Overwintering Container Perennials in a Zone 5 Quonset Greenhouse

A 20×20 ft Quonset greenhouse in Zone 5 can overwinter container perennials with minimal heat.

Heat Strategy

• Install a thermostatically controlled heater (electric or propane) set to 28°F (-2°C).
• Run only when forecast lows drop below 20°F.
• Goal: keep root zone above freezing, not warm.

Insulation Boost

• Wrap each pot in bubble wrap or burlap.
• Group pots tightly; mulch over the tops with straw or leaves.
• Place on pallets or foam board to break ground contact.

Ventilation

On sunny days above freezing, vent the greenhouse to prevent heat buildup and fungal diseases.

Monitor

Check monthly: soil barely moist, no rodent damage, no mold.

With a low-temp heater and pot insulation, your starter stock will emerge ready for spring sales.