September is the month of the transition from summer into fall, and the best time to start preparing the garden for winter. Here in the Pacific Northwest, our weather is still relatively warm, but the days are getting shorter, and by the end of the month the sun is up for less than half the day. No matter how warm the air stays, summer crops slow to a halt as the soil cools off. That cool soil favors germination of cool-season crops, and as gardeners, we need to start preparing the garden soil to handle the inevitable winter rains.
Prepare for rain. Any crops that stay in the ground through the winter help protect the soil from winter rains. As water seeps through the soil, it picks up unused nutrients and can carry them well below the rooting depth of the plants. Crop plants, and cover crops that are planted specifically to feed and protect the soil, help minimize any damage from winter rains and can even improve the soil. They create a physical barrier to the raindrops, depositing the rain more gently on the soil. Their roots help hold the soil in place and create pore space for the water to soak in. The roots also take up nutrients for the plants to store through the winter, releasing the nutrients only when the plants die and decompose the following spring, making those nutrients available again to the spring-planted crops.
Cut back plants. Wait until after the first hard freeze to cut back collapsed perennials such as columbine, Japanese anemones, brunnera, hardy geraniums, hostas, ligularia, phlox, and herbaceous peonies.
Add mulch. If you don’t leave a cover crop on your garden over the winter months, or if you had to cut back too many collapsed plants, add insulation by layering a top layer of mulch to act as a protective blanket between the cold air and the roots. Look for shredded bark of native trees or evergreen boughs. Add in a mixture of shredded leaves, straw, or pine needles for a mixture that encourages organic growth. Make an effort to keep as much organic material on your soil as possible, rather than discarding it.