Washington state spans several climate zones, from mild maritime on the coast to cold, dry continental east of the Cascades. Winterizing a greenhouse in Washington requires attention to local conditions, but many effective measures are low-cost, practical, and scalable. This guide gives step-by-step, budget-focused strategies to keep plants alive, reduce heating costs, and protect your structure through the cold months.
Before investing time or money, identify your greenhouse location, typical winter lows, and what you want to grow through winter. Coastal Western Washington often stays above 20 F on most nights, while Eastern Washington can see well below zero F. Prioritizing gives you the best return on limited funds.
What minimum temperature do your plants tolerate?
How long do you need to maintain that temperature (nights only, entire season)?
How airtight and insulated is your existing greenhouse?
Can you use passive measures first before adding active heat?
Twin-wall polycarbonate panels have good R-value but can be costly. A budget-friendly alternative is horticultural bubble wrap or reflective bubble insulation. The material is lightweight, easy to cut, and can be attached with clips or tape. For older plastic-glazed greenhouses, double-layering polyethylene film with a small inflation fan can provide dramatic thermal improvement.
Use hay bales or straw outside the greenhouse along the windward side as a windbreak. Place them on pallets or a small frame to reduce rot against the greenhouse wall. Straw is inexpensive, widely available in Washington, and can raise the effective exterior temperature by several degrees by reducing convective heat loss.
Measure and cut the bubble wrap to fit panels, leaving a small air gap. Attach with aluminum horticultural clips or UV-resistant tape. For a double layer of polyethylene, create a simple frame and install a small low-RPM fan to keep the space slightly inflated. The cost of a small inline fan and thermostat-controlled outlet is modest and pays back in reduced heating demand.
Thermal mass stores daytime heat and releases it at night. Water is one of the cheapest and most effective mass materials. Use food-grade 55-gallon drums painted flat black on the sun-facing side, or line a row of partially buried masonry blocks or stones inside the greenhouse.
Place barrels near the planting areas and away from ventilation drafts. A single 55-gallon drum stores significant heat; several drums placed strategically can reduce night temperature swings by 4 to 8 F, often enough in maritime Washington to avoid active daytime heating.
Maximize winter solar gain by cleaning glazing and removing shading from trees on the south side. Use reflective materials on the north wall to bounce light back into plant zones. Thermal curtains or horticultural fleece that can be drawn over benches at night add insulation without blocking daytime sun.
Thermal curtains made from inexpensive quilted moving blankets or commercial greenhouse thermal curtains can be suspended on simple pulleys. They should cover the plant zone at night and be opened during the day. Even a DIY setup with curtain hooks, cord, and a small pulley reduces heat loss substantially.
When passive tactics are not enough, add active heat selectively and safely. Consider these low-cost options:
Always prioritize safety: install a carbon monoxide alarm if using combustion heaters, use GFCI-protected circuits for electrical heaters, keep combustibles away from heat elements, and use thermostatic controllers to avoid overheating and energy waste.
Composting converts plant waste into heat. A well-managed compost pile can reach 120 F internally for extended periods. Build a compost bin or trench adjacent to or under the greenhouse and pipe the warm air through simple ducting or perforated piping beneath benches. This approach costs little beyond labor and can supply meaningful overnight warmth while recycling garden waste into mulch.
Layer greens and browns to maintain moisture and aeration. A mix of fresh clippings, manure if available, and straw works well. Turn the pile weekly during the active heating phase. For piped systems, use corrugated tubing or perforated PVC behind benches; position inlet near the hot core and use a small fan to draw warm air toward plant zones.
Cold air holds less moisture. Condensation and high humidity in a greenhouse can increase disease pressure. Manage humidity by:
Proper ventilation also prevents the buildup of ethylene and combustion gases if using heaters. Install manual or automatic vent openers that close at night and open with daytime heat.
Wrap exposed irrigation lines with foam pipe insulation and install simple thermostat-controlled trace heating cables where freezing is a risk. Drain low points and store hoses indoors. Elevate electrical outlets and use weatherproof housing and GFCI protection. Simple rubber mats over cold floors protect water lines and provide a warmer working surface.
Group tender plants close to thermal mass or under hanging plastic mini-tunnels. Use cloches, cold frames, and row covers for seedlings. Move containers off the cold ground by placing them on pallets or benches and wrap pot sides with bubble wrap to protect roots.
Consider shifting plant selection to cool-season greens and overwintering varieties that tolerate lower minima, reducing the need for supplemental heat.
If you have a small budget, prioritize actions by impact per dollar:
Purchase an inexpensive digital thermometer with min-max memory or a basic data logger to track temperature swings over a week. Adjust insulation, thermal curtains, and heaters based on real readings. A cheap programmable thermostat controlling a relay or outlet can automate heating during the coldest hours and save fuel.
Winterizing a Washington greenhouse on a budget is mainly about reducing heat loss and increasing daytime heat capture and storage. Start with airtightness, add thermal mass, use inexpensive insulation, and prioritize passive strategies. When active heat is needed, use thermostatically controlled, safe heaters targeted to the most sensitive plants. Small investments in sealing, bubble wrap, and barrels will often return the highest value and allow you to keep more plants alive with minimal fuel costs.
By understanding your microclimate, planning a few low-cost interventions, and monitoring results, you can get through a Washington winter without breaking the bank while maintaining a productive greenhouse.