About the Greenhouse

One goal of the efforts at Sage Farm is to extend the growing season in this part of New England in order to be better prepared for probable increases in the cost of food when the consequences of peak oil become more obvious.

One aspect of this greenhouse is its ability to actively store heat from the sun. This thermal mass of eleven tons of rock keeps the greenhouse cooler during the day and warmer at night.

To actively get the warmth from the greenhouse to the rocks there is some ductwork, a thermostat, and a fan. The ductwork lies at the bottom of the bin and runs its length and also has an elbow to connect the captured air to the fan. When the temperature in the greenhouse is warmer than the set-point of the thermostat, the fan goes on and draws warm greenhouse air down through the rocks and then returns it between the gaps in the planks of the central aisle.

This thermal mass of stone is directly beneath one of the soil beds and thereby can warm the plants there from below.

In order to better evaluate the thermal effectiveness of the rock storage system on the soil bed directly above it, logging thermal sensors have been used to monitor temperature conditions.

Data from this logging indicates that as early in the season as February 12th the soil bed above the rocks is already warming up. This was very noticeable in 2010

But less so in 2011:


In addition to this thermal performance monitoring assessment, the condition of the plants in early 2010 provide an additional indication of the season extending capability of this growing system:

The other locations monitored, in addition of course to the outdoor temperature were the elevated soil box and the Plexiglas mini-greenhouse.

The elevated soil bed rests on top of a triple box beam

This in turn is covered with concrete blocks:

This in turn is covered by a drainage mat and sand:

Which in turn is covered by potting soil, amendments, and seedlings, so that by February 19, 2011, it looks like:

As with the rock storage system, this elevated soil bed includes not only thermal mass but a thermostat and fan to circulate warm greenhouse air underneath the concrete blocks.

The Plexiglas mini-greenhouse, when in use, is thermally connected to the basement of the house where a small fan draws heat from its peak and blows it into the basement. By thermally linking these two components the mini-greenhouse is prevented from overheating when the sun shines and also is keep warmer on cold nights. Seedlings are thereby protected early in the season, such as on February 19, 2011.

This greenhouse, whose exterior was originally constructed in the late 1970s, has had its interior modified over the years in an effort to evaluate differing approaches to nurture vegetables early in the growing season and thereby extend the growing season without supplemental heat energy but merely the use of fans and electricity to improve the capture and storage of this heat. For the soil beds these fan could be powered by photovoltaics as the need for fan power would be directly proportional to the intensity that the sun was shining.

Concord, MA
February 22, 2011

Greenhouse update, January 2015

As further information about the season extending effectiveness that can be achieved using a heat-storing greenhouse I submit the following photograph of the interior, upper soil bed yielding harvest-able spinach on March 4, 2014.

Figure 1. Spinach seedlings in upper soil bed on 3/4/2014

The greenhouse has varying micro-climes as evidenced by the fact that the spinach seedlings on the same date in March 2014 in the deep soil bed are not as far along as those shown above. Differences in these two micro-climes are due to variations in the amount of incoming solar energy and differences in the amount of thermal mass that needs to be warmed up. These spinach seedling in this lower and outboard soil.

Figure 2. Spinach seedling in lower exterior soil bed on 3/4/2014

Later in the season, these spinach seedlings in the lower exterior soil bed do progress, as shown in the following photograph (Figure 3) taken on April 9, 2014.

Figure 3. Spinach seedlings in lower exterior soil bed on 4/ 9/2014


By then, however, the seedlings in the elevated interior soil bed have grown even more, as shown in Figure 4.

Figure 4. Spinach seedlings in elevated interior soil bed on 4/9/2014


As a further comparison, Figure 5 provides a view of the progress of some spinach seedlings in one of the outdoor season-extending growing bed on May 16, 2014.

Figure 5. Spinach seedlings in Growing Box #1 on May 16, 2014