Introduction
Economic production of vegetable transplants (plugs) requires inexpensive greenhouses or other growing structures, small seedling cell size to maximize the number of plugs per unit area, and rapid production of a marketable plug. While less expensive than glasshouses, traditional double poly-covered greenhouses reduce solar radiation reaching the seedlings and require power to run cooling fans during cloud-free periods. A possible solution to this problem would be to raise the roof and sides of the greenhouse during periods of high solar radiation to allow full utilization of the sunlight and provide natural cooling. The SunpocketTM greenhouse or coldframe design features a roll-up plastic cover. This experiment was designed to compare the growth of seedlings of four vegetable species in the Sunpocket versus a glasshouse, two double-poly greenhouses, and a fiberglass-covered screenhouse.
Methods
'Tall Utah' celery and 'Granada' onion were seeded into 256-cell trays (cell dimensions: 0.75 inch square x 1.2 inch deep) on 1 March, 1987, and placed in a glasshouse to germinate. The glasshouse was set for a 60 F minimum temperature. The rooting medium was a peat-vermiculite mix. Trays of each species were moved to the Sunpocket (50 F minimum); an unheated, fiberglass-covered screenhouse; and two heated houses (50 F minimum) covered with a double layer of 6-mil clear polyethylene (longhouse and prophouse) on 22 March. 'Gem' broccoli and 'Ithaca' lettuce were similarly seeded on 24 March and moved to the final locations on 31 March. Maximum and minimum temperatures were recorded for each house from 26 March to 24 April, when the experiment was terminated. Twenty plants from each tray were selected at random and harvested at the soil line. Fresh weight, stem length, leaf number, and stem diameter were determined and recorded individually.
Results
The highest mean temperature (71.3 F) was recorded in the glasshouse (Table 1). However, the mean daily maximum temperature was greatest in the Sunpocket, reflecting the lack of forced air cooling and rapid temperature buildup when the Sunpocket cover was not raised early enough. Temperatures often exceeded 90 F in the closed Sunpocket, even when the ambient temperature was in the 45 to 55 range. It was not possible to obtain an hourly mean temperature for all the structures but the hourly mean in the Sunpocket was nearly 10 F less than the average of the daily maximum and minimum temperatures. This indicates that, except for brief periods, the daytime temperatures in the Sunpocket were no higher than in the other structures.
Light transmission varied considerably among the structures. Transmissivity was greatest for the glasshouse and only slightly reduced for the recently covered Sunpocket. The older covers on the longhouse and prophouse further reduced transmisssion. The lowest transmissivity occurred with the fiberglass-covered screenhouse. Considering that the Sunpocket covers were rolled up during the majority of the daylight hours, total solar radiation was probably as great for this structure as for the glasshouse.
Onion growth was slowest in the Sunpocket and greatest in the glasshouse (Table 2). For lettuce and celery, growth was also greatest in the glasshouse but slowest in the screenhouse. For broccoli, growth was greatest in the longhouse, least in the screenhouse. Thus, no single environment stands out as promoting the most rapid growth of all species. The degree of variation in plant growth (expressed as the coefficient of variation or CV) differed somewhat among structures, but no one environment was superior for all crops and components of plant growth.
A major problem with late winter and early spring transplant production is that the combination of low light intensity and relatively high daytime temperature produces a spindly or leggy transplant. This problem can be quite severe in houses in which the cover greatly reduces light transmission. One measure of legginess is the height to weight ratio of the transplant: the lower the ratio, the stockier or less leggy the transplant. For celery, the stockiest plants were produced in the Sunpocket. For onion and lettuce, the glasshouse produced the stockiest plants. Ths stockiest broccoli plants came from the longhouse.
That no one house stood out as superior is not surprising as the temperature regimes were similar for all. Light intensity probably did not play a large role either since the trial was carried out during a period of non-limiting solar radiation. However, the energy required to maintain moderate daytime temperatures was lowest in the Sunpocket since forced-air cooling was not needed. If the watering and cover lifting mechanisms of the Sunpocket were automated (time clocks, solenoid valves, and thermostat-regulated cover roll up), this structure would also be no more labor intensive than the others.
Table 1. Effect of greenhouse structures on mean daily maximum and minimum temperatures and transmission of solar radiation, NWREC, Oregon, March 26 - April 24, 1989 Structure Maximum (F) Minimum (F) Transmissivity (%) Sunpocket 85.6 54.5 84 Glasshouse 84.4 58.2 92 Longhouse 85.3 53.6 61 Prophouse 83.8 54.3 60 Screenhouse 79.0 50.1 41 Table 2. Effect of greenhouse structures on growth parameters of four species of vegetable transplants, NWREC, Oregon, April 24, 1989 Crop Location Weight CV Height CV No. of CV Stem diam. CV Ht./Wt. (g) (%) (cm) (%) leaves (%) (cm) (%) Onion Sunpocket 0.65 43 16.3 20 3.0 0 0.325 18 25.1 Glasshouse 1.56 39 30.2 15 3.1 9 0.353 13 19.4 Screenhouse 1.42 28 29.0 13 3.0 0 0.345 8 20.4 Prophouse 0.95 33 23.1 15 3.0 0 0.345 9 24.3 Longhouse 1.03 35 26.7 10 3.0 0 0.318 14 26.2 LSD (0.05) 0.32 2.8 NS NS Broccoli Sunpocket 1.08 18 13.0 7 3.4 15 0.155 10 12.0 Glasshouse 1.35 17 19.3 6 3.3 14 0.155 5 14.2 Screenhouse 0.87 24 15.5 11 2.6 20 0.135 11 17.8 Prophouse 1.58 23 20.1 7 3.6 14 0.152 8 12.7 Longhouse 2.28 31 20.8 8 3.8 17 0.178 13 9.1 LSD (0.05) 0.32 1.3 0.4 0.013 Lettuce Sunpocket 1.36 51 9.7 16 4.1 16 7.1 Glasshouse 2.56 32 15.5 7 4.4 18 6.5 Screenhouse 1.19 25 11.9 12 3.7 13 10.0 Prophouse 2.40 34 15.0 12 4.3 15 6.3 Longhouse 1.98 41 13.2 22 4.5 23 6.7 LSD (0.05) 0.59 1.5 0.5 Celery Sunpocket 1.74 19 12.2 12 3.2 12 7.0 Glasshouse 2.23 26 18.0 12 3.3 14 8.1 Screenhouse 0.98 39 12.2 14 2.6 20 12.4 Prophouse 1.44 36 15.2 18 2.8 13 10.6 Longhouse 1.38 36 11.9 18 3.0 0 8.6 LSD (0.05) 0.38 1.5 0.3