What to Plant in Washington Greenhouses Each Season

Greenhouses extend the growing season in Washington, buffering against winter rains in western counties and the cold dry nights of eastern valleys. Whether you grow food, ornamentals, or propagate cuttings, understanding what to plant each season and why will improve yields and reduce waste. This guide covers specific crops, sowing and transplant timing, environmental controls, and practical tips tailored to Washington’s regional climates.

Understanding Washington’s Climate Zones and Greenhouse Advantages

Washington’s climate ranges from maritime and mild on the coast and Puget Sound (zones 7-9) to continental and semi-arid in central and eastern areas (zones 5-7). A greenhouse moderates extremes, but you must still manage temperature, light, humidity, and ventilation to match crop needs.
A greenhouse provides three practical advantages:

stable temperatures for germination and growth;
control over humidity and disease risk through ventilation and spacing;
the ability to start or overwinter tender plants and extend harvests.

Match crops to how you plan to manage the greenhouse: heated vs unheated, ventilated vs sealed, and whether you will supplement light in low winter sun.

Winter (December – February): Overwintering, Microgreens, and Early Starts

Winter is the time to protect valuable plants, keep production moving, and sow rapid-turn crops that tolerate low light.

Vegetables and Greens

Sow microgreens, salad mixes, and lettuce varieties suited for low light and cool roots. Use seed trays and shallow flats; harvest in 10-30 days.
Sow hardy leafy greens like spinach, tatsoi, mizuna, and winter lettuce under cold frames or in minimally heated greenhouse beds. Sow thinly and provide good air circulation to prevent damping-off.
Start brassicas (broccoli, kale, cabbage) in flats for transplanting in late winter to early spring. Keep seedlings at 50-60 F nights and 60-68 F days for compact growth.

Herbs and Specialties

Grow parsley, chives, and thyme from seed or divisions. Many herbs are slow to germinate but thrive in cool, bright conditions.
Maintain overwintering pots of basil indoors only if you can sustain 65-70 F and bright light; otherwise, grow basil as spring transplants.

Overwintering and Propagation

Overwinter tender perennials (citrus, bougainvillea, some salvias) in the greenhouse. Provide frost protection and reduce feeding and watering.
Take and root softwood cuttings of pelargoniums, fuchsias, and roses in a heated propagator if available.

Practical Winter Takeaways

Use cold frames, row covers, or a small heater to maintain 40-50 F minimum for hardy crops and 55-65 F for tender seedlings.
Maximize light: keep glazing clean and avoid shading; use reflective surfaces to increase available light.
Prioritize ventilation on sunny days to control humidity and disease.

Spring (March – May): Main Planting Season and Warm-Season Starts

Spring is when the greenhouse earns its keep. Start warm-season crops early, harden off transplants, and fill beds.

Vegetables and Fruiting Crops

Start tomatoes, peppers, eggplants, and cucurbits (cucumbers, melons) from seed indoors 6-10 weeks before the last frost. For western Washington, sow tomatoes mid-February to March; for eastern Washington, sow in late February to mid-March depending on microclimate.
Sow peas and early beans in colder-tolerant beds; pea sowing can begin in March for early harvests.
Direct-sow root crops like carrots and beets in wide beds once soil can be worked, or start in flats for earlier transplants.

Herbs and Companion Plants

Start basil, dill, and cilantro from seed as temperatures warm. Basil transplants into the greenhouse in late April to May when nights consistently stay above 50 F.
Interplant marigolds and calendula to help attract beneficial insects and suppress pests.

Flowers and Ornamentals

Sow annual bedding plants (petunias, impatiens, snapdragons) early to produce mature plants for late spring and early summer sales or garden planting.
Divide and pot up perennials as they break dormancy for greenhouse sale or display.

Practical Spring Takeaways

Use soil temperatures (not air alone) to time sowings; most warm-season crops need soil above 60 F for reliable germination.
Harden off greenhouse-grown seedlings gradually over 7-14 days by reducing temperature and increasing ventilation before moving outside.
Stagger sowings every 1-2 weeks for continuous harvest of fast crops like lettuce, radishes, and herbs.

Summer (June – August): Heat Management and Succession Planting

Summer in a Washington greenhouse can mean managing heat, light, and water to keep plants productive and disease-free.

Heat-Tolerant Crops and Succession Planting

Plant or transplant heat-tolerant varieties of tomatoes, peppers, eggplants, and okra. Indeterminate tomatoes do well if pruned and supported; determinate types yield concentrated harvests.
Start a second succession of lettuce and Asian greens in shaded benches or under shade cloth for summer harvests.
Sow bush beans for quick harvests; they tolerate heat better than peas.

Watering, Ventilation, and Shade

Install shade cloth rated 30-50% to reduce sun scorch and overheating; remove or adjust as fall light decreases.
Use drip irrigation or soaker hoses to maintain consistent moisture and reduce foliar disease. Water early in the day.
Increase ventilation: open vents, use exhaust fans, and employ horizontal airflow fans to reduce temperature and improve pollination.

Pest and Disease Management

Monitor for aphids, whiteflies, spider mites, and fungal diseases. Use sticky traps, biological controls (introduce predatory insects), and spot-treat with approved organic or chemical controls when necessary.
Sanitize benches and pots between plantings to reduce carryover of pathogens.

Practical Summer Takeaways

Plan for cooling: shade, ventilation, and evaporative cooling when necessary.
Continue succession sowings for continuous food production.
Harvest and preserve excess produce: blanch and freeze, make sauces, or dry herbs.

Fall (September – November): Late Crops, Bulbs, and Preparing for Dormancy

Fall is for harvesting, planting overwintering crops, and preparing the greenhouse for winter.

Fall Planting and Extending Harvests

Sow fall lettuces, spinach, and kale for greenhouse harvests into late fall and early winter. Move crops into protected benches or use row covers to extend production.
Plant garlic cloves in October in western Washington and early November in colder eastern pockets, using raised beds or pots in the greenhouse to protect against winter wet.
Start seed potatoes in late summer to early fall for early spring tuber production in cold areas.

Bulbs, Houseplants, and Tropicals

Force bulbs (tulips, hyacinths) in pots for winter and early spring blooms. Begin chilling schedules in mid-fall for outdoor-forced bulbs.
Bring tender houseplants and tropicals in before first freezing nights. Reduce watering and inspect for pests before moving indoors.

Preparation and Cleanup

Remove spent crops and debris to reduce disease vectoring.
Check seals, vents, and heaters. Clean gutters and ensure roof glazing is intact.
Compost healthy plant residues away from greenhouse to prevent pest and disease harboring.

Practical Fall Takeaways

Use the greenhouse to bridge the gap between field crops and winter.
Protect moisture-sensitive crops from heavy fall rains by using raised beds and well-draining mixes.
Plan next season now: order seed, plan crop rotations, and schedule repairs.

Year-Round Propagation and Specialty Crops

Greenhouses are ideal for propagation, overwintering, and specialty production.

Propagate ornamentals from cuttings: use bottom heat (75-80 F rooting temp) and misting to speed rooting. Rooting hormone boosts success.
Grow microclimates for citrus and figs in a heated corner of the greenhouse; keep temperatures above 50 F and provide high light and humidity.
Produce baby greens, edible flowers, and microgreens year-round under grow lights during short winter days to sell at farmers markets or for home consumption.

Seasonal Checklist (Quick Reference)

Winter: microgreens, spinach, brassica starts, overwinter tender perennials, root cuttings.
Spring: sow tomatoes/peppers/eggplant, start cucurbits, transplant brassicas, sow herbs and bedding plants.
Summer: manage heat, succession sowing of lettuce and beans, maintain irrigation and ventilation.
Fall: sow fall greens, plant garlic, force bulbs, move tender plants indoors, clean and repair.

Troubleshooting and Best Practices

Temperature swings: Insulate north walls, use thermal mass (barrels of water) to stabilize night temps, and use thermostatic heaters for consistent minima.
Low light winter: use supplemental LED grow lights on a timer, but balance photoperiods — many crops need rest periods.
High humidity and disease: increase airflow, widen bench spacing, prune to improve light penetration, and avoid overhead watering at night.
Nutrient imbalances: use balanced, soluble fertilizers for container crops and conduct periodic soil tests for in-ground greenhouse beds.

Final Practical Takeaways

Plan sowing dates by counting back from your desired transplant or harvest date and adjust for your local microclimate and greenhouse heating level.
Stagger plantings to avoid labor peaks and ensure steady production.
Match cultural practices (temperature, humidity, light) to the crop: cool-season crops favor lower temps and shorter photoperiods, warm-season crops demand higher soil and air temps and more light.
Prioritize hygiene: clean pots, benches, and tools between uses to reduce pest and disease pressure.
Track everything: maintain a simple greenhouse journal with sow dates, varieties, fertigation records, and pest incidents to refine timing and varieties year to year.

A well-managed greenhouse in Washington can provide fresh produce, beautiful ornamentals, and a year-round propagation space. Use seasonal recommendations above, tailor them to your greenhouse type and location, and refine your schedule each year based on experience and recorded results.