Vegetable Production in "Solar Pods"

The Willamette Valley of western Oregon is well known for production of a wide range of quality vegetables. Commercial, large-scale production of warm-weather vegetables such as tomatoes and melons is possible but is limited by competition from more favorable growing areas. However, for the home gardener or small market gardener, production of almost any vegetable crop except tropicals is possible in the long, mild growing season of the Willamette Valley. Major limiting factors are late spring frosts, insufficient heat for some warm-weather crops, and low night temperatures.

Production of the warm weather crops can be greatly enhanced by the use of greenhouse-grown transplants, unheated glasshouses and cold-frames, and various plastic structures. For cool-season but non-winter hardy crops such as leafy greens, root crops, and cole crops, the growing season can also be extended with protective structures. Some of these crops, e.g. lettuce, can be successfully overwintered with minimal frost protection.

In 1980, the Collins Foundation funded a two-year trial of passive solar heating methods for extending the growing season of both warm- and cool-season vegetables. One system studied was the "Solar Pods" of Solar Survival, Inc.

Methods

The Solar Pods consisted of 1.22m x 2.44m raised-bed cold frames covered by a double glazing of fiberglass. The sides and bottom of the Pods were insulated with 5 cm of plastic foam insulation. Effective growing height for plants was 56 cm, limiting the type of vegetables which could be grown before removal of the fiberglass cover in May. During the first winter, one of the two Pods contained a half-exposed 55-gallon black drum filled with water as a potential passive solar heat sink. During the second winter, the drum was replaced by solar panels from The Vegetable Factory, Inc. which contain a liquid with a melting point of 29°C. Pods were generally opened during sunny periods if the ambient temperature exceeded 13°C. This prevented excessive heat buildup and allowed greater solar radiation to reach the crops. Pod covers were always open from May through September.

Results

Pods strikingly increased mean maximum air temperatures. For example, in January 1981, the mean daily maximum air temperature recorded at the North Willamette Station weather recording facility was 10°C, but the Pod maximum averaged 29°C. Effect of the Pods on mean low temperatures was quite small, a 1.7°C average increase. However, the Pods provided substantial frost protection on the coldest nights: the lowest temperature ever recorded in the Pods was -4°C on a night when ambient air temperature fell to -11°C. Frost sensitive crops such as lettuce and beans were not injured, except along the outer edges of the growing area.

Soil temperatures at two-inch depth also were affected. Pod maximum soil temperatures averaged 12 C higher than ambient; low temperatures were increased 4°C. Soil temperatures in the Pods never reached the freezing point and were usually adequate for growth of cool-season crops.

Cropping History

A complete cropping history for the Solar Pods is in Table 1. Crops grown in the Pods in 22 months included bush bean (5 crops), carrot (4), Chinese cabbage (1), cucumber (1), head lettuce (4), leaf lettuce (5), bunching onion (2), parsley (2), bell pepper (1), radish (4), spinach (3), squash (1), and tomato (2). No more than seven of these crops occupied the Pods at any one time. All Pod crops were grown from seed. Use of transplants would have decreased production time considerably for several crops. Total usable production for the two Pods was 223 kg in slightly less than 22 months or 25.6 kg/m /yr.

Solar Pod costs in late 1980 were $160 each for materials and plans, plus shipping and assembly, or in excess of $53/m of growing area. However, similar structures could be constructed from local materials for less than $100 material cost or about $32/m.

Discussion

A large number of crops are well adapted to Pod culture. The Pods are most effectively used to start vegetable transplants in early spring or to grow cool season crops through the winter. Neither the water-filled drum nor the solar panels significantly affected Pod air or soil temperatures. A solid melting at 50 to 70°F may be needed for effective heat storage.

The primary advantages of the Pods are that they provide significant frost protection and raise air and soil temperatures during daylight hours sufficiently that good crop growth can occur. Pod drawbacks include 1) extremely high initial cost, 2) daily care is needed to lift and close Pod covers to prevent excessive heat, 3) crop height is limited, 4) poor light transmission through the fiberglass covers. Given the high initial cost and labor requirement, Solar Pods are economically feasible only for year-around home gardening or for production of very high value crops.

Table 1. Solar Pod Cropping History, 1980-1982                                             Planting   Harvest                        Yield for two  date       date      Crops                  pods (kg)   Comments                           12/17/80   2/27/81  'Michihli' Ch. cabbage     4.1    Good quality and appearance.                       'Ithaca' head lettuce      4.6    Only fair quality, bitter.                       'Waldmann's', 'R-1'        5.7    Only fair quality, soft,                       'Deep Red, and  (total)           elongated. 'Deep Red' failed                      'Oak Leaf' leaf lettuce           to achieve normal red color.                       'Melody' spinach           2.0    Small leaf size, fair quality.                      'Inca' radish              1.8    Excellent quality.  2/27/81    7/09/81  'Pixie' tomato             7.8    Normal quality, yield. In                                                        production 7 weeks early.                       'White scallop' squash    13.7    Excellent yield and quality.                                                        First harvest 5/10                      'Bush whopper' cucumber    6.5    Good quality. First harvest                       	                              on 5/16.                      'Nantes' carrot            6.7    Small carrots, normal top growth.                      'Tendercrop'bean           2.7    Good quality but should have                                                        been harvested earlier.                      'Ithaca' lettuce           6.2    Normal field quality.                       'Banquet' parsley          1.1    Good quality.  2/27/81    5/05/81  'Inca' radish              3.2    Good quality, appearance.  5/05/81    7/09/81  'Inca' radish              2.7    Reduced bulb size, quality. Too hot?  7/09/81    9/29/81  'Ithaca' lettuce          10.0    Immature.                      'Nantes' carrot           19.3    Immature.                      'Melody' spinach          11.8    Planted too early for best quality.                       'Spartan Arrow' bean      16.1    Good yield and quality bean.                      'Banquet' parsley          0.8    Needed more time.                      'Japanese bunching' onion  9.1    Good quality.   9/30/81-10/18/81 (Soil sterilized with Vapam)  10/18/81   3/10/82  'Tendercrop' bean          2.7    Quality adequate. Yield                                                        limited by low temp.                      'Melody' spinach           5.0    Fair quality.                      'Nantes' carrot            5.0    Excellent crop, some bolting.                      'Ithaca' lettuce           5.6    Adequate quality.                      'Japanese bunching' onion  7.6    Good quality.                       'Inca' radish              4.6    Overmature.  3/15/82    7/20/82  'R-5' lettuce              3.8    Good yield and quality.                      'Waldmann's' lettuce       3.4    Good yield, slightly pale                      'Spartan Arrow' bean       8.3    Excellent quality and yield  3/15/82   10/05/82  'Keystone Giant' pepper    4.0    Good yield, small size.                      'GSV80-25' tomato         11.7    Good quality, fruit like 'Pixie'.  7/20/82   10/05/82  'Spartan Arrow' bean      11.5    Good quality and yield.                       'Nantes' carrot            5.6    Good quality.                      'Waldmann's' lettuce       4.3    Good quality, deep color.  _________________________________________________________________________________________  Total production by crop:   bean, 41.3; carrot, 36.6; Chinese cabbage, 4.1;                              cucumber, 6.6; head lettuce, 26.4; leaf lettuce, 20.8;                              bunching onion, 16.7; parsley, 1.9; pepper, 4.0;                               radish, 12.4; spinach, 18.8; squash, 13.7; tomato                                19.5.  Total production:           222.6 kg on 4.83m2 (usable space) in 1.80 years =                               25.6 kg/m2/year fresh weight yield.  

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