Overwintering roses in pots, in a cold climate garden, requires a few simple steps for success. As a gardener with a zone 5 garden that receives 3-4 feet of snow per winter coupled with harsh winter winds, I am acquainted with the trials and tribulations of wanting to preserve those precious perennials and roses in pots until the next gardening season begins.
First lets dispel a common myth; freezing temperatures are responsible for plant loss. Actually lack of moisture, not cold temperatures is the usual culprit for plant loss in cold climate areas. If you leave a potted rose outside in a container, and the temperatures are in the low 20’s no amount of insulation will keep the soil from freezing in the pot; therefore, success stems from dealing with this fact.
Tips for Overwintering Plants in Pots
- Know the hardiness of the plant: To successfully over-winter a plant in a pot outside know what the hardiness zone is. In my zone 5 garden I can successfully over-winter most zone 5 plants, but a few just don’t make it. My prize marginal plants are sheltered from winter in a warm greenhouse. Roses which are own-root, not grafted, have a very high success rate overwintering outside, and have fewer pest problems due to the freezing weather which keeps many pests in check.
- Choose the right container: In harsh winter areas use only plastic or fiberglass pots for over-wintering plants in. The larger the pot, the better chance of survival over the winter. Soil acts as a very nice insulation blanket. Also root bound plants will have a difficult time with freezing temperatures as they typically have a low soil to root mass ration in the pot.
- Choose a location for overwintering plants in pots: Place plants in groups in an area in your garden that receives full sun. If you have a large vegetable garden, you can also dig a trench 4-6 inches deep and bury the pots. This will give an extra layer of insulation. The full sun location will allow plants to thaw quicker in the spring.
- Save your fall clean-up for the spring: Trimming plants back hard will expose the crown of most plants to harsh winter temperatures and drying winter winds. The foliage that you would usually trim back acts as a nice blanket over the winter. For large shrubs trim back any branches which may break under a snow load, and for ornamental grasses and most perennials trim back to 6-8 inches high. For roses resist the urge to trim back hard, leaving thinning for the spring. Winter trimming can cause new growth to emerge which will freeze back once winter is in full swing resulting in cane die-back.
- Water well until it snows: Even as the temperature begins to drop water all plants in pots outside once a week until the snow starts to fall. Blustery fall winds, and cool nights can sap moisture from container plants causing the root-ball to dry up. Ideally the root ball of a potted plant should be moist before the first hard freeze.