps, then cut the clumps into sections with a spade, shovel, sharp knife, or pruning shears. Replant sections in well-amended soil and keep moist while new roots develop.
Harvest tomatoes. Continue picking your summer tomatoes. Dig or pull up any plants that have finished producing or have succumbed to disease; add only undiseased plants to your compost pile.
Make your beds. Zones 7-9, 14-17: Before planting a new lawn or flower or vegetable beds, prepare the soil. Dig down 10 to 12 inches with a shovel or rotary tiller, then till in a 4- to 6-inch layer of compost or other organic matter. You can also add a complete fertilizer to lawn areas.