Row covers increase daily mean and maximum air temperatures, but their effect on minimum air temperatures is not clear cut. Manufacturers of these materials often claim that covers provide a significant degree of frost protection. However, M. H. Keveren (Plastics in horticultural structures. Rubber and Plastics Research Assoc. of Great Britain, 1973) has established that, on cool nights, temperatures under plastic row covers may be lower than the ambient. This "temperature inversion" was related to the thermal conductivity of the plastic, the degree of condensation on the cover, and the amount of air movement.
Temperature measurements have been made under row covers at the North Willamette Research & Extension Center for several years, both in winter and in spring. In every experiment, row covers increased the mean minimum air temperature recorded for the length of the experiment. However, there were exceptions to this trend, primarily on cool, clear nights. The following is a compilation of mean minimum temperatures recorded in each of these experiments, along with every instance of exceptions to the general trend. Exceptions are underlined. All temperatures in degrees Fahrenheit.
Table 1. Effect of floating row covers on minimum air temperatures, 14 April -1 June, 1983. Crop: muskmelon Date Soil Mulch Mulch+Reemay Mulch+Vispore Mulch+Xiro Mean 44.1 44.1 47.4 46.7 46.6 4/15 NR 34 34 37 34 4/16 NR 38 39 40 38 4/23 NR 39 38 40 39 4/30 NR 48 48 52 51 All temperatures measurements made 1.0 inch above the soil surface. NR = not recorded Table 2. Effect of floating row covers on minimum air temperatures, 5 April-1 July, 1984. Crop: muskmelon Date Soil Mulch Mulch+Reemay Mulch+Vispore Mean 46.6c 47.2bc 47.5 ab 48.0 a 5/08 43 43 42 43 5/13 44 39 43 41 5/22 46 43 46 46 5/25 52 51 51 51 5/27 52 53 51 52 6/04 50 50 48 51 6/10 53 52 52 57 All temperature measurements made 1.0 inch above the soil surface. Table 3. Effect of Kimberly Farms row tunnels on minimum temperatures, 7 May-23 June. 1986. Crop: tomato, melon, broccoli Date Mulch Mulch+Tunnel Mean 48.8 52.6 ** 5/12 40.0 39.8 NS 5/16 43.0 40.0 * Temperature measurements made 6 inches above the mulch surface. Table 4. Effect of floating row cover materials on minimum air temperatures, 10 June-19 June, 1986. No crop Date Soil Agronet Reemay Vispore Agryl P-17 Kimberly Farms Mean 49.3 d 49.8 cd 52.7 a 51.5 bc 52.0 ab 52.4 a 6/18 50.0 49.5 51.5 52.5 50.5 52.0 Temperature measurements made 1.0 inch above the soil surface. Table 5. Effect of floating row covers and black mulch on minimum air temperatures, 9 May-6 June, 1986. Crop: cucumber Date Soil Mulch Reemay Vispore Reemay+Mulch Vispore+Mulch Mean 49.1 c 49.3 c 51.4 a 50.7 b 50.7 b 51.3 a 5/10 39 39 42 41 40 37 5/11 36 35 41 39 37 36 5/12 39 39 41 39 38 38 5/13 48 48 47 48 48 49 5/23 47 47 48 46 53 48 Temperatures measurements made 1.0 inch above the soil or mulch. Table 6. Agryl P-17 floating cover and tunnel effects on minimum air temperatures, 8 April-22 May, 1987. Crop: muskmelon Date Soil Mulch Floating Tunnel Mean 44.1 b 43.9 b 46.8 a 46.2 a 4/08 45.7 46.0 46.0 45.7 Temperatures measured 1.0 inch above the soil or mulch surface. Table 7. Agronet floating cover effects on minimum air temperatures, 1 June-10 June, 1987. Crop: potatoes Date Soil Agronet Mean 49.8 51.1 ** 6/02 54.0 53.3 * Temperatures measured 1.0 inch above the soil surface. Table 8. Effect of covers on minimum air temperatures, 26 November, 1986 - 21 January, 1987. Crop: ornamental nursery stock Date Check Agronet Agryl P-17 Lutradur Reemay Tufbell Vispore Mean 30.3 d 32.2 c 33.5 b 34.4 a 34.7 a 32.9 c 33.1 b 12/17 28 28 30 30 30 29 29 12/18 30 30 30 32 33 32 31 01/10 29 29 30 31 31 30 29 01/15 29 29 30 31 31 30 29 Temperatures measured 9 inches above surface of can yard, in plant canopy. Table 9. Effect of covers on minimum air temperatures, 10 December, 1987-22 February, 1988. Crop: ornamental nursery stock Date Check 4-mil white poly Typar Agryl P-17 Mean 34.0 c 35.0 b 35.9 a 35.4 b 12/26 28.5 28.5 29.0 28.5 01/01 30.5 31.0 30.5 31.5 01/03 32.5 31.5 30.0 33.0 01/05 30.5 31.5 30.5 30.0 01/06 31.5 32.5 31.0 33.0 Temperatures measured 9 inches above surface of can yard, in plant canopy.
Temperatures were recorded for a total of 403 nights during these experiments. On 30 occasions, equal or lower temperatures were recorded under row covers than over bare soil. Usually the differences were very small and may not have exceeded experimental error. However, in three instances the temperature was 2 or more degrees F lower under a cover. Although not occurring often, these instances tend to confirm the findings of Keveren. "Temperature inversion" under covers did not appear to be related to the type of cover as instances occurred both with polyethylene and non-woven fabric materials. Higher-weight materials generally gave more protection.