Cold-Hardy Container Gardening – Suitable Woody Plants

Are you looking for a cold-hardy woody plant to grow in a half wine barrel or large planter? Because container plants have far less insulation from winter temperatures than plants growing in the ground, the rule of thumb is to select plants that are marked as two USDA hardiness zones colder than the zone in which you are planting. For regular gardening in the ground, the Walla Walla area can be considered Zone 6 or 7, depending on how cold a given winter gets. For cold-hardy containers, we recommend plants labeled Zone 4 or lower to minimize risk of loss.

Depending on the planter size, here are some recommended cold-hardy woody plants for Walla Walla gardeners (though we cannot guarantee survival of any plants wintered over in planters):

• Junipers (evergreen)
• Hicks yew (evergreen)
• Dwarf Scots pines (evergreen)
• Dwarf globe blue spruce (evergreen)
• Big sagebrush (evergreen)
• Dwarf ginkgo cultivars
• Dwarf magnolia cultivars
• Contorted filbert cultivars
• Staghorn sumac cultivars
• Ninebark cultivars

• Chokeberry
• Serviceberry shrubs
• Dogwood shrubs
• Some viburnum cultivars
• Weigela cultivars
• Some blueberry cultivars
• Dwarf birch cultivars
• Spiraea cultivars
• Some raspberry and blackberry cultivars
• Dwarf lilac cultivars

Some additional considerations:

• Pottery containers—even those marketed as “frost-resistant” or “frost proof”—always have some risk of cracking or crumbling due to the expansion and contraction of moisture during winter freezing. Glazed pots are typically more resistant to cracking than unglazed pots. Keep potting medium from becoming waterlogged to help prevent expansion during freezing.
• Packing plenty of fine-textured mulch around pots up to the rim before cold temperatures hit can help give container plants more insulation.
• Evergreens are still actively carrying out photosynthesis during the winter and therefore still need to be watered occasionally. Be sure to use cold water to avoid cracking pottery containers, and avoid overwatering to prevent root rot.
• Drainage is especially important for container plants during all seasons. Although some plants may have unique potting medium needs, coarsely textured potting mixes will ensure the best drainage. Half wine barrels and other repurposed containers will need to have holes drilled in the bottom to ensure an escape route for water.
• Small trees should be planted in containers that are several times larger than the tree’s root ball to ensure sufficient space and soil insulation.