Timely Tips

What to Do in September 2026
Weekly Gardening Timely Tips from
Cornell Cooperative Extension of Westchester County

September 6 – 12

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 plantcommonly 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

September 13 – 19

Perennials and Flowers: Living ground covers, such as dianthus and creeping phlox, are becoming increasingly popular. When ground cover is used, a thin layer of organic mulch (approximately one inch thick), such as shredded bark or leaves should be applied between plants to help suppress weeds as the plants become established. Keep mulch from collecting against the base of the plants where it can promote rot. Consider using free fallen leaves that don’t mat down. These leaves can provide shelter for overwintering.

Flowers: Dig up cold-sensitive plants such as zonal geraniums well in advance of the first frost. Trim the foliage by one-half to one-third, and lightly trim roots. Pot them up to grow as houseplants or store them in a cool place. Water stored plants occasionally to keep the plants from shriveling. Dig caladiums when the foliage begins to yellow or at the first frost. Wait until frost kills the tops of tuberous begonias, canna lilies, dahlias and gladiolas before lifting them from the soil. In the warmer parts of our region (USDA zone 7A), cannas, dahlias and gladiolas may overwinter in the ground with protection.

Fruits and Vegetables: To ensure home grown greens throughout the fall, continue to sow lettuce, kale, spinach and other greens. A tip for picking kale is to remove the bottom leaves only.This allows the top of the kale plant to continue growing.Once frost threatens, cover plants with a cold frame or a fabric to protect growth and production until severe cold sets in. Keep up with the harvest of maturing vegetables and tree fruit such as apples, pears (pick while slightly unripe), peaches, plums, and brambles such as blackberries and raspberries. Start (or add to) a compost pile. Use a ratio of one part healthy “greens” – like disease-free spent vegetables and other garden plants, weeds without seeds or other propagules – and 2 parts “browns” – like fallen leaves, woodchips, and/or shredded paper or cardboard.

Trees and Shrubs: Tree and shrub growth is beginning to slow in preparation for winter. Avoid the use of fertilizer, especially now. Woody plants should not be fertilized at any time unless there is a demonstrated need as indicated by a soil test. Fertilization and pruning, which encourage new growth, at this time may disrupt the hardening off process and leave plants vulnerable to winter injury. Growth that appears in fall could be injured or die when freezing temperatures arrive.

Lawns: If a soil test indicates that a lawn requires fertilization, early fall is the best time for high maintenance cool-season grasses. This can reduce the chance of nutrient-laden runoff that adds to water pollution. For low maintenance lawns, if needed, fertilize only at this time. Older, well-established lawns may not require fertilization at all, especially if clover is mixed into the lawn, clippings are returned, and leaves are mulched in whenever possible. If required by soil test, apply fertilizer as recommended.

Houseplants: To acclimate plants to the indoors, bring them in about a month before turning on the heat. Ready them for the move by cutting off overgrowth, repotting them if needed and dealing with any pests. Quarantine them for a week in a garage or screened in porch for full observation for the presence of pests. Contact your local Cooperative Extension for advice.

September 20 – 26

Perennials and Flowers: Many spring bulbs may be planted now, except for tulips which are most successful with an early October planting. Consider planting bulbs around ferns, late emerging perennials and other plants that can hide bulb foliage when it withers in late spring. Some annuals and perennials that have gone to seed will “seed around” and produce free plants for next spring if left in place.

Fruits and Vegetables: For a spring spinach harvest, sow seeds of hardy varieties such as ‘Melody,’ ‘Tyee,’ and ‘Winter Bloomsdale’ or other winter hardy varieties under a row cover now and surrounded with a thick layer of mulch applied when the ground begins to freeze. They can also be started in a cold frame, insulated by mulch around the first hard freeze. They’ll overwinter under the mulch layer for an early spring crop. Keep up with the fruit harvest. Remove overripe, damaged, or diseased fallen fruit which may contain pests.

Trees and Shrubs: Fall is the best time to plant deciduous spring-flowering trees and shrubs such as redbud, serviceberry, summersweet, and more. Needled evergreens, such as cedar, pine, spruce, and yew, can also be planted through the end of the month. Wait until spring to plant broadleaved evergreens, Japanese maples and marginally hardy trees and shrubs. Water any new plantings as needed until there is a light freeze. Contact your local Cooperative Extension for advice.

Lawns: To encourage “deep” rooting of a newly seeded lawn, gradually reduce watering over a few weeks. Allow the soil surface to dry once the grass has filled in enough to obscure most of the ground. Water the newly established turf as needed up until a light freeze. Begin cutting the grass when it is about 4 to 4½ inches in height. Remove no more than one-third of the grass in height so at least 3 to 3½ inches will remain after mowing.

Houseplants: If your plants have practically doubled their size during their outdoors vacation this summer and are not going to fit in their usual space, try to make accommodations for a new place indoors with good sun and airflow. Alternatively, divide or prune them back before you bring them in.

September 27 – October 3

Perennials: Consider leaving some plants standing this fall to help feed the birds and provide winter interest in the garden. These include purple coneflowers, rudbeckia, sedum, and switchgrass. Document the garden now with photos, to assess how it is performing. What worked? What didn’t? Early fall is also the perfect time to divide and plant many spring flowering perennials to fill in areas and remedy problems.

Flowers: Mums and fall annuals can add “pop” and interest to the flower garden now. For the best color display keep mums moist at all times. Keep in mind that most mums sold at this time of year at grocery stores or big box stores are not perennials and will not regrow in the spring. Planning ahead, there’s still time to sow seeds of hardy annuals such as pansies, sweet alyssum, and viola in gaps in the flower borders.

Fruits and Vegetables: Did insects, mites or diseases get the best of some crops this year? Bag infested plants and send them to a municipal composting site where the process will heat up enough to kill insect eggs and disease spores. Alternatively, leave them in the sun to heat up and dry out before discarding (avoid composting diseased plants at home). If it makes more sense to call it quits for a main-season crop, there’s still time to put in some fast maturing fall greens. Consider growing a cover crop instead or getting a jump on fall cleanup. Look for resistant varieties to plant next year. Contact your local Cooperative Extension for advice.

Trees and Shrubs: Don’t worry if pines and other conifers are dropping some inner needles. It’s normal this time of year. Take a walk around the property, looking for dead or diseased trees and shrubs. Remove dead stems and diseased branches. If the job requires a ladder, consider hiring the services of a professional. Wait until November through March to prune oak trees to reduce the risk of oak wilt.

Lawns: You’ll likely be mowing for another month or two. Don’t bother bagging grass clippings and some leaves that can be mowed to disappear into the turf. These will return some nitrogen so the grass will need less fertilizer, and also add organic matter that facilitates nutrient cycling to the soil.

Houseplants: Water until some drains out of the bottom of the pot. Giving plants just enough water for the soil to get wet but never drain can lead to salt buildup and reduced root growth. A telltale sign of potential salt buildup problems is a white crust on the sides of the pot or in the soil. If the soil is very dry, it may require repeated deep watering (where water drains out of the bottom) or a complete submersion in water for several minutes. Compost and other potting soil ingredients can become hydrophobic when fully dried out, meaning it becomes difficult for the soil to retain any water at all. Shorter days and weaker daylight slow down most houseplants’ growth, so they don’t need as much water or fertilizer. Start to cut back on these now.

General: If you have an ornamental pond or other water element, cover it with netting so falling leaves don’t make a mess or clog the pump.

Last updated September 1, 2026