Spring gets the credit, but September is the month experienced gardeners plan around. The soil is still warm from summer, the air has finally cooled and the rain tends to come back into rhythm. Roots keep growing long after the leaves stop, and everything that goes into the ground now spends the next two months establishing below the surface instead of fighting the heat above it. For landscape maintenance in Kennett Square and across the Brandywine Valley, this is where next year’s garden is decided.
Plant trees and shrubs now, not next April.
Roots keep growing until the soil drops to roughly 40 degrees, which in Chester County usually means well into November. A tree or shrub planted in September gets eight to ten weeks of root growth with none of the summer stress a spring planting faces in its first July. Set the root flare at or slightly above grade, never buried, and give each new planting about an inch of water a week until the ground freezes. Fall rain rarely delivers that on its own.
Give the turf its most important month.
Late August through the end of September is the window for core aeration and overseeding cool-season turf. Aerating first opens the soil so seed lands in contact with it instead of sitting on the thatch. Tall fescue overseeded at 4 to 5 pounds per 1,000 square feet fills thin areas well, and the seedbed needs light, daily watering for the first two to three weeks. This is also the follow-through on the grub activity we flagged in August. Grubs feed close to the surface now and are still small, so turf care done this month does far more than anything attempted in spring.
Divide the perennials that have outgrown their space.
Daylilies, hostas, Siberian iris and ornamental grasses that have gone crowded or hollow in the center can be lifted and divided now. Aim to finish about six weeks before the first hard frost, which here typically arrives in late October. Peonies are the exception to handle with care. If you’re moving or dividing them, replant with the eyes no more than 2 inches below the surface. Set them deeper and they’ll reward you with foliage and no flowers.
Buy bulbs now, plant them later.
The best selection of tulips, daffodils and alliums is on the shelves in September, but the soil isn’t ready for them yet. Hold them somewhere cool and dry and plant once the soil falls below 60 degrees, usually late October into November. The rule of thumb is a hole three times as deep as the bulb is tall, which puts tulips at 6 to 8 inches.
Put the pruners and the fertilizer away.
Pruning or feeding shrubs now pushes tender new growth that won’t harden off before the first cold snap. Hold off on both until the plants are fully dormant. Leave the seed heads on Echinacea and Rudbeckia standing too. They carry the garden’s structure into winter and keep the goldfinches coming back.
September is a working month for the EHS professional gardening team, and much of what we plant, divide and seed now is what EHS clients enjoy most next May. If your property could use that kind of head start, we’re glad to talk about yours.
Ann Waters is Head Horticulturist at Eastern Horticultural Services. She is a University of Delaware graduate and completed the two-year Professional Gardener Training Program at Longwood Gardens.
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