If my plants were toddlers, they’d start doing one of two things right now: melting down in fiery tantrums or sitting still with drooping eyelids. Either way, I’d know it’s close to bedtime. No more roughhousing, or the kids will be too excited to sleep.
It’s the same in the garden and the greenhouse. My plants are slowing down and I don’t want to do any fertilizing or pruning that would stimulate new growth when it’s getting on toward dormancy time.
You can think of dormancy as a sort of sleep, a renewing rest for plants when resources grow short or conditions get harsh. In southern Africa, plants have evolved to go dormant during the dry season and perk up again when it rains. Where I garden in the American Midwest, most plants go dormant in the winter, when it’s cold and both sunlight and liquid water are in short supply.
Nearly all the trees and shrubs in my neighborhood will shed this year’s leaves, some in a red-and-gold outburst of color. They’ll let much of their sugary sap sink down into their roots to store for next year. Perennials will let this year’s top growth dry up and decay while they concentrate on keeping their roots alive in the soil. Annual plants will die altogether—except for their fallen seeds, lying dormant until spring.
Even plants that don’t go through obvious changes get sleepy. Houseplants in the conservatory will slow their growth as the days grow shorter, needing no fertilizer and much less water as they rest. Evergreens such as boxwoods and pines also slow down, but they’re light sleepers: Since they keep their leaves, any warm winter day will rouse them to do a little photosynthesis.
If everything is getting ready to go to sleep, it may seem odd that September is a good time to plant most shrubs and to divide perennials. But planting them into well-prepared soil is like putting on their PJs. It won’t wake them up to grow stems or leaves; instead they’ll focus on their roots, safe in the insulating soil, until they go dormant, and be ready to grow and bloom above ground next spring.
I’m planning to renovate a major bed this September, moving a bunch of perennials and several shrubs. To tuck them in, I’ll water them all well until the soil freezes. For garden plants, especially evergreens and anything recently planted, watering in autumn is like a goodnight kiss.