"Solar Trenches" for Production of Warm-Season Vegetables (1981)

North Willamette Research and Extension Center (NWREC) report

Delbert Hemphill
OSU Dept. of Horticulture


The purpose of this experiment was to evaluate the effectiveness of "solar trench" techniques for extending the season of vegetable production and bringing warm weather crops into earlier production. The solar trench consisted of a V-shaped furrow 12 inches deep and 18 inches wide at the top. The bottom and sides were lined with 1.5 mil black plastic mulch for weed control. After seeding or transplanting through holes cut into the black plastic, the trench was covered with 4 mil perforated clear plastic. The purpose was to form a miniature greenhouse, trapping heat and possibly giving some frost protection.


Two sets of trenches were formed and three tons/acre of lime and 1,000 pounds/acre of 10-20-10 were broadcast. The first set of north-south oriented trenches was prepared for planting on April 22, 1.980. Viaflo tubing was placed in the trench bottom for drip irrigation. Pepper 'Keystone Giant', tomato 'Pikred', muskmelon 'Early Dawn', and zucchini 'Elite' transplants, seeded in a heated glasshouse on March 13, were set into the trench and an equal number of planting holes were direct-seeded with the same varieties. The planting was replicated three times with four transplants and four direct-seeded plants of each species per replication. In-trench spacing was two feet for tomato and pepper, and three feet for melon and squash. Trenches were six feet apart.

The second set of trenches was planted on May 8. Pepper transplants had been seeded on March 31, tomatoes on April 8, zucchini and muskmelons on April 24. Varieties were the same as for the first planting. In addition to the trenches, black plastic-covered flat beds were also planted. Thus, for the second planting date there was a four-way comparison among transplanted trenches, transplanted beds, direct-seeded trenches, and direct-seeded beds.

The clear plastic trench covers were slit above each plant when the plant tip reached the plastic. The covers were not removed until July 11 when day-time high air temperatures beneath the plastic consistently exceeded 90° F. Both air and soil temperature were recorded until removal of the plastic covers. No yield comparisons were made, but date of first bloom and harvest, and peak harvest were recorded for each treatment. Additional NPK fertilizer was applied through the irrigation system.


Temperature Effects
Maximum air temperatures recorded beneath the clear plastic trench covers always exceeded the maximum air temperature six inches above the flat beds. However, differences were small during periods of cloud cover. During hours of clear skies, the maximum recorded beneath the clear plastic often exceeded 100°F in late June and early July, but the highs recorded over the flat beds never reached 90°F before plastic removal on July 11. The mean high temperature under the clear plastic exceeded that over flat beds by an average of 16°F during the April 22 to July 11 period (Table 16). In contrast, the clear plastic covered trenches had almost no effect on low air temperatures. Thus, the trench walls provided very little insulating effect. As seen in Table 17, the trenches had very little effect on soil temperature at two-inch depth.

Zucchini responded well to trench culture. Vegetative growth was greatly promoted. In the second planting, flowering of transplants was eight days earlier in the trench than on flat beds and harvest commenced five days earlier. Direct seeded plants bloomed 15 days earlier in the trench and harvest commenced six days earlier. Some scorching of leaves occurred before the covers were removed. Fruit set was poor until the covers were removed, because pollinators would not penetrate the slitted plastic.

Transplanted tomatoes bloomed one week earlier in the trench and ripened fruit 10 days earlier. Direct seeded plants failed to respond to trenching, perhaps because the bloom period did not start until shortly before the plastic covers were removed. As with zucchini, poor pollination was a problem in the trenches before uncovering. Lack of air movement may have been a factor. When the plastic was slit, but not completely removed, the plants rapidly emerged. Flowers blooming beneath the plastic had poor set, probably related to lack of air movement and high temperatures, while flowers blooming above the plastic were affected by very low air temperatures through the first week of July.

Melon transplants and direct-seeded plants bloomed nine and six days earlier, respectively, in trenches. First harvest was nine and 13 days earlier, respectively, in trenches. Overall yields were not good because of poor fruit set under the plastic. Plants in the first trench successfully matured a second crop before first killing frost on October 23.

Peppers did not respond well to trench culture. No earliness was noted. Plants were stunted and pollination was poor before plastic removal. Direct seeded peppers generally failed to emerge.

General Comments
The solar trench is most valuable very early in the season, when comparison of the trench with traditional culture is limited by frost danger. Leafy crops or those which do not depend on either insect or wind pollination may be better adapted than the crops studied in this experiment. Poor pollination and lack of growing room were the biggest problem for each crop. No unusual disease or nutritional problems were noted except for the stunting and yellowing of peppers.

  Table 16. Weekly Air Temperature Summary                                                                Mean weekly temperature, °F                 Week ending     Flat bed high   Trench high    Flat bed low   Trench low  4/29                 68              82             44             46  5/06                 70              84             42             43   5/13                 60              69             47             47   5/20                 68              85             45             47  5/27                 57              68             45             46   6/03                 64              75             47             48   6/10                 68              85             50             53   6/17                 64              86             50             53  6/24                 71              95             51             53   7/01                 75              95             47             51  7/08                 73	             94             54             57  Mean, 11 weeks       67              83             47             49         Table 17. Weekly Summary of Soil Temperature at Two Inch Depth                                            Mean weekly temperature, °F                 Week ending      Flat bed high   Trench high    Flat bed low   Trench low  4/29                  68               71            56             56  5/06                  71               73            56             57   5/13                  63               63            55             57   5/20                  71               74            57             59  5/27                  59               60            52             52   6/03                  67               69            55             56   6/10                  70               74            59             62   6/17                  66               71            58             60  6/24                  74               80            62             64   7/01                  75               80            60             61  7/08                  75               79            63             65  Mean, 11 weeks        69               72            58             59          Table 18. Dates of First Bloom, First Fruit Harvest, and Peak Harvest         Crop	     Treatment	           First bloom   First harvest  Peak harvest  Zucchini  First trench,transplant     6/01            6/29        7/01-8/15              First trench, direct seed   6/16            6/24        7/15-8/15             Second trench, transplant   6/10            6/27        7/25-8/30            Second trench, direct seed  6/17            7/02        7/25-8/30            Bed, transplant             6/18            7/02        7/25-8/30            Bed, direct seed            7/02            7/08        7/30-9/05    Tomato    First trench, transplant    6/02            8/11        9/01-9/20            First trench, direct seed   6/18            8/18        9/05-10/01            Second trench, transplant   6/18            8/18        9/05-10/01            Second trench, direct seed  7/01            8/28        9/15-10/15            Bed, transplant             6/25            8/28        9/15-10/15            Bed, direct seed            7/05            8/30        9/15-10/15    Muskmelon First trench, transplant    5/14            8/11        8/20-9/05            First trench, direct seed   5/23            8/15        8/30-9/15             Second trench, transplant   6/01            8/18        8/30-9/15             Second trench, direct seed  6/12            8/25        9/10-9/30            Bed, transplant             6/10            8/27        9/10-9/30            Bed, direct seed            6/18            9/08        9/15-10/10    Pepper    First trench, transplant    6/25            7/15        7/30-10/22            First trench, direct seed    --              --             --            Second trench, transplant   6/30            7/21        8/05-10/22            Second trench, direct seed  7/15            7/30        8/15-10/22            Bed, transplant             7/01            7/21        8/10-10/22            Bed, direct seed             --              --             --