Yes, tomatoes in an unheated greenhouse do fine if nights stay above 50°F (10°C) for the whole growing run. Below that mark, plants stall, refuse to set fruit, and may even suffer cold damage to leaves and stems.
Most folks who grow unheated greenhouse tomatoes plant too early and wonder why nothing grows for weeks. The plants sit there, sulk in the chill, and lose the head start they had on the open garden outside.
I tested my own timing for three years before I locked in the right window. My Zone 6 plants go into the greenhouse on April 15, which is four weeks earlier than my open garden frost date of May 15.
That four-week jump gives me red fruit by early July versus mid-August in the open garden. The trade-off is I have to watch night temps like a hawk and act fast on cold snaps.
Tomato cold tolerance has three clear lines you need to know by heart. Fruit set stops below 55°F (13°C) at night, which means no new tomatoes form on your vines.
Growth stalls below 50°F (10°C) even if old fruit keeps ripening on the plant. Cold injury starts below 40°F (4°C) and leaves turn purple from stress on the leaves and the lower stem.
Any frost at or near 32°F (0°C) will kill the plant outright in one bad night. So your goal is to keep nights above 50°F (10°C) inside the greenhouse all the way through the growing run.
I learned the cold injury lesson the hard way one April when temps dropped to 38°F (3°C) inside. My six plants turned purple and stalled for two full weeks before they bounced back to normal growth.
Track your nighttime minimums with a min/max thermometer inside the greenhouse from day one. A 15 dollar model logs the lowest reading each night so you can spot trends and act before damage hits.
Add a layer of frost row cover over your plants when forecasts call for 45°F (7°C) or colder. That cheap blanket buys you another 3 to 5°F (2 to 3°C) of warmth at the leaf level for the night.
Variety choice plays a huge role in your success with early greenhouse tomatoes of this kind. Some types shrug off cool nights better than the standard slicers found in any seed rack.
Pick cold-tolerant types like Stupice, Glacier, or Siberian for the first plants you put in. They set fruit at 5°F (3°C) cooler temps than a Big Boy or beefsteak type would tolerate.
I tested Stupice against a standard slicer one spring and the Stupice set fruit two weeks sooner. That trial sold me on cold-tolerant types for my whole early crop in the greenhouse from then on.
Plan your timing, watch your temps, and pick the right type for your first batch. You will pull ripe tomatoes weeks ahead of any neighbor with a plain open garden in your area.
Read the full article: Cold Frame Gardening: Complete Guide
