Pete Johnson grows salad greens in an unheated greenhouse. It occurred to him that if he could heat the soil in the greenhouse, it should be possible to both promote the rate of growth of his greens, and extend the growing season. His SARE project involved constructing a mechanism to heat the soil; in addition, he conducted a second experiment to examine the efficacy of Reemay and slitted plastic plant row covers.
Mr. Johnson buried eight 6-inch pipes in the ground, four at a depth of three feet, and four at a depth of one foot. The pipes were packed in sand, overlain with compost and topsoil, and underlain with insulating foam. He attached the pipes to a manifold and air pump, and ran another pipe up to the apex of the roof, so that hot air accumulating near the ceiling could be circulated down through the buried pipes. A thermostat was attached, and set to start the pump whenever the temperature near the peak reached 75°F.
Mr. Johnson planted mizuna in this area, and in an adjacent control area, on March 27, and he monitored temperatures at several depths below ground for about a month. By April 20 the soil 14 inches down was 6°F warmer than that at the same depth in the control area. He harvested the mizuna on that day, but found that the yield in the experimental area did not differ significantly from that in the control. He did not continue this experiment after that point.
Regarding the row covers, Mr. Johnson reports that the slitted plastic held in too much moisture, thus causing his greens to rot. He found that the Reemay enabled him to harvest his mizuna a full week earlier (26 days after seeding, compared to 33 days for the control plots), but did not improve yields. He noted, however, that the greens grown under Reemay were more tender, and consequently more susceptible to damage during washing. Also, color development, such as in red lettuce and certain mustards, was less pronounced among plants grown under Reemay.