Timely Tips

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

September 21 – 27

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.

Gardening Misinformation October 8, 2025, 7:00 pm to 8:00 pm

Speaker: Kathleen Strahan, Horticulture Community Educator

Do you ever search for help when taking care of your plants, but only get more confused by the answers you find? Join Cornell Cooperative Extension of Westchester County for a webinar program all about gardening misinformation. Learn what misinformation is, how to find reliable sources of information online, how to tell fact from fiction on social media, and debunking common garden myths.

Register here:https://cornell.zoom.us/webinar/register/WN_rQBCrhwdTFyRy5zsCen7ig#/registration

Beech Leaf Disease: Nematodes Attacking Beech Trees! October 22, 2025, 6:00 pm to 7:00 pm

Speaker: Magery Daughtrey, Senior Extension Associate

The new Beech Leaf Disease has been in southeastern New York since 2019—and it has been spreading. Learn about the new invasive exotic nematode that causes the foliar infections, and how the disease progresses until even large trees may be killed in 6-10 years. There are no guaranteed measures for stopping the advance of Beech Leaf Disease in the wild on the native American beech, but research has shown some benefit to treatments on landscape ornamentals (usually European beech). This talk will cover the history of the disease, the symptoms, the geographic range, and our local NY observations. Injection treatments and sprays for management of the disease will be discussed.

Register here:https://cornell.zoom.us/webinar/register/WN_bRl_MypRR6yHlXI9M2LS5w#/registration

Last updated September 20, 2025