We built our season extending 'solar pod' in 1996 based on the book Solar Gardening by Leandre and Gretchen Poisson (a book with many ideas for season extending besides the solar pod). It has held up well for over 25 years and become an integral component of our garden.
This 4'x8' mini-greenhouse is built primarily of exterior grade plywood, 2"x4"s, and .04 inch thick fiberglass glazing. The covering is comprised of two layers of 40 mil fiberglass with angel-hair fiberglass in between. The solar pod sits on a cedar 2"x8" base that can be extended for taller crops by adding another 2"x8" box.
We have experimented with various plants in our solar pod, discovering our best results with spinach and peppers. We discovered that if we plant spinach in this pod in November, by late March we have a beautiful crop of spinach that can be harvested for nearly 4 months. In our climate, small spinach plants can remain dormant during December and January in an unheated but insulated cold frame. Depending on the weather, by February, there is enough light and heat to stimulate growth in these young plants, such that by March, the leaves are large enough to harvest.
In our northern Illinois climate, soil temperature is too low to plant spinach until around mid April, so using the solar pod over winter gives us a very early harvest. However, the best thing about planting in this manner is how daylight length will work to our advantage. An unprotected, mid-April planting of spinach will produce until late June, at which time it is no longer harvestable because it bolts and goes to seed.
The tendency to bolt is caused by long daylight hours as well as by high temperatures. These two criteria apply as well to spinach over-wintered in the solar pod, meaning the spinach in the solar pod will also bolt in late June. The result is that we can harvest spinach at least two months longer from the solar pod than from the unprotected planting. During the first year we planted spinach in this manner, we harvested 2-3 bushels of spinach out of this 4'x8' space over the months of March - June. This approach represents another permaculture application – in this case, working with growing cycles of nature to maximize abundance.
These photos were taken on March 4, 2004. At this time, the spinach had already been harvested twice.
In June, the spinach plants were preparing to go to seed, but by this time had produced a significant volume of harvestable leaves.
We discovered that spinach can grow to a remarkable size in this mini-environment.
This photo was taken on November 6, 2004 - almost a month past our normal early October freeze date. The pimenton peppers in the 2 gallon bucket were perfect, with no insect or frost damage. The plants were still lush and productive even though we had already had a few freezing nights and the day length was rapidly waning. The short day length reduced the plant's ability to produce further fruit, meaning this was the last pepper harvest, even though the plants were still growing.
Notice in this picture the dill and endive outside the wooden box. These plants sprouted on their own from seed dropped by an earlier crop. On the other side of the solar pod was a spinach and lettuce patch that had also sprouted on its own. In these volunteer patches, only minor weeding was required to produce a healthy salad crop through the end of November when cold and inadequate light eventually drove the plants dormant.
Letting plants grow where they wish in this manner is another application of permaculture - in this case, the principle is - don't do any more work than is necessary.