When to Start Seedlings in Idaho Greenhouses for Spring Transplanting

Overview: Why timing matters in Idaho

Idaho covers a wide range of climates and elevations, from the wet, cool panhandle in the north to the arid Snake River Plain and high mountain valleys. That means “when to start seeds” is not a single date; it is a calculation based on the expected outdoor transplant date for your location, the crop’s needs, and whether your greenhouse is heated, unheated, or uses supplemental heat. Starting too early wastes time and space and produces long, leggy plants that weaken before transplant. Starting too late reduces the harvest window. This guide gives the practical rules, crop-specific timing, greenhouse considerations, and step-by-step calculations you can use to plan a successful spring seedling program in Idaho.

Understand your local last frost date and microclimate

Every seed start schedule begins with an estimate of the average last frost date for your garden. Idaho ranges widely:

North Idaho (Coeur d’Alene, Sandpoint): average last frost mid-May to early June.
Panhandle and mountain valleys: often late May to mid-June or later at higher elevations.
Treasure Valley (Boise, Meridian, Nampa): average last frost from about April 15 to May 1.
Magic Valley (Twin Falls, Jerome): last frost early to mid-May.
Eastern Idaho (Idaho Falls, Pocatello): last frost mid-May, with local elevation variation.

Use your own historical records or county extension resources to refine these dates. Then adjust for microclimate: south-facing slopes, thermal mass in the greenhouse, or cold pockets can shift the effective transplant window by 1-3 weeks.

Heated greenhouse vs unheated greenhouse vs cold frame

When planning seed start dates, classify your structure:

Heated greenhouse: you can maintain daytime germination and growth temperatures year-round. This lets you start warm-season crops earlier than outdoors by 4-8 weeks.
Unheated greenhouse or high tunnel: provides frost protection and warmer days but nights track outdoor lows. Useful for hardening and for cold-tolerant crops; not reliable for early starts of peppers and eggplant without supplemental heat.
Cold frame: shortens the window only slightly; best for hardening off and starting cold-tolerant crops like lettuce and brassicas a little earlier.

Adjust start dates accordingly: heated greenhouse = earliest, unheated = intermediate, cold frame = latest.

Basic calculation: work backwards from transplant date

To find your seed-sowing date, follow this rule:

Determine your expected outdoor transplant date (based on last frost, soil temperature, crop tolerance, and location).
Add the number of weeks the crop needs to reach transplantable size under your greenhouse conditions (germination + grow time to true leaves and appropriate cell size).
Subtract the hardening-off period (typically 7-14 days) if you need seedlings to be hardened before transplant.

In practice: Sowing date = Transplant date – (growth time + hardening time).
Growth times below are general; use seed packet “days to transplant” or track your own averages.

Typical seed-to-transplant timing and starting windows for Idaho regions

Below are practical guidelines. Adjust the calendar by your local last frost date.

Warm-season crops that need longer greenhouse time (tomato, pepper, eggplant):
Tomatoes: 6-8 weeks from sowing to strong transplants. In heated greenhouse start 6-8 weeks before your planned field transplant date; in unheated, start 4-6 weeks before if nights are cool, or use heat mats to speed germination.
Peppers and eggplant: 8-10 weeks; peppers especially benefit from heat mats and steady warm greenhouse temps. Start earlier than tomatoes if planning early transplant.
Cucurbits (cucumber, squash, melon):
Usually best started 3-4 weeks before transplant because they dislike long root restriction and become rootbound quickly. Many growers direct-seed into warm soil, but in Idaho you can start 2-3 weeks early in a heated greenhouse for early harvest.
Brassicas (cabbage, broccoli, kale):
Cold-tolerant; 4-6 weeks from sowing to transplant. Can be started early in unheated greenhouses or cold frames in late winter or very early spring for May transplants.
Leaf crops (lettuce, spinach, arugula):
3-5 weeks. Start early in unheated greenhouse or cold frame for a continuous harvest; these handle cool nights well.
Onions and leeks:
Long lead time: 8-12 weeks or more depending on variety. In Idaho, many growers start onion seed in late winter (January-February) in heated space for spring transplant.
Herbs:
Varies by species. Basil and other warm-season herbs need 4-6 weeks and warmth; start later. Hardy herbs can be started earlier.

Examples by region and structure:

Boise, heated greenhouse, tomato outdoor transplant around mid-April to May 1: sow tomato seed mid-February to early March.
Coeur d’Alene, heated greenhouse, tomato outdoor transplant mid-May to early June: sow tomato seed mid-March to mid-April.
Magic Valley, unheated greenhouse, brassicas transplant late April to mid-May: sow brassica seed early to mid-March.

Germination and growing environment: temperatures, light, medium

Germination and seedling quality depend on controlled conditions.

Germination temperatures (approximate):
Tomatoes: 70-80 F.
Peppers: 75-85 F.
Cucurbits: 75-95 F.
Brassicas: 60-75 F.
Lettuce: 60-70 F.
Onions: 65-85 F.
Greenhouse growing temps: Aim for daytime 65-75 F for most seedlings; night temps 55-65 F. A few degrees cooler at night helps sturdier growth. Use heat mats for seeds requiring higher soil temps.
Light: Provide 14-16 hours of light per day if natural light is short. Seedlings stretched toward light indicate insufficient intensity or distance. Maintain lights 2-3 inches above young seedlings and raise as they grow.
Medium: Use a sterile, fast-draining seed-starting mix. Avoid garden soil to reduce damping-off risks.

Practical greenhouse management during seedling stage

Watering: Bottom-water trays or water from the base to encourage roots downward and avoid damping-off. Keep medium moist but not saturated.
Airflow and ventilation: Provide circulation fans and occasional fresh air to strengthen stems and reduce disease. Vent greenhouse on warm days.
Fertilizing: Start with a weak balanced fertilizer (1/4 strength) after first true leaves appear, then increase gradually to 1/2 to 3/4 strength.
Pot sizing and potting on: Start in cell trays, transplant to larger cells when seedlings have 2-4 true leaves and roots begin to fill the cell. Tomatoes, peppers benefit from intermediate pots (2-3″ cells) before final transplant.
Disease prevention: Use sterile medium, avoid overwatering, ensure airflow, and inspect daily. Rotate seed trays and clean benches between crops.

Hardening off and timing for transplant

Hardening off is essential in Idaho’s spring conditions.

Duration: 7-14 days is typical. Cold-hardy brassicas may need only 3-7 days; tender peppers and eggplant need 10-14 days.
Procedure: Gradually expose seedlings to outdoor conditions. Start with a few hours in a sheltered, sunny spot in the greenhouse or under shade cloth, increase exposure each day, reduce watering slightly, and lower night temps gradually if possible.
Target for transplant: Seedlings should show active root growth, compact and sturdy stems, and be accustomed to daytime sun and cooler nights. Avoid transplanting soft, etiolated plants or those that have not been hardened.

Soil temperature and field conditions for transplanting

Warm-season crops need warm soil:

Tomatoes and peppers: soil should be at least 50-55 F for safe transplanting, ideally 60 F or higher for best establishment.
Cucurbits: prefer soil 60 F+.
Brassicas, lettuce, spinach: tolerate cooler soil (40-50 F) and can be transplanted earlier.

Check soil temperature with a probe at planting depth in the morning and afternoon for accurate readings. In Idaho, warm-season crops often cannot go into the field until late April to mid-May depending on location.

Troubleshooting common problems

Leggy seedlings: insufficient light or excess heat. Increase light and lower night temps.
Damping-off: overwatering, poor drainage, crowded trays. Use sterile mixes and improve air circulation.
Slow growth: inadequate nutrients or low temps. Provide light fertilizer and consider supplemental heat.
Root-bound seedlings: pot on earlier or use larger cells for long-lead crops like peppers and onions.

Quick planning checklist

Estimate average last frost for your area.
Choose heated vs unheated schedule.
For each crop, determine days to transplant and hardening needs.
Count back from transplant date to get sowing date.
Prepare seed-starting supplies: sterile mix, trays, heat mats, lights, ventilation.
Monitor soil and air temps; adjust heating and ventilation.
Harden off seedlings 7-14 days before transplant.

Practical calendar examples (adjust to your last frost)

Warm-season crops, Boise heated greenhouse: tomato sow mid-February for late April transplant; pepper sow late January to mid-February for late April transplant.
Cool-season crops, North Idaho unheated greenhouse: brassica sow early March for May transplant; lettuce sow late February to March for continuous spring harvest.
Onions, statewide heated greenhouse: sow January-February for April-May transplant or direct sets depending on variety.

Final takeaways

The right sowing date depends on your location, structure type, and crop. Always work backwards from the expected transplant date.
Use heat mats and consistent light to speed germination and produce compact seedlings for warm-season crops.
Invest time in accurate soil temperature checks and hardening off; these steps reduce transplant shock and increase field survival in Idaho spring conditions.
Keep records of dates, temperatures, and outcomes year to year. Idaho microclimates reward local knowledge: what works in one valley may need to shift a few weeks in another.

Use these principles and the crop-specific ranges provided to build a seed-start calendar tailored to your greenhouse and garden location in Idaho, and you will maximize the growth window and success of your spring transplants.