Many varieties of thyme are hardy to USDA zone 4. Although usually gardeners do the bulk of their planting in the spring, some gardeners choose to plant thyme in the autumn to give it a head start on the next growing season. As long as you ensure the thyme has several weeks to establish before the first autumn frost, you can successfully plant thyme in the fall.
Prepare the sunny growing area in the autumn when you anticipate at least three weeks of growing time before the first autumn frost. Cultivate the soil with the garden spade down to a depth of approximately 4 inches. Add 1 to 2 inches of compost to the top of the soil to enrich it. Till the compost in with the loose soil to incorporate it. Rake the soil surface smooth.
Prepare holes for planting the thyme plants with the trowel. Space the plants 1 foot apart and make the holes deep enough so the thyme plants will be at the same depth as they are in the temporary containers.
Remove the thyme plants from the temporary containers and place them into the prepared holes. Add soil around the roots and tamp the soil down gently around the thyme plants.
Provide enough water to saturate the soil thoroughly immediately after planting the thyme plants. Keep the soil evenly moist during the first two weeks while the thyme plants establish in the soil.
Trim the thyme back after the first frost so it is approximately 3 inches above the soil level.
Apply gravel mulch over the roots of the thyme to insulate the thyme roots. The light color of the gravel will reflect sunlight and keep the roots at the proper temperature.