The greenhouse is unheated so you will have to know at what outside temperature the greenhouse will stay above freezing. On a windy night, the greenhouse will lose heat faster. I have an unheated sunroom attached to the house. I determined I have about a 12-15 degree buffer from the outside temp in the spring.

As to when to put plants in there will depend somewhat on the temperature the greenhouse can maintain at night. While most will tolerate cold temps, the warm season crops, like tomato, won't like it. I would wait on warm season crops until the greenhouse can maintain temperatures above 45 or so at night.

Make sure to be able to vent on a sunny day. Overheating can also damage the plants. Our closed, unused greenhouses at work are already getting into the 90's on sunny days.

Vent on cloudy days if the greenhouse feels very humid, the plant foliage is wet or there is a lot of condensation or dripping.

Thank you for the info. So as long as the greenhouse temp stays above 32degrees my cole crops will be fine?

If you are in a real hurry, you can add a litttle heat for those crispy nights by making up a string of light bulbs (4 bulbs at 60 watts ea.) One in each corner area will help keep the temps up. Another idea is to purchase a heat cable (used to keep the ice from building up on the roof of the house). Just run it around the inside base of the greenhose and plug it in at night. You may want to consider purchasing a "Max/Min" thermometer which will indicate the low temps you're getting overnight inside the greenhouse. Very useful !! Good Luck.

A heating cable..nice idea. Yesterday(Apr.19th) featured an uncommonly strong east wind which toppled my tiny greenhouse. Actually ,other than losing a small favourite pot the damage was minimal although I now know I need a better barrier for those unexpected gales. R squared