In the North, the ground will have become frozen by some point in December. If you have not already done so, you will need to afford winter protection to any plants that you think need it: You may not get another chance once a deep layer of snow forms.
It is a different matter altogether in the South, where December brings welcome relief from the heat to plants and gardeners alike. Take advantage by growing plants that you can't grow for most of the year and by undertaking tasks requiring vigorous physical activity.
Here are regional gardening tasks to perform to get the winter off on the right foot.
All Regions
You are at the end of another gardening year. This is a great time to take stock of your garden to determine which aspects of it you love most and which aspects you do not care for so much. You may wish to make changes for the next gardening year that give you more of the former and less of the latter. Hopefully, you have been taking photos of your garden and recording your thoughts about it in a journal all year long, so that the process of evaluation does not rely solely on memory.
If you have been storing bulbs, corms, or tubers, check them to make sure that they are neither rotting nor totally drying out.
Midwest
December can be a frigid month in the Midwest. Maintenance and inspection are your primary chores.
Inspect windbreak fences for damage. Repair any damage promptly.
Inspect trees and shrubs for bark damage. If you find any, you most likely have a problem with voles, rabbits, or deer and need to take action.
Spray the foliage of broadleaf, evergreen shrubs with an anti-desiccant to prevent dehydration.
Once snow falls, remove it from paths to the garden. This gives you better access to the winter garden, enabling you to clear away fallen limbs, inspect shrubs for damage, etc.
If you need help winterizing your garden, please don't hesitate for help. We would love to offer help and suggestions!