I have now been using my new greenhouse for five months now and have found it to be a huge improvement on my old one. Even though I haven’t added any insulation or thermal mass as yet, it stays much warmer at night, which was always the problem in the old one. This has speeded up the germination and growth of my spring seedlings enormously and I’m now overwhelmed with new plants. When I add the insulation and thermal mass I think I will be able to realize an unfulfilled gardening ambition to grow bananas (I already have one in there). It has also been a great teacher and has given me a better understanding of how greenhouses actually work. I already knew the theory of course, but there is a big difference between reading about it and actually feeling it.
I finished installing the glazing just after the winter solstice, when the days are short (under 10 hours) and the sun is at its lowest point in the sky and easily obstructed by evergreen trees (deciduous trees aren’t a problem). At this time of year the sun doesn’t even hit my greenhouse until 11:00 and is gone by 4:00, which means that it doesn’t have a lot of opportunity to build up heat (it only reaches about 80 degrees F). Fortunately there isn’t a lot going on because the short day length restricts plant growth severely. Generally all you need to do is keep the place comfortably above freezing so tender plants can survive the winter. Here in the Santa Cruz mountains the weather is mild enough that this doesn’t require a lot of heat, but in colder climates some serious thermal mass and insulation may be needed to keep it from freezing at night. To make your winter greenhouse more productive you might try growing various salad greens, as most of these can tolerate quite low temperatures.
In February and March the greenhouse becomes the center of the garden and fills to capacity with the seedlings that will make up the spring garden (it is also the nicest place to work at this time). Even though it can be just as cold outside as it was in December, the conditions inside are very different because the sun is higher in the sky (which means less shade) and the days are longer (it gets sun by 10:00). This means the greenhouse warms up much more rapidly and can reach 100 degrees F if not vented.
In April and May the greenhouse is full of plants and the warmer weather means that cooling becomes more of a concern than heating. As the sun gets stronger the temperature in a closed greenhouse can easily reach 120 degrees F and forgetting to open the vents can damage plants. The vents aren’t just to control temperature however, they are also needed for air circulation, as warm, still humid air encourages diseases such as damping off. Vents should be opened any time they won’t make the greenhouse too cold.
In summer the days are long, the sun is high in the sky (there is little shading from trees) and it gets very hot inside the greenhouse every day. At this time of year the value of the greenhouse depends upon the climate. In warmer areas it may well get too hot for healthy plant growth and often stands empty (though you can try using shade cloth and wide open venting to keep the temperature down to tolerable levels). In cooler climates it can still be important for growing heat loving crops (this is the case where I live as summer nights can be too chilly for melons or okra to be really happy).
In fall, as the sun drops lower in the sky again, deciduous trees can be a problem for as long as they have their leaves. In the reverse of spring the days shorten and shade increases so there is less heating potential. However this isn’t usually a big problem as the thermal mass of the greenhouse still holds a lot of accumulated heat from the summer. You aren’t usually starting many seedlings at this time, though you may bring in tender plants (tomatoes or peppers) from the garden to extend the harvest season (try rooting suckers from your favorite plants in July or August and potting them up for this).
