September is a month of suspended animation. Theres nothing new coming up in the garden, but its still too early for fall planting. The days are getting shorter, but it will be weeks before we can stop watering. Often we come home from summer vacations to gardens that look sad and neglected but dont have the energy for so much exertion so late in the season.
Add late-blooming plants to fill in gaps in your perennial beds. For the back section of the bed, choose leggy autumn-blooming anemones in pink or white, cheerful red or gold Helenium (sneezeweed), towering white Boltonia, golden Heliopsis (false sunflower), or the unusual pea-flowered Lespedeza. In the middle of the bed ,plant brightly colored asters or the shrubby blue Caryopteris (Worcester Gold has the added attraction of golden foliage). And for edging, pick the bright blue-flowered plumbago (Ceratostigma plumbaginoides).
Ornamental grasses are at their best now. Use them to fill large or small gaps in your beds, or as the dramatic centerpiece to a planter. Try combining one of the dark grasses like Pennisetum Red Riding Hood or Carex Toffee Twist with a multicolored coleus. Add a blue plumbago or cherry red millionbells and finish it off with some variegated ivy for a stunning but easy display.
Continue to deadhead annuals and summer-blooming perennials to promote further flowering. Cut back to ground level valerian, babys breath, shasta daisies, Husker Red penstemon and Stokes aster. Cut the stems of Russian sage in half, but leave lily stems intact to nourish the bulbs.
Stop feeding your roses, but continue weekly spraying with fungicide right up until the first frost to prevent black spot and powdery mildew. Many fungicides should not be used when temperatures are over 85 degrees, so check the label before spraying. Continue to spray other plants that are prone to powdery mildew such as lilacs, peonies, euonymus, coreopsis, black-eyed-Susans, bee balm, and zinneas. Minimize mildew by watering deeply, keeping the water off of leaves.
Shrubs should not be fed or pruned again before winter except to remove any diseased or broken branches. Check around the base of Anthony Waterer spireas and remove unwanted suckers.
Although fertilizer manufacturers try to get us excited about lawn care in spring, fall feeding is more beneficial to our cool-season grasses, when weather conditions naturally encourage strong root production and turf density. Apply to 1 pound of water-soluble nitrogen per 1000 square feet. (The amount of water-soluble nitrogen your fertilizer contains will be stated on the package.) Organic fertilizers will give you a healthier, more drought-tolerant lawn with less risk of damage due to over-application.
September is also the best time for seeding new lawns and repairing old ones. Before seeding, prepare bald patches by applying to 1 inch of organic matter (bagged manure or compost are fine) plus lawn fertilizer at the rate suggested on the label. Dig over the area to aerate the soil and incorporate the amendments, spread the seed by hand and tamp down the area gently with your feet to ensure good contact between the seed and soil. Sprinkle the area lightly with water every day until the new grass is at least two inches tall. Once the new lawn is established, water deeply once or twice a week.
September is usually a dry month. Keep up regular watering, especially shrubs, annuals and late-blooming perennials. Plants that are especially in need of regular watering include late-blooming perennials, annuals (which generally have shallow roots), and recently-planted shrubs and trees.