Since you state temperature in C and we in the US use F, I did not take the time to convert your highs with mine here in Mississippi (zone 8b). If the outside temperature here (measured in the shade) is 90 F, and we probably have 90-120 days when it reaches that temperature, the temperature in the greenhouse will be 100-110 unless it is well ventilated and has fans to move the air. Since my greenhouses have their vents and doors closed in the fall/winter months, on a sunny day, when the temperature outside is 45 F, it is common for the greenhouse temperature to be 70 F without artificial heat. You are correct that the floor will be cooler than the upper part of the greenhouse. All my heaters are near the floor and automatically come on when the temperature reaches 50 F. The floor may be 50 F, but the upper part of the greenhouse will be closer to 60-65 F.
I start several vegetable plants from seeds in February and continue through March. The outside temperature would average 45-50 F for highs and 30-35 F for lows. I start several varieties of tomato, onion, lettuce, and herbs and a single variety of asparagus. They will either be transplanted into large pots or a raised garden in April, when there is no danger of frost, or grown in ever-increasing sizes of pots to sell in May. I am a hobbiest grower. Ken
