How to Overwinter a Greenhouse (Keep Growing in Winter)
Insulate, heat, select cold-hardy crops, and manage humidity to grow lettuce, kale, and spinach through Utah winter.
TL;DR
- Time: 6 hours setup (one-time)
- Cost: $$
- Yield:
- Difficulty: Intermediate
Supplies
Tools
Steps
Pro Tips
Passive solar (dark barrel, south-facing glazing, good insulation) works in Utah because our winters are sunny. A sunny day heats a greenhouse dramatically; plan to vent.
Winter greens grow slowly but they're incredibly cold-hardy. Kale survives to 20°F. Spinach to 10°F. Don't baby them.
Seed tape (pre-spaced seeds on paper) saves time and ensures proper spacing. Make your own or buy commercial.
Mulch the floor with straw or compost. It insulates the soil and makes cleanup easier.
Don't fertilize winter crops heavily. They grow slowly and don't need much nitrogen. One feeding with half-strength liquid fertilizer is plenty.
Warnings
Overheating is as dangerous as freezing. Close vents late, open early. On a sunny 50°F day, interior temps can hit 100°F in 2 hours.
Damping-off fungal disease thrives in cool, humid, stagnant air. Ventilate on warm days and space plants widely for air movement.
Research & Sources
- Year-Round Greenhouse Production for Utah
Dr. Dan Drost, USU Extension Horticulture (2024) - Cold-Hardy Crops and Winter Growing
Dr. Heidi Kratsch, USU Extension (2023) - Greenhouse Environmental Control
Utah State University Extension (2024) - Sevier County Winter Gardening
Sevier County Extension (2023)
Frequently Asked Questions
Do I really need to heat a greenhouse in winter?
Not if you're only growing extremely cold-hardy stuff (kale, spinach, mâche). But below 35°F, growth stops. Heat speeds things up. Decide based on what you want to grow and your budget.
What's the difference between a greenhouse and a cold frame?
Greenhouses are larger and usually heated. Cold frames are small, passive solar. Both work for winter growing; greenhouses let you grow more.
Can I use a greenhouse for summer vegetables too?
Yes, but you need serious ventilation and shading (30–50% shade cloth) or temps will exceed 100°F and kill plants. Summer greenhouse growing is advanced. Most people focus on spring, fall, and winter.
How much electricity does a 1500W heater use?
Running continuously, about 36 kWh/month ($4–5 depending on your rate). Most thermostats cycle on/off, so actual usage is less. It's a small cost for fresh winter greens.
