What to Grow in a Seattle-Area Greenhouse in Fall and Winter

Seattle’s maritime climate gives gardeners a head start and a safety net: relatively mild winters, high humidity, and frequent cloudy days. A greenhouse in the Seattle area extends the growing season and makes winter production productive rather than marginal. This article gives practical, detailed advice on which crops succeed in a fall and winter greenhouse around Seattle, how to manage temperature, light, water and pests, and a season-by-season schedule with variety and technique recommendations.

Seattle winter realities and greenhouse goals

Seattle winters are cool, cloudy, and wet. Outdoor temperatures commonly range from the mid-30s to mid-40s Fahrenheit at night and 40s to low 50s by day in the coldest months. Day length is short and overcast conditions reduce light intensity. A greenhouse in this region can:

protect crops from wind and heavy rain,
moderate temperature swings (with passive solar heat gain),
allow controlled humidity and pest exclusion,
and–with modest supplemental heat and light–support near-continuous production of cool-season vegetables and year-round herbs and microgreens.

The primary operating constraints are light and occasional freezing nights. To maximize success choose crops adapted to cool temperatures and low light, insulate the greenhouse, prioritize ventilation and humidity control, and use supplemental lighting and heat only where needed.

Key environmental targets for fall and winter production

Temperature

Daytime target for cool-season crops: 55 to 70 F.
Nighttime target: 40 to 50 F for hardier greens; 50 to 60 F if you want to grow warm-season crops like tomatoes or peppers.
Freeze protection: an unheated greenhouse often holds a few degrees above ambient. Insulate glazing with bubble wrap or row covers when expecting sub-28 F extremes (rare in Seattle but possible in cold snaps).

Light and photoperiod

Natural light is limited. Give leafy crops a minimum of 10-12 hours light; 12-14 hours is better for faster growth.
Use full-spectrum LED fixture(s) if you plan to maintain steady production–aim for even coverage and 20-40 watts of quality LED lighting per square foot as a general guideline.

Humidity and ventilation

Aim for 50-70% relative humidity. High humidity increases fungal disease risk; good air circulation, vents, and small fans reduce problems.
Water early in the day and avoid prolonged leaf wetness.

Soil and nutrients

Use a loose, well-draining mix with good organic matter for containers and raised beds–pH 6.0-6.8 for most vegetables.
Slow-release fertilizer at planting plus regular liquid feeds for water-hungry crops. Reduce fertilizer strength for cool-season growth to avoid leggy, weak plants.

What to grow: best crops for Seattle greenhouses in fall and winter

Seattle-area greenhouse winter success favors cool-season, quick-maturing and long-cropping plants. Below are categories with varieties, timing, and tips.

Leafy greens (high priority)

Leafy greens are the backbone of winter greenhouse production: fast, shade-tolerant, and continuously harvestable.

Lettuce: Look for winter-hardy types such as Winter Density, Rouge d’Hiver, and other dense, cold-tolerant romaines and butterheads. Grow as cut-and-come-again or individual heads. Sow every 2-3 weeks for continuous harvest.
Spinach: Bloomsdale Long Standing and other savoy-leaf varieties tolerate cold and slower bolting in low light. Sow in fall and again in late winter.
Swiss chard: Bright Lights or Fordhook Giant produce reliably through cool months; tolerate partial cold.
Asian greens: Mizuna, tatsoi, komatsuna and baby bok choy grow quickly and tolerate cool temps–excellent for stir-fries and greens mixes.
Arugula and mustard greens: Very fast (harvest in 3-4 weeks), great for cut-and-come-again.

Brassicas (for fall planting and overwintering)

Broccoli, cauliflower and cabbage are best started in late summer or early fall for mature heads before or during early winter; some varieties (purple sprouting broccoli, overwintering broccoli) tolerate cooler growth and will produce in late winter/early spring.
Kale (Lacinato, Nero di Toscana, and winter-hardy curly types) thrives in low temperatures and often sweetens after cold exposure.

Roots and bulbs

Radishes: Extremely fast–10-30 days depending on variety. Use daikon in larger beds or French breakfast and Cherry Belle for quick turnarounds.
Carrots: Use short or Nantes-type varieties (e.g., ‘Nantes’, ‘Chantenay’, ‘Little Finger’) in deep, loose soil. Germination is slower in cold soil; pre-warm seed flats or use bottom heating to speed establishment.
Beets and turnips: Reliable and tolerant. Baby beets and hakurei salad turnips are great in mixed plantings.
Garlic: Plant cloves in fall (October-November) for summer harvest. Protect with mulch and let bulbs overwinter.
Overwintering onions and shallots: Plant sets in fall for early next-year harvests.

Herbs and perennial edibles

Parsley, cilantro, chives, and thyme do well in cool conditions.
Rosemary and bay can be overwintered in a greenhouse if temperatures are kept above freezing and light is supplemented.
Mint and oregano tolerate cool, moist greenhouse conditions but confine them as they are vigorous.

Microgreens, shoots, and sprouts (fast cash crop)

Microgreens and baby shoots are ideal for winter: minimal light needs, fast turnover, and high yields in small space.

Good choices: radish, broccoli, arugula, pea shoots, sunflower.
Turnaround: 7-21 days, depending on species. Succession sow every week for steady harvest.

Warm-season and fruit crops (with caveats)

Tomatoes, peppers and cucumbers can be grown in winter only with reliable supplemental heat and significant added light. If you have vigorous LED lighting and maintain greenhouse temps above 60 F at night and 70-75 F by day, you can produce small volumes of cherry tomato varieties and compact pepper cultivars.

Better winter fruit choices: dwarf citrus (Meyer lemon) and figs in a greenhouse corner, but they require high light and careful humidity/temperature management.

Pest and disease management in winter greenhouses

Even in colder months pests can be a problem–aphids, spider mites, fungus gnats, slugs and mildew. Common strategies:

Sanitation: remove old plant debris, disinfect benches, and keep pathways clear.
Exclusion: seal gaps, screen vents, and inspect new transplants before introduction.
Biological control: introduce beneficials (ladybugs, lacewings, predatory mites) where aphids or mites appear; use Bacillus thuringiensis for caterpillars if needed.
Cultural controls: avoid overwatering, rotate crops, and maintain airflow to prevent fungal diseases.
Traps: sticky traps for flying insects and beer traps or copper tape for slugs.

Practical greenhouse management tips and tricks

Insulate: line lower walls and part of glazing with bubble wrap in December-February to reduce radiant heat loss. Insulate bench edges and pipework.
Thermal mass: add water barrels painted black or large rocks to store heat during the day and slowly release at night.
Ventilation: even in winter, open vents on sunny afternoons to prevent humidity spikes and fungal outbreaks.
Row covers and cloches: use inside the greenhouse for extra protection on cold nights and to create microclimates for seedlings.
Staging and heat zoning: put the most cold-sensitive plants together near heaters or inside smaller cold frames; place hardier crops on cooler benches.
Seedling care: bottom heat (propagation mats at ~70-75 F) speeds germination in cold months without heating the entire greenhouse.

Example fall-to-winter planting calendar (Seattle area)

September-October
Start brassica transplants (broccoli, cabbage) for fall maturity or overwintering.
Sow carrots, beets, and parsnips in beds where soil is prepared and loose.
Plant garlic cloves and onion sets for next summer.
Begin lettuces, spinach, and Asian greens in succession (every 2-3 weeks).
November-December
Continue succession sowing of lettuces, mizuna, and arugula.
Sow microgreens and maintain weekly harvests.
Plant overwintering broccoli varieties if not already done.
Move tender herbs and potted citrus into the greenhouse; provide supplemental light.
January-February
Sow spinach, more lettuce (choose very cold-tolerant varieties), and hardy kale.
Start early peas in sheltered corners if planning an early spring crop–use supports.
Begin tomato and pepper seeds under strong supplemental lighting in late February if you plan to grow them in a heated greenhouse for early summer fruit.

Varieties to consider (practical shortlist)

Lettuce: Winter Density, Rouge d’Hiver, Buttercrunch.
Spinach: Bloomsdale Long Standing.
Kale: Lacinato (Dinosaur), Winterbor.
Chard: Bright Lights or Fordhook Giant.
Radish: Cherry Belle, French Breakfast.
Carrot: Nantes, Chantenay, Little Finger.
Turnip: Hakurei.
Broccoli: Calabrese; purple sprouting for late winter/spring.
Herbs: Parsley, cilantro, chives, thyme, rosemary.

Final takeaways and priorities

Prioritize cool-season greens and fast crops (microgreens, baby leaves, radishes) for reliable winter yields with minimal heating and lighting.
Invest in insulation, airflow, and targeted supplemental lighting rather than heating the entire greenhouse indiscriminately.
Use succession sowing and intensive spacing to maximize production in limited space.
Keep humidity under control and use sanitation and biologicals to manage pests.
Plant garlic and overwintering onions in fall to harvest in summer–this is one of the best uses of a Seattle greenhouse over winter.

A Seattle-area greenhouse can be a productive, year-round food source when you match crop choice to light and temperature realities, use smart microclimates, and focus on the crops that reward you fastest in low-light cool seasons. With modest investments in insulation, ventilation and targeted supplemental lighting, you can harvest fresh salad greens, herbs and roots through the darkest months.