Overwintering Container Plants in Cold Climates: Lilac, Delphinium, and Spruce

Gardening in Colorado Springs presents unique challenges, especially when winter temperatures plummet to -20°F or lower. While a lilac rated to -40°F can survive the cold above ground, the real danger for container plants lies beneath the soil. In pots, soil temperatures typically run a full hardiness zone colder than the ground, and roots remain active throughout winter, needing access to water and nutrients. If the root ball freezes solid for extended periods, the plant cannot sustain itself.

Protecting Container Roots from Freezing

The key to overwintering potted perennials like royalty lilac, delphinium, and spruce is preventing the soil from freezing. Since you cannot move the heavy pots, focus on insulating them in place.

Effective Insulation Materials

Use household insulation—such as fiberglass batts or rigid foam boards—in at least two layers wrapped completely around the container. Cover the insulation with heavy-duty plastic sheeting (not just a drop cloth) to keep it dry; wet insulation loses its effectiveness. A thick layer of mulch (straw, shredded leaves, or wood chips) on top of the soil adds further protection.

Monitoring Soil Temperature

If you have a probe thermometer, check the soil on the coldest nights. If it’s freezing despite the insulation, add more layers. At -20°F, even well-insulated pots may need supplemental protection.

Ideal Placement

If possible, shift pots to a sheltered spot—a covered patio against the house, out of wind and frost. Even a few degrees of warmth radiating from the structure can make a difference. If you can enlist help with a hand truck, moving containers into an unheated garage, shed, or basement is the single best strategy.

Quick Checklist for Winter Container Care

• Wrap pots with two layers of household insulation + plastic.
• Mulch heavily on the soil surface.
• Place in the most protected location available.
• Monitor soil temperature during extreme cold snaps.
• Water sparingly—only when soil is thawed and dry—to prevent desiccation.

With these steps, your lilac, delphinium, and spruce have a strong chance of emerging healthy in spring.