Greenhouse growing
Greenhouse temperature guide for seeds and seedlings
By Tools for Gardeners · Last reviewed 2026-10-04
Most seeds germinate best when the growing medium is warm enough for that crop, but seedlings usually grow better with cooler nights and strong light. As a starting point, keep tender warm-season seedlings around 65 to 75°F during the day and 60 to 65°F at night. Cool-season seedlings can use cooler conditions. Always use the crop's range, watch the plant as well as the air, and vent before the greenhouse overheats.
A greenhouse gives you more control over temperature, but it does not remove weather risk. Sun, wind, humidity, pot size, the temperature of the growing medium, and the condition of the seedlings all affect the result. This guide gives you useful ranges for planning, then shows when to add heat, open vents, or protect plants from a freeze.
Germination temperature by crop
Seeds need moisture, oxygen, and a suitable temperature to germinate. The right temperature is the temperature of the seed-starting mix, not only the air around the tray. A heat mat can warm the mix while the greenhouse air stays cooler, which is useful for warm-season crops.
| Crop | Optimum germination temperature | Useful range | Temperature note |
|---|---|---|---|
| Lettuce | 75°F | 40 to 80°F | Germinates in cooler conditions than most warm-season crops |
| Spinach | 70°F | 45 to 75°F | Warm conditions can slow or reduce germination |
| Onion | 75°F | 50 to 95°F | Slow-growing seedlings need a long indoor window |
| Parsley | 75°F | 50 to 85°F | Keep the mix evenly moist while waiting for emergence |
| Beet | 85°F | 50 to 85°F | Can germinate across a broad range |
| Cabbage | 85°F | 45 to 95°F | Germinates warm, then grows as a cool-season seedling |
| Tomato | 85°F | 70 to 95°F | Bottom heat can improve speed and consistency |
| Pepper | 85°F | 65 to 95°F | Benefits from warm mix temperatures |
| Eggplant | 85°F | 75 to 90°F | Needs more warmth than most cool-season crops |
| Cucumber | 95°F | 60 to 95°F | Sow only a short time before transplanting |
| Squash | 95°F | 70 to 95°F | Avoid keeping the mix wet and cold |
| Melon | 90°F | 75 to 95°F | Warmth and bright light are both important |
These figures follow the seed and seedling temperature table from Penn State Extension's seed and seedling biology guide. Seed packets remain the first reference for a specific variety. If germination is slow, check mix temperature with a thermometer before adding more water or fertilizer.
A common mistake is to leave a heat mat on after the crop has emerged. Warm roots can help a tomato or pepper sprout, but excessive warmth combined with weak light produces tall, soft seedlings. Once most seeds in a tray have emerged, move the tray to bright light and use the seedling range instead of the germination target.
Seedling temperatures after emergence
Seedlings need a balance between growth and strength. Warm days encourage growth, while cooler nights help limit stretching. The greenhouse should not be managed like a living room. A warm, still greenhouse with insufficient light can produce long stems and thin leaves even when every seed germinates.
| Crop | Day temperature | Night temperature | Typical indoor time |
|---|---|---|---|
| Broccoli | 60 to 70°F | 50 to 60°F | 5 to 7 weeks |
| Cauliflower | 60 to 70°F | 50 to 60°F | 5 to 7 weeks |
| Lettuce | 55 to 65°F | 50 to 55°F | 5 to 7 weeks |
| Onion | 60 to 65°F | 55 to 60°F | 10 to 12 weeks |
| Tomato | 65 to 75°F | 60 to 65°F | 5 to 7 weeks |
| Pepper | 65 to 75°F | 60 to 65°F | 6 to 8 weeks |
| Eggplant | 70 to 80°F | 65 to 70°F | 6 to 8 weeks |
| Cucumber | 70 to 75°F | 60 to 65°F | 3 to 4 weeks |
| Squash | 70 to 75°F | 60 to 65°F | 3 to 4 weeks |
| Melon | 70 to 80°F | 65 to 70°F | 3 to 4 weeks |
The ranges are targets, not pass or fail lines. A tomato seedling that briefly reaches 62°F at night is not automatically damaged. A tray that stays at 80°F day and night may still look acceptable for a few days, but it will need stronger light, careful watering, and more frequent checks for stretching and disease.
Penn State Extension recommends using ventilation to lower greenhouse temperature as transplants mature and approach hardening off. Reduce temperature gradually rather than moving tender plants directly from a warm greenhouse into cold outdoor air.
Day and night temperature management
Measure temperature at plant height with a thermometer that records the minimum and maximum. A thermometer mounted high on a greenhouse wall can read warmer than the leaves and growing medium. If the greenhouse has several benches, check the warmest and coolest areas. Sun-facing walls and corners often behave differently from the center aisle.
During the day:
- Open roof vents, side vents, or doors before the greenhouse reaches the crop's upper limit.
- Use a small circulating fan to reduce stagnant pockets, but do not aim a strong stream directly at tender seedlings.
- Watch the growing medium. A sunny greenhouse can dry small cells quickly even when the air feels humid.
- Shade only when needed. Permanent heavy shade can reduce the light that keeps seedlings compact.
At night:
- Close vents early enough to hold stored heat, but keep some air movement to limit condensation.
- Use a thermostat-controlled heater when a crop requires temperatures above the forecast low.
- Keep leaves from touching cold greenhouse glass, which can create a colder surface than the room average.
- Put a second thermometer at tray level if frost is possible.
The goal is a stable range with a normal day and night difference. Large swings are harder on young plants than a small, steady deviation from the target. Thermal mass such as full water containers can slow temperature changes, but it does not replace a heater or frost cover during a hard freeze.
Frost protection thresholds
Frost risk is based on the temperature where plants are located, not simply the official outdoor forecast. A forecast of 36°F can still produce frost in a sheltered low spot, while wind or cloud cover can change the result. Greenhouse plastic reduces wind and traps daytime heat, but an unheated greenhouse can approach outdoor temperature overnight.
Use these practical thresholds as warning points:
| Temperature near the plants | What it means | Action |
|---|---|---|
| 40°F and below | Warm-season growth slows and chilling risk increases | Close the greenhouse, check the heater, and group tender plants away from walls |
| 36 to 32°F | Frost is possible on exposed surfaces and tender crops may be damaged | Add a cover over plants, use safe supplemental heat, and monitor plant-level temperature |
| Around 32°F | A light frost can damage tender leaves | Do not rely on the greenhouse alone if the structure is unheated or poorly sealed |
| 28°F or colder | A hard freeze can kill most warm-season crops | Use a reliable heat source and an inner cover, or move valuable plants to a protected space |
These are protection thresholds, not planting dates. Cool-season crops such as broccoli, cabbage, kale, and lettuce can tolerate more cold than tomato, pepper, cucumber, squash, basil, and melon. A mature plant may also tolerate a brief cold event better than a new transplant. Check the forecast, the crop, and the actual temperature at the leaves before deciding that a crop is safe.
A frost date finder gives you location-based spring and fall freeze estimates. Use it to plan the season, then use the forecast and a thermometer for the actual night. Frost dates are statistical planning references, not a guarantee that every night before or after the date will stay above freezing.
Temperature ranges by crop group
Thinking in crop groups makes a mixed greenhouse easier to manage.
Cool-season crops
Broccoli, cabbage, cauliflower, kale, lettuce, spinach, and onions prefer cooler seedling conditions. They can often handle daytime temperatures around 55 to 70°F and nights around 45 to 60°F, depending on the crop and growth stage. Excess heat can lead to weak growth, rapid bolting, or poor establishment after transplanting.
Keep these crops in the coolest bright area of the greenhouse. If the structure becomes hot in spring sunshine, vent early rather than waiting for leaves to wilt. The seed starting calculator can help you count back from your local frost date, but the crop's temperature tolerance still determines when a seedling is ready to move outside.
Warm-season crops
Tomatoes, peppers, eggplants, cucumbers, squash, melons, beans, and basil need more warmth. Germinate them in warm mix, then keep the seedlings bright and avoid cold, wet roots. Tomatoes and peppers generally grow well around 65 to 75°F by day and 60 to 65°F at night. Eggplant, cucumber, squash, and melon usually prefer warmer conditions.
Do not use warmth to compensate for a planting date that is too early. A tomato can survive in a heated greenhouse while still becoming rootbound before outdoor conditions are safe. Count backward from your frost date and allow enough time for hardening off. The planting calendar connects that timing to outdoor transplant and direct-sowing windows.
Flowers and herbs
Annual flowers vary widely. Basil behaves like a warm-season vegetable, while parsley and some hardy flowers prefer cooler conditions. Check the packet when the temperature range is important. If the packet gives no temperature information, start with the range for a similar crop and adjust from the seedlings' growth and appearance.
Ventilation and overheating warnings
Overheating can happen on a clear spring day even when the morning feels cold. A small greenhouse can rise far above outdoor temperature soon after sunlight reaches the plastic or glass. Open vents before the plants wilt. Waiting for visible wilting means the crop has already experienced stress.
Watch for these warnings:
- The inside temperature climbs rapidly after sunrise.
- Leaves are limp even though the mix is moist.
- Leaf edges turn pale, dry, or scorched.
- Seedlings stretch despite adequate light.
- Condensation stays on the structure and leaves for hours after sunrise.
- The air near the roof is much hotter than the air at tray level.
Ventilation removes excess heat and moisture. It does not mean exposing plants to a cold draft all day. Open vents gradually on cool mornings, close them as the outdoor temperature drops, and avoid placing a tender tray directly beside an open door. A fan that circulates air can help, but it cannot lower temperature as effectively as exchanging hot air with cooler outdoor air.
A simple temperature check routine
Check the greenhouse at four points during a cold or changeable week: before sunrise, late morning, midafternoon, and before closing for the night. Record the air temperature, the minimum and maximum reading, and which crop is in the warmest spot. After a few days, you will know where the structure loses heat and where it overheats.
When a reading is outside the crop's range, change one factor at a time. Vent first for excess heat. For cold, close openings, add an inner cover, and use safe heat designed for greenhouse conditions. For a tray with cold roots, move it from a concrete floor or exterior wall onto an insulated bench. Do not place household heaters where water, plastic, or crowded plants can create a fire hazard.
Frequently asked questions
What temperature is too cold for seedlings?
It depends on the crop. Warm-season seedlings become stressed as temperatures approach 40°F, and a light frost near 32°F can damage tender leaves. Cool-season seedlings tolerate lower temperatures, but cold, wet media can still slow roots and increase disease risk.
Should I use a heat mat in a greenhouse?
Use bottom heat when the seed mix needs to be warmer than the surrounding air, especially for peppers, eggplants, tomatoes, cucumbers, squash, and melons. Remove or reduce bottom heat after emergence if the tray is becoming too warm or the seedlings are stretching.
Can a greenhouse get too hot for seedlings?
Yes. A greenhouse can overheat quickly in direct sun. Vent before plants wilt, monitor maximum temperature at plant height, and use shade only when ventilation cannot keep the crop within range. Warmth without enough light commonly produces tall, weak seedlings.
When should I move greenhouse seedlings outside?
Move them when they are the right size for the crop, outdoor temperature and soil conditions fit the crop, and they have been hardened off. Use the seed starting calculator for the calendar window, then adjust for the actual forecast and the plant's condition.
What to do next
Use the frost date finder to identify your local spring and fall freeze dates, then enter the crop in the seed starting calculator. For a full outdoor schedule, open the planting calendar. If you are starting plants for the first time, read when to start seeds indoors for crop-by-crop indoor timing.
Sources
- Penn State Extension: Seed and Seedling Biology provides the germination and seedling day and night temperature tables used in this guide.
- University of Minnesota Extension: Starting seeds indoors covers indoor seed starting, light, air temperature, and transplant preparation.
- University of Minnesota Extension: A Minnesota guide to garden timing explains the relationship between soil temperature, air temperature, crop type, and frost risk.
