Timely Tips

What to Do in September 2025
Weekly Gardening Timely Tips from
Cornell Cooperative Extension of Westchester County

August 31 – September 6

Perennials: If all the color in the garden fades at the end of summer, try adding late-blooming perennials to extend the season. Asters, purple coneflowers, goldenrod, New York ironweed, perennial sunflowers, rudbeckia and other late bloomers are all great options (try plants like American burnet in moist areas). These will also provide food for beneficial insects. In addition, plant perennials with variegated and colored leaves to brighten up the garden. Many of these plants can also provide shelter for overwintering pollinators. Early September is a good time to seriously assess perennials. It is too early to start digging up and dividing plants as some perennials are still blooming. However, walk through the garden, make notes, and take pictures to plan which plants require dividing or partial removal.

Flowers: Many flowers that are annuals in our climate are perennials in more temperate areas, including coleus, geraniums and impatiens. Take cuttings of healthy insect and disease-free plants now and grow them indoors, they will have color all winter and provide good-sized plants ready to go outside in the spring.

Fruits and Vegetables: Now is a good time to plant commonly grown fall crops: arugula, beets, cabbage, carrots, fennel, kale, kohlrabi, lettuce, peas, parsley, radicchio, radishes, spinach, turnip and more. Tuck a few plants of smaller vegetables into available gaps in the main season crops if these are still going strong. September is a big month for ripening fruit. Check tree fruit and other fruiting plants regularly to harvest at peak condition.

Trees and Shrubs:As the weather cools through the end of the month plant needle-leaved evergreens, such as cedars, pines, spruces and yews and extra-hardy containerized broad-leaved evergreens (American holly, andromeda and mountain laurel). It’s better to wait until early spring to plant most broadleaved evergreens (including those mentioned above if they are balled and burlapped), Japanese maples and marginally hardy trees and shrubs. Water any new plantings as needed until soil evaporation slows. Unless removing dead, damaged, or diseased parts, avoid pruning as this may encourage new growth. Pruning can disrupt the hardening off process, leaving plants vulnerable to winter injury. This new growth may be injured or die when freezing temperatures arrive.

Lawns: Early fall as the weather begins to cool is the perfect time to attend to the lawn. If there is no longer heat stress in the area, start reseeding and other tasks now. Start by dethatching and aerating the lawn. Prepare the soil well for a new lawn by performing a soil test. A soil test will demonstrate if the lawn requires any soil amendments. Grass seed must be planted, or sod lain in loose soil with good soil contact. Use high quality disease-resistant grass varieties. Shade mixes require at least four hours of direct sunlight, sun mixes at least six hours of direct sun. Overseed established lawns as needed to increase turf density.

Houseplants: Start preparing outdoor houseplants to move back indoors for the winter. Address pest or disease problems to avoid bringing them into the home environment. Clean pots and possibly sanitize houseplant pots as well. Gently clean leaves with a dishcloth and water to remove insect residues, possible insect eggs, and any other unsightly messes. Prepare spots inside the house as well, providing proper light, air movement, and temperatures as needed by each plant.

General: Invasive spotted lanternfly (SLF) adults are actively feeding on many plants as they mate and begin to lay eggs. The insects are especially attracted to tree of heaven and grape vines, but their eggs can be deposited on many surfaces in addition to trees. If you live in, plan to travel, or have recently returned from a place where SLF is established, be sure to check your vehicle and any items kept outdoors for hitchhiking lanternflies or their eggs, and remove all before traveling (a flashlight and mirror may be helpful in hard to see spaces). For more information on spotted lanternfly, seehttps://westchester.cce.cornell.edu/horticulture-environment/invasive-nuisance-species/invasive-pests/spotted-lanternfly

Last updated September 1, 2025