Do you need to heat a greenhouse in the Pacific Northwest?

Usually only lightly. The marine Pacific Northwest west of the Cascades runs mild by American standards — Seattle sits in USDA Zone 9a and Portland splits 8b (west side) and 9a (east side) on the current USDA hardiness zone map, both a step up from the 8b Seattle held on the prior map. Compare that to a Minnesota or Maine winter and a Pacific Northwest greenhouse rarely needs the heavy insulation or backup-heater sizing a real cold-climate build does. The actual weak point is somewhere else entirely: light and moisture, not temperature.

Why does gray mold (botrytis) show up so often in Pacific Northwest greenhouses?

Because the region's normal winter weather is close to a lab recipe for it. Botrytis cinerea, the fungus behind gray mold, spreads fastest between about 60-70°F with relative humidity above 85%, per Pacific Northwest Pest Management Handbooks and NC State Extension research on greenhouse ornamentals. A closed-up Pacific Northwest greenhouse during the Seattle/Portland wet season — mild temperatures, vents shut against the rain, humidity climbing since there's little dry outside air to exchange with — sits inside that exact window for weeks at a time. Removing spent blooms and dead plant material promptly and running an exhaust fan on a humidity-triggered cycle, not just a temperature-triggered one, does more to prevent it than any fungicide.

How much light does a greenhouse get in a Pacific Northwest winter?

Less than almost anywhere else in the continental US that isn't Alaska. Outdoor daily light integral (DLI) can drop below 5 mol/m2/day on a dark, cloudy, short winter day in the northern US, and a greenhouse's own glazing and frame commonly cut light transmission by another 35-50% before it reaches a plant's leaves. Do the math and a Seattle or Portland greenhouse can sit at roughly 2.5-3.5 mol/m2/day at the darkest point of the year — well under the 10 mol/m2/day floor Michigan State University Extension found lets plants develop full root systems and reach transplant size on a normal timeline.

Do Pacific Northwest greenhouses need supplemental grow lights?

For winter production, generally yes, more than they need a heater. Since the region's outdoor winter DLI already runs under 5 mol/m2/day before greenhouse glazing losses are even factored in, most growers trying to start seedlings or grow leafy greens through December and January are working well below the 10 mol/m2/day threshold where growth reliably stalls. A supplemental LED fixture closes that gap far more directly than adding heat does — see our grow light picks below for options sized to a hobby structure.

What's the biggest mistake Pacific Northwest greenhouse growers make?

Buying and insulating for cold that mostly doesn't come, then leaving the vents and grow lights as an afterthought. Because the region's winter lows are mild relative to the rest of the northern US, it's easy to assume the greenhouse challenge is the same shape as a colder state's, just smaller — more insulation, a bigger heater. In practice the harder problem is the opposite: a well-sealed structure trapping humid, mild, dim air is exactly the condition gray mold needs, and dim light alone can stall winter growth even in a perfectly warm structure. Ventilation sized for humidity control and a grow light budget usually matter more here than another layer of insulation.

Greenhouse Gardening in the Pacific Northwest: Low Light & Damp-Winter Guide (2026)

Quick Answer: A Pacific Northwest greenhouse rarely needs serious winter heat — Seattle sits in USDA Zone 9a and Portland splits 8b/9a, both mild by northern-US standards. The real challenges are the opposite of a cold-climate build: winter daily light integral (DLI) can fall to roughly 2.5-3.5 mol/m2/day inside a glazed structure, well under the 10 mol/m2/day floor plants need for normal growth, and the region’s mild, humid, rain-soaked winters (Seattle 37.49in/year, Portland 36in/year, concentrated October through spring) create the exact 60-70°F/85%+ humidity window botrytis gray mold needs to spread. Budget for grow lights and humidity-triggered ventilation before another layer of insulation.

Most of this site’s regional guides fight an extreme: Florida’s heat and humidity, Arizona’s dry desert sun, Minnesota’s -20°F winters. The Pacific Northwest doesn’t fight an extreme at all, and that’s exactly what makes it easy to get wrong — a mild climate reads as an easy climate, but “mild” here means dim and damp for months on end, not warm and dry.

Pacific Northwest greenhouse growing by the numbers:

9a / 8b-9a — current USDA hardiness zones for Seattle and Portland respectively, both a step milder than the prior hardiness map, per Seattle Times reporting on the USDA’s updated 2023 zone data.
36-37.5 inches — average annual rainfall for Portland and Seattle, most of it falling October through spring during the region’s wet season.
60-70°F / 85%+ RH — the temperature and humidity window where botrytis (gray mold) spreads fastest, per Pacific Northwest Pest Management Handbooks and NC State Extension.
Under 5 mol/m2/day — typical outdoor winter daily light integral (DLI) on a dark, cloudy, short day in the northern US, before a greenhouse’s own glazing cuts transmission by another 35-50%.

The problem isn’t cold

A hobby greenhouse kit built for real winter cold — thick insulation, a heavily sized heater, deep anchored footings — is solving a problem the Pacific Northwest mostly doesn’t have. Seattle’s shift to Zone 9a and Portland’s 8b/9a split mean extreme winter lows in the 20-25°F range for most of the region, nothing close to the -20°F to -30°F our Minnesota guide covers or even the harder freezes a greenhouse for winter build is rated to shrug off elsewhere. A standard kit from that roundup, lightly heated on the coldest nights, comfortably handles a Pacific Northwest winter’s actual temperature floor. Spending the same budget on more insulation instead of a grow light or a humidity-triggered exhaust fan solves the wrong problem for this specific climate.

The real problem: light

Daily light integral — the total usable light a spot receives over a full day — collapses in a Pacific Northwest winter. Outdoor DLI can fall under 5 mol/m2/day on a dark, cloudy, short day this far north, and a greenhouse’s own structure and glazing material commonly cut another 35-50% of that light before it reaches a leaf. Run the numbers and a Seattle or Portland greenhouse can sit around 2.5-3.5 mol/m2/day at the darkest point of December and January — less than half the 10 mol/m2/day floor Michigan State University Extension has found lets seedlings develop full root systems and reach transplant size on schedule; below that floor, MSU’s own research greenhouses saw transplant time roughly double. That’s a light problem no amount of heat fixes.

Full-Spectrum LED Greenhouse Grow Light

Supplemental lighting for winter DLI shortfall · sized for a hobby 6x8-8x8 structure

Closes the gap between a Pacific Northwest winter's roughly 2.5-3.5 mol/m2/day indoor DLI and the 10 mol/m2/day floor most seedlings and leafy greens need to grow on a normal schedule.
Run on a timer alongside natural daylight rather than replacing it entirely — see our best greenhouse grow lights picks for models sized to different structure sizes.
Matters more here, budget-for-budget, than an upgraded heater — this region's winter temperature floor is mild; its winter light floor is not.

Check price on Amazon →

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The other real problem: gray mold

Botrytis cinerea, the fungus behind gray mold, spreads fastest between roughly 60-70°F with relative humidity above 85%, according to Pacific Northwest Pest Management Handbooks and NC State Extension research on greenhouse ornamentals. That’s close to a description of a closed-up Pacific Northwest greenhouse on a typical wet-season day: mild outside air, vents shut against the rain rather than opened for exchange, and humidity climbing because there’s little dry air outside to trade it for. Seattle and Portland both see the bulk of their 36-37.5 inches of annual rainfall from October into spring, which is exactly the stretch when growers are most tempted to keep everything sealed. The fix isn’t more heat or a dehumidifier alone — it’s ventilation triggered by humidity, not just temperature, plus promptly removing spent blooms and dead plant material where the fungus gets its start. Our greenhouse dehumidifier and greenhouse fan picks both matter more in this climate than they would somewhere drier.

What to prioritize, in order

For a Pacific Northwest hobby greenhouse, the spending priority runs opposite to a cold-climate build: ventilation and humidity control first (a fan that responds to humidity, not just heat), supplemental grow lighting second (to close the winter DLI gap), and a modest heater last, sized for occasional light-frost nights rather than a real cold snap. That ordering surprises growers who assume “mild climate” means “easy, low-budget greenhouse” — it’s a different budget shape, not necessarily a smaller one, and skipping the light and humidity fixes to save on them is the most common regional mistake.

The bottom line

A Pacific Northwest greenhouse’s fight isn’t cold, it’s dim and damp: winter DLI can run under half the 10 mol/m2/day growth floor, and the region’s mild, wet-season conditions sit squarely inside botrytis gray mold’s favorite range. Skip the heavy-insulation upgrade a Minnesota or Maine build needs and put that budget into a grow light and a humidity-triggered exhaust fan instead — that’s the actual dividing line between a Pacific Northwest greenhouse that limps through winter and one that grows through it.