Introduction
Tomato and muskmelon production in the Willamette Valley is limited by cool springs, a short growing season, and relatively cool nights, even during mid-summer. Cultural techniques to increase the mean air and soil temperatures around plants, such as plastic mulch and row covers, have hastened development and increased yield of both crops.
Removal of mulch residues following crop harvest is a large factor in total crop production costs and disposal of the plastic presents additional environmental problems and costs. Bio- or photodegradable films have been used as mulches for several years but have not been practical due to variability in degradation rate. Recently, new degradable mulch materials have shown promise in several production areas, but have not been evaluated under Willamette Valley environmental conditions.
Another new development in mulch technology is the advent of wavelength-selective (WS) films. These films provide much of the weed control benefits of black plastic mulch, but warm the soil more than do the black films. Thermic films, such as the IR-303 thermofilm developed as a greenhouse cover, somewhat increase minimum temperatures compared to standard polyethylene films. These films may have a role as either floating or hoop-supported row covers.
The purposes of this trial were: 1) to compare several WS and photodegradable mulches with bare ground and standard black plastic ground mulch in promoting early and total yield of tomato and muskmelon; and 2) to compare combinations of standard black plastic mulch and row covers with standard black plastic mulch alone, for production of muskmelon.
Methods
In 1989, 'Pikred' tomato was seeded in 5-cm pots in an unheated greenhouse on 6 April; 'Superstar' muskmelon was seeded on 17 April. The plot area of Willamette silt loam was prepared by rotary tillage in mid-May following a broadcast application of 10N-8.7P-8.3K fertilizer at 1,000 pounds per acre. Mulches were applied to the appropriate plots after laying drip irrigation tubing and both crops were transplanted on 17 May. Plot size was 7.5 meters of bed with 10 plants per plot. Mulch treatments were replicated four times for each crop in randomized block design. Treatments consisted of a bare ground check, a standard 1-mil embossed black polyethylene, three formulations (2b, 19b, 131b) of Plastigone photodegradable polyethylene, and two formulations (IRT 64, IRT 70) of WS films from AEP industries.
Soil temperature was sensed by thermocouples placed 5 cm below the soil surface and was recorded for the period 8 June to 29 June on a Speedomax 250 multipoint recorder for two replications of selected treatments. Plots were harvested at least weekly from first fruit ripening until late September. Early yield was defined as that recorded during the first four weeks of the harvest season. Only USDA Grade No. 1 fruit were included in the yields.
In 1990, the same cultivars were seeded on 4 April (tomato) and 16 April (melon). Transplanting was on 16 May. Mulch treatments for tomato and muskmelon consisted of a bare ground check, an embossed 1.25-mil black polyethylene, three formulations of 1.25-mil WS films from AEP Industries (IRT 64, 70, and 76), a brown WS film (AL-OR, 0.8-mil) from Polyon-Barkai, and a 0.6-mil clear perforated polyethylene film (Polydak). Temperatures were monitored continuously at two inches soil depth from 1 June until 13 June in all mulch treatments.
Additional treatments for muskmelon only included the embossed black plastic combined with either a 2-mil perforated IR-303 Thermofilm (Polyon), Vispore micro-perforated polyethylene, Lutrasil 17 nonwoven polypropylene, or Lutrasil 30 polypropylene. The Thermofilm was hoop-supported while the other materials were allowed to float on the crop surface. Covers were removed on 19 June. Both soil and air (two inches above the soil surface) temperatures were measured from 15 June through 20 June in the cover treatments. The number of recorder channels was insufficient to take replicated readings from all mulch and cover treatments over the same time period.
Results
In 1989, all mulches tested increased both the mean maximum and the mean minimum soil temperatures (Table 1). Due to a limited number of temperature probes, soil temperatures were measured for only one of the photodegradable mulches. Both the mean maximum and mean minimum temperature under this mulch were about the same as under the standard mulch. Both IRT mulches increased soil temperatures above those under the standard mulch. The increase was particulary striking for the IRT 70 mulch which transmits approximately 70 percent of the infrared radiation but only 12 percent of the photosynthetically active radiation. Neither mulch transmitted enough light to permit weed growth.
As expected, the standard and the IRT films maintained their integrity during the entire growing season, with little or no brittleness or holes and tears. The three photodegradable mulches degraded significantly during the season, with the 2b formulation nearing loss of any weed control effect by the end of the harvest period (Table 1). However, all three mulches maintained integrity long enough to prevent significant weed competition with the crop and hasten development of the melons.
Mulch treatment had no significant effect on the early yield of tomato; however, the total number of marketable fruit harvested per plant increased with all mulches compared to the bare ground control (Table 2). In contrast, the mean fruit weight was decreased by all mulches compared to the control, resulting in no significant diffences among treatments in total weight harvested per plant. Neither the number of fruit per plant, the mean fruit weight, nor the total weight per plant varied significantly among types of mulch.
All mulches increased early muskmelon yield, as no fruit were harvested from the bare ground check plots during the first four weeks of the harvest season (Table 3). This is typical of the difference that mulch usually makes in the production of melons in the Willamette Valley. Among mulches, the IRT and the standard films tended to produce a greater early yield than did the degradable films. Total melon yield was very low for the control as compared to any of the mulches. Among the six mulches, total yield tended to be highest for the IRT films. Mean fruit weight did not vary significantly with treatment. Although not statistically significant, both early and total melon yield tended to be highest with the IRT 70 film.
Table 1. Effect of mulches on mean maximum and minimum soil temperatures (C), 8-29 June, 1989, and visual ratingsz of mulch degradation at selected days after mulch installation, NWREC, Oregon Days Mulch Mean max. Mean min. 64 91 141 None 27.4 dy 14.6 c -- -- -- Standard 28.6 cd 17.6 b 1.0 c 1.0 c 1.4 c Plastigone 2b 30.2 bc 17.6 b 2.6 a 3.4 a 3.9 a Plastigone 19b -- -- 1.5 bc 2.0 b 2.9 b Plastigone 131b -- -- 2.0 ab 2.0 b 2.8 b IRT 64 30.9 b 17.8 b 1.0 c 1.0 c 1.1 c IRT 70 34.3 c 18.3 a 1.0 c 1.0 c 1.5 c zFive point scale with 1=no change; 2=slight brittleness; 3=brittle, small holes or tears; 4=large longtitudinal or transverse tears; 5=complete loss of mulching benefit. yMean separation within columns by Duncan's MRT, 5% level. Table 2. Effect of wavelength-selective and photodegradable mulches on yield per plant of tomato, 1989, NWREC, Oregon Early Total Mulch No. Wt. (kg) No. Wt. (kg) Mean fruit wt. (g) None 7.1 1.7 41 bz 10.5 259 a Standard 6.8 1.8 68 a 14.5 214 b Plastigone 2b 8.0 1.9 60 a 12.9 216 b Plastigone 19b 5.6 1.3 59 a 12.8 216 b Plastigone 131b 7.1 1.8 64 a 15.2 236 b IRT 64 6.8 1.5 63 a 14.0 222 b IRT 70 5.7 1.7 66 a 15.2 232 b NSD NSD * NSD * NSD;*: No significant differences; significant differences at the 5% level. zMean separation within columns by Duncan's MRT, 5% level. Table 3. Effect of wavelength-selective and photodegradable mulches on yield per plant of muskmelon, 1989, NWREC, Oregon Early Total Mulch No. Wt. (kg) No. Wt. (kg) Mean fruit wt. (g) None 0.0 bz 0.0 c 0.3 c 0.5 b 1365 Standard 1.0 a 2.0 ab 3.4 b 5.7 a 1686 Plastigone 2b 0.8 a 1.4 ab 3.7 ab 5.9 a 1571 Plastigone 19b 0.8 a 1.2 ab 3.3 b 5.2 a 1559 Plastigone 131b 0.6 ab 1.0 bc 3.6 ab 5.3 a 1501 IRT 64 0.9 a 1.9 a 4.0 ab 5.8 a 1487 IRT 70 1.4 a 2.2 a 4.2 a 6.2 a 1429 * * ** ** NSD NSD;*;**: No significant differences; differences significant at the 5% and 1% levels, respectively. zMean separation within columns by Duncan's MRT, 5% level.
Among the mulch treatments in 1990, the highest maximum soil temperature occurred on plots mulched with the IRT 76 and clear Polydak films (Table 4). Mean maximum soil temperature was increased significantly over bare ground for all mulches except the standard black plastic. The mean daily low temperatures were increased significantly by all mulches compared to bare ground, with very little difference among types of mulch.
Among the 1990 row cover treatments, the greatest mean maximum air temperature occurred with the Thermofilm tunnel and the two Lutrasil floating covers (Table 5). The Vispore cover also increased the mean maximum temperature compared to the plots with only a black ground mulch. All covers increased mean maximum soil temperature compared to mulch-only plots, with the greatest increase occurring with the lighter-weight Lutrasil cover. Apparently, this cover allowed more transmission of solar radiation and greater soil warming. Mean minimum temperatures were also increased by row covers. The highest mean minimum air temperature was recorded beneath the Vispore covers, while the highest minimum soil temperature was recorded beneath the Thermofilm tunnel. Since the temperatures were recorded for different time periods, one cannot directly compare soil temperature means for the mulch-only treatments with those for the mulch-cover treatments.
Early tomato yields were not significantly affected by mulch treatment in 1990 (Table 6). However, total yield for the growing season was usually slightly greater on mulched plots compared to bare ground. Differences among mulches were not significant. Mean fruit size was not affected by mulch and the increased yield on mulched plots was solely attributable to an increase in the number of fruit harvested. Although there was considerable weed growth under and through the clear mulch, the weeds apparently did not compete significantly with the rapidly developing and relatively upright-growing tomato plants. Some weed growth also occurred under the IRT 76 film but, again, yield was not reduced.
In comparing tomato yields between the two years, fruit size was about the same, but the number of fruit produced per plant was much greater in 1989 than in 1990. This can be attributed to a relatively late season in 1990 due to unusually cold, wet weather in May and June and to the very severe and early incidence of both early and late blight in 1990. In both years mulches failed to increase early fruit production but increased total fruit production, especially in 1989. In both years, plants grown on bare ground produced much less canopy growth and the plants appeared stunted throughout the growing season. While some of this is probably due to differences in soil and canopy air temperatures, the non-mulched plants may also have suffered from moisture stress, as there was no barrier to evaporation of applied water. The stressed bare ground plants, in both years, produced and ripened sufficient early fruit that mulch did not hasten fruit ripening. However, over the course of the season, the mulched plants, with greater vine growth, produced and ripened more fruit. Bare ground plants were probably under relatively more moisture stress compared to mulched plants in 1989 than in 1990 since 1989 had a drier growing season.
Tomato response to both mulch and row covers has been considerably less dramatic than melon response to the same treatments under Willamette Valley conditions. Some of our earlier work with tomatoes and mulches has shown significant increases in early tomato production for mulched plants, but this was not the case in 1989 and 1990. Among the plastic mulch films, the standard black plastic appears to be as effective as any of the wavelength-selective films for tomato production in this area.
Table 4. Effect of ground mulches on mean minimum and maximum soil temperatures, NWREC, Oregon, 1-13 June, 1990 Treatment Mean minimum, oF Mean maximum, oF ____________________________________________________________________________________________ Bare ground check 54.8 71.3 Std. black mulch 57.9 74.2 IRT 64 57.2 78.3 IRT 70 58.0 83.3 IRT 76 57.2 85.3 AL-OR brown 57.1 77.5 Clear perforated 57.5 85.2 LSD (0.05) 0.7 5.3 Table 5. Effect of ground mulch-row cover combinations on mean minimum and maximum air and soil temperatures, NWREC, Oregon, 14-19 June, 1990 Treatment Mean minimum, oF Mean maximum, oF ______________________ ______________________ Air Soil Air Soil Std. black mulch 50.3 59.7 92.8 87.2 Mulch + Thermofilm 54.7 65.7 116.8 91.2 Mulch + Lutrasil 17 52.0 61.5 116.3 96.0 Mulch + Lutrasil 30 53.3 62.3 117.1 91.3 Mulch + Vispore 55.2 62.0 112.7 92.5 LSD (0.05) 2.3 0.9 4.1 1.1 Table 6. Effect of plastic ground mulches on yield and earliness of 'Pikred' tomato, NWREC, Oregon, 1990 No. fruit/plant Mean fruit weight (g) Yield (T/A) Treatment Early Total Early Total Early Total Bare ground check 7 26 249 216 4.9 15.2 Std. black mulch 8 31 264 224 5.8 18.3 IRT 64 green mulch 7 33 231 214 4.0 18.4 IRT 70 green mulch 8 30 226 223 5.0 17.7 IRT 76 green mulch 8 31 216 216 5.1 17.7 AL-OR brown mulch 8 34 217 203 4.8 18.1 Perf. clear mulch 9 30 235 222 5.4 17.3 LSD (0.05) NSz 6 NS NS NS 2.9 __________________________________________________________________________________________________________ zNS: no significant differences among treatments.
Early melon yields in 1990 were generally highest for the mulch plus cover treatments (Table 7). However, early yield on plots with only the IRT 70 and 76 mulches were equal to those obtained with the combination of the black mulch and the Thermofilm tunnel. Mean fruit weight during the early part of the season was greater for any WS mulch or mulch-cover combination when compared to standard black plastic. But among the WS mulches and the mulch-cover combinations, mean fruit size was usually smaller for treatments producing greater numbers of early-ripening fruit. Thus, the IRT 64 and AL-OR films, which produced relatively few early fruit compared to several other treatments, produced the largest fruit during the early part of the season.
For the entire growing season, the greatest yields occurred with the combination of black mulch and the Thermofilm tunnel. Total yield with mulch plus Vispore and mulch plus Lutrasil 30 did not significantly exceed that for several of the better mulch-only treatments. Among the mulch-only treatments, total yield tended to be highest with the standard black plastic mulch. The greatest mean fruit size for the entire season was obtained with the AL-OR mulch. Differences among other treatments were relatively minor.
Although producing greater numbers of fruit, most mulch treatments also produced larger fruit. The one exception was the Polydak clear mulch. While weed control was acceptable under other mulches, the Polydak mulch promoted weed growth which probably competed significantly with crop growth. No fumigants or herbicides were used in this trial in order to compare the effectiveness of the mulches without confounding effects of herbicide treatments.
Growers in this area have successfully used standard clear polyethylene mulches under vine crops without use of fumigants or herbicides. Emerging weeds have generally been killed by the high temperatures under the mulch. However, with the unusually cold and damp weather in May and June, insufficient temperatures were generated to kill emerging seedlings. Also, the perforation of the Polydak mulch may have produced lower air temperatures under the mulch.
Some weed growth was also noted under the IRT 76 film, but it did not appear to interfere significantly with crop growth. However, it is interesting to note that neither early nor total yield on the 76 film exceeded that with the IRT 70 film. One season's data are hardly conclusive, but it interesting to speculate that the 70 formulation may be adequate for Willamette Valley conditions and that the 76 film may permit too much weed growth in some situations without the benefit of increased melon vine growth.
The relatively small melon size for the entire season for the tunnel and Lutrasil 17 treatments may be due to the greater number of fruit produced with these treatments.
For the mulch-cover treatments, early fruit production correlated strongly with mean maximum soil temperature, but not with minimum soil temperature or air temperature. For mulch only treatments, early fruit production also correlated strongly with mean maximum soil temperature except for the Polydak mulch where weed competetion probably played a role in the lower than expected yields. Total season yields did not correlate with the early-season temperature measurements. This is not surprising as canopy development strongly affects both air temperatures recorded just above the soil surface and soil temperature. Mulch effects should be negligible after the plant canopy has completely covered the bed surface and, of course, covers are effective only as long as they are in place, usually only the first four to six weeks after transplanting.
These results reaffirm the necessity of using a ground mulch for successful muskmelon production in the Willamette Valley. Some of the WS mulches appear to be more effective than black mulch and are nearly as effective as row covers for promoting early fruit ripening. The thermic films appear promising for promoting excellent vine development and high total yields.
Both the WS and the photodegradable mulches appear promising for production of warm weather crops in the Willamette Valley and deserving of further trial. The WS films increased soil temperatures more than did the standard film, indicating the possibility of improved crop growth and development in many growing seasons. The photodegradable films, particularly the early-degrading 2b formulation of Plastigone, appear to break down to very small particles which do not need to be removed from the field in preparation for the next crop.
Table 7. Effect of plastic ground mulches and row covers on the yield and earliness of 'Superstar' muskmelon, NWREC, Oregon, 1990 No. fruit/plant Mean fruit weight (g) Treatment Early Total Early Total Bare ground check 0.0 1.2 -- 1483 Std. black mulch 0.1 2.3 1200 1659 IRT 64 green mulch 0.2 1.5 2169 1650 IRT 70 green mulch 1.1 2.2 1711 1652 IRT 76 green mulch 1.1 2.0 1578 1611 AL-OR brown mulch 0.4 1.6 2186 2098 Perf. clear mulch 0.3 1.0 1662 1418 black mulch + tunnel 1.1 4.6 1927 1408 black mulch + Lutrasil 17 2.4 3.1 1442 1391 black mulch + Lutrasil 30 1.7 2.3 1632 1679 black mulch + Vispore 1.8 2.5 1688 1770 LSD (0.05) 0.5 0.6 459 326