Every crop has four temperatures that matter, and confusing them is behind most disappointing greenhouse results. The survival minimum is the point at which tissue is damaged or killed. The growth minimum is much higher: the point below which the plant is alive but adding no usable growth. The optimum range is where growth is fastest. The heat ceiling is where a specific physiological process fails, and for fruiting crops that process is usually pollination rather than growth.
The distinction between survival and growth minimums is the one that costs people a winter. Holding a greenhouse at 40 degrees Fahrenheit keeps almost everything in the table alive. It does not make anything grow except the hardy greens, which is why a frost-free greenhouse in deep winter is a holding pen rather than a production space.
| Crop | Survives to | Grows above | Optimum day | Optimum night | Trouble above |
|---|---|---|---|---|---|
| Kale | 10 | 40 | 60 to 70 | 45 to 55 | 80 |
| Spinach | 15 | 40 | 55 to 68 | 45 to 50 | 75 |
| Mache and claytonia | 10 | 38 | 50 to 65 | 40 to 50 | 70 |
| Swiss chard | 20 | 45 | 60 to 75 | 50 to 55 | 85 |
| Arugula | 20 | 40 | 55 to 68 | 45 to 50 | 75 |
| Bok choy | 25 | 45 | 55 to 70 | 48 to 55 | 80 |
| Lettuce | 25 | 42 | 60 to 70 | 45 to 55 | 75 |
| Scallions | 15 | 42 | 60 to 72 | 50 to 55 | 85 |
| Peas | 26 | 45 | 60 to 70 | 50 to 55 | 80 |
| Carrots | 20 | 45 | 60 to 70 | 50 to 55 | 85 |
| Beets | 25 | 45 | 60 to 72 | 50 to 55 | 85 |
| Radishes | 25 | 45 | 55 to 70 | 45 to 55 | 80 |
| Parsley | 20 | 45 | 60 to 72 | 50 to 55 | 85 |
| Cilantro | 25 | 42 | 55 to 68 | 45 to 55 | 75 |
| Strawberries | 15 when dormant | 48 | 65 to 75 | 50 to 60 | 85 |
| Geraniums | 32 | 48 | 65 to 75 | 55 to 60 | 90 |
| Petunias | 32 | 50 | 65 to 78 | 58 to 62 | 92 |
| Tomatoes | 33 | 55 | 70 to 80 | 60 to 65 | 95 |
| Peppers | 33 | 58 | 70 to 82 | 62 to 68 | 95 |
| Eggplant | 35 | 60 | 72 to 85 | 65 to 70 | 95 |
| Green beans | 33 | 58 | 70 to 80 | 60 to 65 | 90 |
| Cucumbers | 35 | 60 | 72 to 85 | 65 to 70 | 95 |
| Melons | 38 | 62 | 75 to 88 | 65 to 72 | 98 |
| Basil | 40 | 58 | 70 to 85 | 62 to 68 | 95 |
What actually goes wrong above the ceiling
The heat ceiling is not a growth limit, it is a specific failure, and knowing which failure tells you what to do about it.
- Tomatoes and peppers above 95 degrees F: pollen becomes non-viable, so flowers open and drop without setting fruit. The plant looks healthy and produces nothing. Ventilation and shade are the fix, and hand pollination in the cooler morning hours recovers part of the loss.
- Lettuce above 75 degrees F: bolting. The plant switches to flowering, the leaves turn bitter as latex content rises, and the head is finished. Bolt-resistant varieties buy a few degrees, shade and succession sowing buy more.
- Spinach above 75 degrees F: bolting, and faster than lettuce. Spinach is effectively a cool-season-only greenhouse crop at mid latitudes.
- Cucumbers above 95 degrees F: bitter fruit and misshapen fruit, driven by uneven water uptake as much as by heat directly.
- Brassicas above 80 degrees F: loose heads, strong flavor and increased aphid pressure, because aphid reproduction accelerates faster than its predators do.
- Almost everything above 100 degrees F: stomata close to conserve water, which stops photosynthesis entirely. The plant is not growing, it is surviving, and it will wilt with wet roots because transport cannot keep up.
The day to night differential, and why growers set it deliberately
Commercial growers do not aim for a constant temperature. They run a deliberate difference between day and night setpoints, typically 8 to 15 degrees Fahrenheit, because that differential controls stem elongation. A small differential produces tall, stretched plants. A larger differential produces shorter, sturdier ones with thicker stems and more branching, at the cost of slightly slower total growth.
Practically, for a hobby house, this means setting a heater thermostat to a night figure below the day figure rather than one number for both. A controller with two setpoints, or a dual-output controller paired with a timer, does this. It is one of the few free levers in greenhouse growing: the same crop, the same light, the same water, better transplants.
Working out which crops your greenhouse can actually hold
Read the chart against the heater you have, not the one you want. Take your calculated heater output from the heater BTU calculator, find the temperature differential it can actually sustain against your design low, and add that to your design low. That figure is your real winter minimum, and everything in the table with a growth minimum above it will hold but not grow.
| Regime | Inside minimum | What grows | What only survives |
|---|---|---|---|
| Unheated | Within 8 to 12 degF of outside | Kale, mache, spinach, claytonia, scallions in mild spells | Everything else, and tender crops die |
| Frost protection | 38 to 42 degF | Hardy greens, slowly. Overwintering herbs hold their leaves. | Geraniums, citrus, tender perennials, stored tubers |
| Zoned bench heat only | House at outside, bench at 75 to 80 degF | Germination and propagation at any time of year | The rest of the house stays a holding space |
| Cool house | 45 to 50 degF | All salad and root crops, brassicas, most ornamentals | Tomatoes, peppers, cucumbers, basil |
| Warm house | 60 to 65 degF | Everything, including fruiting crops through winter | Nothing, but the running cost is roughly eight times frost protection |
The most economically sensible answer for most hobby houses is the third row: heat a small zone rather than the whole volume. A small infrared unit or a regulated heat mat over 12 square feet of bench costs a fraction of holding 180 square feet at the same temperature, and propagation is where the value is anyway.
Holding and knowing the temperature
See what each crop needs in detail on its own page under by crop, work out the heat required in the heater BTU chart, and read the winter strategy in winter greens in an unheated greenhouse and heating a greenhouse in winter.