When to Start Seeds in Washington Greenhouses

This guide explains when to start seeds in greenhouses across Washington state, how to match sowing dates to local climate and crop needs, and practical greenhouse techniques to produce strong, healthy transplants. It focuses on differences between western and eastern Washington, offers concrete sowing windows for common crops, and provides step-by-step recommendations you can use to plan your season.

Why timing matters in Washington

Washington contains a broad range of climates: maritime lowlands in the west, rain-shadowed semi-arid valleys in the central interior, and colder mountain and high-elevation pockets. That diversity changes frost dates, soil warming, day length and humidity — all of which affect germination, seedling vigor, and transplant success.
Greenhouses let you manipulate temperature, humidity and light to get a jump on the season. But greenhouse-grown seedlings that are started too early can become stretched, root-bound or out-of-sync with outdoor conditions; started too late, and you miss ideal planting windows. The goal is to time seed starting so transplants are robust, stocky, and ready to go into their final location when conditions are appropriate.

Basic principles for seed timing

• Start seeds so seedlings are the right size for planting at the expected transplant date, not as a fixed calendar date.
• Use crop-specific target ages: many warm-season crops need 4 to 10 weeks of greenhouse growth before transplant; cool-season crops and direct-sow crops have different rules.
• Base your schedule on local last-frost or soil temperature milestones rather than fixed calendar dates. Last-frost dates vary widely across Washington.
• Consider hardening off and acclimation time: seedlings typically need 7 to 14 days of gradual exposure to outdoor conditions before planting.

Washington timing zones (practical summaries)

This section gives broad last-frost ranges and soil-warming behavior for practical planning. Use these as starting points and adjust for your microclimate.

Western Washington (Puget Sound and coastal lowlands)

Last frost typically: mid-April to early May in low-elevation coastal and Puget Sound sites.
Soil warms sooner than eastern Washington because of maritime moderation, but air is cooler and cloudy spring weather is common. Greenhouses allow you to start warm-season crops earlier but expect cool outdoor nights some weeks after the last frost.

Eastern Washington (inland valleys and plains)

Last frost typically: late April to mid-May in lower-elevation river valleys; mid-May to early June in higher-elevation inland basins and plateaus.
Days warm quickly and soil temperatures rise faster. Wind and low humidity increase transplant shock risk; robust acclimation and irrigation plans are important.

High elevations and mountainous valleys

Last frost often occurs late May to June or even July at high elevations. Growing windows are short; choose crops accordingly and prioritize later seed starting close to planting.

Crop-specific starting windows and rules of thumb

These are practical, crop-specific ranges. “Weeks before transplant” refers to the recommended time from seed sowing in the greenhouse until the intended outdoor transplant date.

• Tomatoes: start 6 to 8 weeks before last expected transplant date. Seed germination 70 F to 80 F (21 C to 27 C) is ideal. Plan for sturdy plants 6 to 8 inches tall with 2-3 sets of true leaves.
• Peppers and eggplant: start 8 to 10 weeks before transplant. These germinate slowly at cooler temperatures and need warmth (75 F / 24 C) to germinate quickly.
• Brassicas (broccoli, cabbage, cauliflower): start 6 to 8 weeks before transplant for spring crops; for fall planting, start 10 to 12 weeks ahead. These tolerate cool soils and often transplant as small sturdy seedlings.
• Leaf greens (lettuce, spinach, arugula): start 3 to 5 weeks before transplant or direct sow. Many greens can be transplanted young to avoid leggy growth.
• Onions and leeks: start 10 to 14 weeks before transplant for sets or transplants; onions grown from seeds need a long lead time.
• Cucurbits (squash, cucumber, melon): start 2 to 3 weeks before transplant or plan to direct sow after soil warms above 60 F (16 C) for cucumbers and 65 F (18 C) for squash. Long greenhouse life makes cucurbits root-bound and fragile; short greenhouse time reduces shock.
• Beans and peas: typically direct sow peas early (as soon as soil can be worked); start bush beans in greenhouse only in cooler interior sites if you need an earlier harvest, 2 to 3 weeks before transplant.

A practical planting schedule example

Below is a numbered set of steps you can adapt to location and crop.

1. Determine your local median last frost date and soil warming date. Use local extension resources or historical records to estimate a conservative last frost window.
2. Decide transplant dates by crop based on the rules above. For example, if you plan to transplant tomatoes around May 15 in western Washington, seed them in the greenhouse around March 20 to April 1.
3. Back-calculate sow dates: subtract the weeks required for greenhouse growth from your transplant date. Add time for germination based on seed packet recommendations.
4. Build in hardening off: schedule 7 to 14 days of gradual exposure to outdoor conditions before final planting.
5. Stagger sowings for succession harvests: sow fresh batches of lettuces every 2 to 3 weeks, tomatoes or peppers only once for the main season.

Greenhouse environment recommendations

To give seedlings the best chance of success, control these variables:

• Temperature: maintain germination temps recommended on seed packets; after germination, grow most seedlings at day temperatures of 60 F to 70 F (15 C to 21 C), with night temps not below 50 F (10 C) for cool-season crops and not below 60 F (15 C) for warm-season crops.
• Light: provide full-spectrum supplemental lighting if natural light is insufficient. Seedlings become leggy without 12 to 16 hours of quality light.
• Humidity and ventilation: maintain humidity to prevent damping-off but provide good airflow. Ventilate on sunny days to control heat buildup.
• Soil mix: use a sterile, fast-draining seed-starting mix with good water retention; avoid garden soil in flats.
• Heating: heat mats or bottom heat accelerate germination for cold-sensitive seeds (peppers, tomatoes).
• Fertility: start with low or no fertilizer until first true leaves appear; then apply dilute fertilizer regularly to keep seedlings compact.

Hardening off and transplant timing

Transplants should be hardened off on a schedule:

• Gradually reduce temperature and watering, increase airflow and sunlight over 7 to 14 days.
• Start with a few hours outdoors in dappled shade and increase exposure daily.
• Pay special attention to wind and sun intensity in eastern Washington; give additional protection for the first 3 to 7 days after transplant.

Practical transplant size targets:

• Tomatoes: 6 to 8 inches tall, well-rooted but not pot-bound, 2-3 true leaves beyond the seed leaves.
• Peppers: sturdy plants 4 to 6 inches tall with several true leaves.
• Brassicas: compact 3 to 5 inches tall, with 3-5 true leaves.
• Cucurbits: 2-3 true leaves; avoid long pot-bound root systems.

Pest, disease and troubleshooting

• Prevent damping-off: use clean trays, avoid overwatering, provide airflow and remove infected seedlings immediately.
• Watch for leggy seedlings: increase light intensity and lower temperatures slightly; pinch back elongated growth if needed or transplant early.
• Root bound seedlings: pot up or repot into larger cells rather than keeping in tiny plugs too long.
• Nutrient deficiencies: pale leaves usually indicate low nitrogen; apply dilute balanced fertilizer after the first true leaves appear.

Fall and winter greenhouse sowing

Washington greenhouses can extend seasons. For fall crops:

• Start brassicas for fall plantings in late summer (6 to 8 weeks before field transplant) to mature as days shorten.
• Overwintering lettuce or greens in heated coldframes or low-tech greenhouses can be seeded in late summer for late-fall harvests or started in late winter for early spring harvests.
• Consider vernalization requirements for crops like onions and some brassicas if transplanting for specific harvest windows.

Quick checklist before you sow

• Confirm last frost and soil temperature expectations for your exact site.
• Choose seed-start timing per crop: 2 to 14 weeks before transplant depending on crop.
• Prepare sterile seed mix, clean trays, labels and heat source if needed.
• Plan hardening-off period and protect first-week transplants with row cover or temporary shade.
• Schedule staggered sowings for continuous harvests and avoid keeping seedlings in trays too long.

Practical takeaways

• Use the greenhouse to control germination and early growth, but time sowing to produce appropriately sized, hardened seedlings at transplant.
• Tailor sowing times to your microclimate: western lowlands can start some warm crops earlier than cold interior or high-elevation sites.
• Warm-season crops require longer greenhouse lead times; cucurbits do better with minimal greenhouse time.
• Hardening off is non-negotiable — it is often the difference between transplant success and failure.
• Maintain records: note sow and transplant dates, soil temps, and outcomes to refine timing year to year.

Starting seeds in Washington greenhouses is a mix of art and precision. Match crop biology to your local climate and greenhouse capabilities, plan backward from desired transplant dates, and manage the greenhouse environment to produce stocky, healthy seedlings ready to thrive outdoors.