What to Grow in a Washington Greenhouse for Early Spring Harvests

Growing an early spring crop in a Washington greenhouse can turn the long, wet winter into an advantage: stable moisture, moderated temperatures, and protection from frosts and heavy rains. Whether you are in maritime western Washington (mild, wet winters) or continental eastern Washington (colder nights, drier air), a greenhouse gives you control over temperature, ventilation, and moisture to harvest fresh greens and roots weeks to months earlier than outdoor beds.
This article gives concrete recommendations for crops, sowing times, environmental targets, cultural practices, and a sample planting schedule so you can maximize early-season yield. Advice is practical and region-specific where relevant.

Why a greenhouse matters in Washington

A greenhouse in Washington changes three limiting factors for early spring growth: temperature stability, light control, and protection from precipitation.

• Temperature stability: Even an unheated greenhouse raises daytime temps several degrees above outside and reduces radiational heat loss at night. With a small heater or thermal mass, you can reliably keep seedbeds above freezing and accelerate germination.
• Light control: Early spring daylight is increasing but still weak. Greenhouses capture available light and allow the use of reflective surfaces and supplemental light if needed.
• Dryness and disease control: Controlling irrigation and ventilation reduces waterlogged soil and foliar disease compared with outdoor crops that are constantly wet from rain.

Understanding your microclimate within Washington is critical: western Washington can start much earlier (January-February for some crops) while eastern Washington may need extra heat or waits until March-April for reliable growth.

Best crops for early spring harvests

Choose fast-maturing, cold-tolerant crops and “cut-and-come-again” varieties that let you harvest multiple times from one planting. Below are categories and specific crops that perform well in Washington greenhouses for early spring harvests.

Leafy greens and salad crops

Leafy greens are the backbone of early greenhouse harvests. They germinate at cool soil temps and produce quickly.

• Lettuce (leaf and butterhead): sow for baby leaves or mature heads. Expect baby leaf harvest in 25-35 days; heads in 45-70 days.
• Spinach: cold-hardy; baby leaves in 20-30 days, larger harvests in 35-50 days.
• Arugula, mustard greens, mizuna, tatsoi (Asian greens): very fast, spicy salad additions. Baby leaves in 14-21 days.
• Swiss chard and kale: slightly slower but very productive and tolerant of cool temps.
• Salad mixes and mesclun blends: sow dense for quick, continuous salad harvests.

Quick root crops and crunchy vegetables

Roots provide color and texture and can be grown densely for baby roots.

• Radishes: among the fastest, 20-30 days to baby radish.
• Baby carrots and small-rooted carrot varieties: use shallow, deep containers with loose soil; harvest baby carrots in 45-60 days.
• Beets and turnips: baby beets and turnips in 30-45 days.
• Scallions and bunching onions: green onion harvests in 30-60 days depending on seeding and sets.

Peas and early vining crops

Peas (snow and sugar snap) tolerate cool soil and taste best early in spring. They will climb if given support and can be sown early in greenhouse to get an early set before transplanting outdoors or continuing to grow inside.

Herbs and microgreens

• Parsley, cilantro, chives, dill: cilantro and parsley prefer cool temps; cilantro bolts in warmth.
• Microgreens (radish, broccoli, sunflower, pea shoots): harvest in 7-21 days and provide rapid, high-value returns.

What to avoid for earliest harvests

Tomatoes, peppers, eggplants and other warm-season fruiting crops take long to flower and set fruit and usually are not part of true early spring harvests unless you have a heated greenhouse and supplemental light. Focus greenhouse space early on quick crops, then transition to heat-loving seedlings later.

Environmental targets and cultural practices

For consistent, early success, control these variables.

Temperature

• Germination targets: many greens germinate between 50 F and 70 F soil temperature. Some crops like lettuce germinate well down to 40 F but more slowly.
• Daytime growth: 50 F to 68 F is comfortable for cool-season crops. Rapid growth occurs closer to 60 F to 65 F.
• Night temperatures: avoid repeated dips below 32 F for seedling trays. For hardier crops like spinach and kale, nights around 35 F to 40 F are tolerable with protection.

If colder extremes are expected, use row cover fabric, thermal mass (water barrels), or a small thermostat-controlled heater.

Light

• Natural light in Feb-March in Washington is limited. Place benches to maximize light exposure from south-facing glazing.
• Rotate crops or use reflective white surfaces to increase light interception.
• Supplemental LED lighting can speed growth if you want larger heads earlier, but many growers find natural light plus greenhouse capture is sufficient for baby greens.

Soil, fertility, and containers

• Use a well-drained, fertile growing mix with good organic matter. pH 6.0 to 7.0 suits most vegetables.
• Fertilize for leafy growth: higher nitrogen will promote leaf production. Use a balanced liquid feed or compost tea every 10-14 days for intensively cropped trays.
• Containers: shallow flats for microgreens and baby salad; deeper beds or 8-12 inch pots for roots and larger heads.

Watering and disease control

• Drip irrigation or bottom-watering reduces foliar disease. Avoid keeping foliage constantly wet.
• Maintain good airflow with vents and fans to reduce humidity pockets that lead to Botrytis and damping-off.
• Sterile seed starting mix and clean trays prevent early damping-off disease.

Pest management

• Monitor for aphids, slugs, fungus gnats, thrips, and whiteflies. Early spring pest pressure in a closed greenhouse is often lower, but pests explode when conditions become favorable.
• Control methods: sticky traps, biological controls (ladybugs, predatory mites), hand removal, and sanitation.

Succession planting and spacing recommendations

To harvest continuously, use succession planting and tighter spacing for baby leaves.

• Succession interval: sow salad mixes and lettuce every 10-14 days for a steady supply.
• Cutting method: use “cut-and-come-again” by harvesting outer leaves and letting plants regrow.
• Spacing examples: lettuce (4-6 plants per sq ft for baby leaves; 1 per 8-10 inches for heads), radish (2-3 per inch in rows for baby harvests), spinach (thin to 4-6 per inch for baby harvests).
• Microgreens: sow broadcast in flats and harvest within 7-21 days; perfect for filling gaps between longer crops.

Sample schedules by region

Below are practical schedules for greenhouse growers in western and eastern Washington. Adjust for your specific microclimate and the year.

Western Washington (Puget Sound, coastal): sample Jan-April plan

• January: start microgreens and salad mixes under glass; sow living mulch/cover in cold frames. Prepare seed trays and soil.
• February: sow lettuce (for baby leaf), spinach, arugula, mizuna, Swiss chard. Start peas in pots for early blooms.
• March: sow radishes every 10-14 days; sow beets and baby carrots in deep containers; sow herbs like cilantro and parsley.
• April: begin succession sowing and transplant early peas to larger supports; start heat-loving seedlings (tomato/pepper) in a heated corner if desired but not for harvest this spring.

Eastern Washington (inland valleys, colder nights): sample Feb-May plan

• February: start microgreens and early salad mixes in trays with bottom heat if nights are below freezing.
• March: sow lettuce, spinach, arugula inside the greenhouse; use row cover at night or thermostatic small heater to keep temperatures stable.
• April: sow radishes, beets, and baby carrots. Plant peas in containers with trellis.
• May: expect vigorous growth and begin heavy harvesting of greens; succession sow for continuous supply.

Practical variety suggestions

While many cultivars will work, experienced greenhouse growers favor specific varieties for reliability.

• Lettuce: “Winter Density”, “Rouge d’Hiver”, leaf mixes.
• Spinach: “Bloomsdale Long Standing”, “Space”.
• Arugula: “Astro” or wild arugula mixes.
• Radish: “Cherry Belle” for quick rounds, “French Breakfast” for thin roots.
• Peas: “Oregon Sugar Pod”, “Sugar Ann” (dwarf sugar snap variety).
• Microgreens: radish, broccoli, sunflower, pea shoot blends.

Sample 8-week greenhouse plan for continuous early harvests

1. Week 1: Sow microgreens and a tray of mixed salad leaves.
2. Week 2: Sow a concentrated patch of lettuce for baby leaf harvest; sow radial strips of radish.
3. Week 3: Sow spinach and one tray of peas for early flowering.
4. Week 4: Thin and harvest first microgreens; sow successive lettuce and radish patches.
5. Week 5: Start harvesting baby leaves; transplant early peas to lower benches with support.
6. Week 6: Sow beets and baby carrots in deeper containers for delayed harvest; continue succession sowing.
7. Week 7: Harvest second round of microgreens and radishes; side-dress leafy trays with dilute fertilizer.
8. Week 8: Begin larger lettuce head harvests and continue staggered sowings every 10-14 days.

Adjust spacing and sowing density depending on whether you want baby leaves or mature heads.

Common mistakes and troubleshooting

• Overcrowding: sowing too dense for mature heads causes poor air flow and disease. Use dense sowing only for baby leaf crops.
• Excessive humidity: closed greenhouses with no ventilation promote fungal diseases. Install vents or automatic vent openers.
• Underestimating night cold: even if days are warm, nights can freeze. Use thermal mass and row cover for critical nights.
• Poor drainage: many greenhouses get soggy; use raised beds, well-draining mixes, and avoid pooling water.

Quick reference: fast crops and days to harvest

• Microgreens: 7-21 days.
• Radish: 20-30 days (baby), 25-40 days (mature).
• Arugula, mizuna, mustard: 14-25 days (baby).
• Lettuce (baby leaf): 25-35 days.
• Spinach: 30-45 days.
• Peas: first edible shoots in 40-60 days from seed.

Use these as guidelines; exact timing depends on temperature, light, and variety.

Practical takeaways

• Prioritize fast, cold-tolerant crops: leafy greens, microgreens, radishes, baby roots, peas, and hardy herbs for the earliest spring harvests.
• Use succession sowing every 10-14 days to maintain a continuous supply.
• Target daytime temps of 50 F to 65 F for cool-season crops and avoid repeated sub-freezing nights for young seedlings.
• Manage humidity with ventilation, avoid overhead watering, and use clean seed mixes to prevent disease.
• Tailor your start dates to western vs eastern Washington microclimates: western growers can begin earlier; eastern growers should plan for supplemental heat or slightly later sowing.

A Washington greenhouse is a powerful tool for beating the calendar and enjoying fresh, homegrown produce weeks earlier than outdoor beds. With the right crop choices, attention to temperature and light, and a simple succession plan, you can supply early spring salads and sides from your own greenhouse.