Stale seedbeds are commonly used by organic vegetable farmers to reduce in-season weed density. The primary purpose of this study was to evaluate the efficacy of soil solarization (clear plastic) with subsequent flaming for stale seedbed preparation. A secondary objective was to compare the efficacy of solarization with tarping (black plastic). Solarization is an established weed management practice in warmer climates, but its efficacy in the humid continental Northeast USA was unknown. We hypothesized that solarization during May-June in Maine, USA would increase weed emergence, and could thereby contribute to depletion of the germinable weed seedbank and, with subsequent flaming, creation of an improved stale seedbed. We expected that firming soil with a roller prior to solarization would further increase weed emergence. Across four site-years of replicated field experiments and two on-farm trials we found that, contrary to expectations, 2 weeks of solarization reduced apparent weed emergence (density) in comparison to nonsolarized controls by 83% during treatment, and 78% after 2 weeks of observation following plastic removal and flaming. Rolling did not significantly affect weed density. Soil temperatures were elevated in solarized plots, reaching a maximum of 47◦ C at 5 cm soil depth, compared to 38◦ C in controls. Weed community analyses suggested that solarization might act as an ecological filter limiting some species. Addressing our secondary objective, two replicated field experiments compared the efficacy of solarization with tarping applied for periods of 2, 4, and 6 weeks beginning in July. Across treatment durations, solarization was more effective than tarping in one site-year, but tarping outperformed solarization in the other; this discrepancy may be explained by differences in weed species and soil temperatures between experiments. Overall, solarization and tarping are promising stale seedbed preparation methods for humid continental climates, but more work is needed to compare their relative efficacy.
Vegetable growers, including organic growers, commonly use stale seedbed periods prior to sowing high-value crops. Creating a false or stale seedbed,
Soil solarization using clear plastic mulch was developed in the 1970s as a method to control soil borne pathogens [
Tarping, also known as occultation [
However, in one study conducted during the fall in Israel, tarping outperformed solarization [
The primary objective of this study was to test whether solarization combined with flaming could improve the efficacy of stale seedbed establishment in the Northeast USA. A secondary objective was to compare the weed control efficacy of solarization to tarping. Field experiments were conducted in 2015-2017 to test the following hypotheses:
1. Springtime soil solarization will increase weed emergence;
2. Firming soil with a roller will further increase weed emergence;
3. The seedbank depletion resulting from solarization and rolling will reduce weed emergence in a subsequent stale seedbed created by flaming; and
4. During mid-summer, solarization will be more effective than tarping for stale seedbed establishment.
To test Hypotheses 1 to 3, replicated field experiments were conducted over four site-years near Orono, Maine, USA (Table
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Rogers 2015 |
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Pushaw-Boothbay complex; 4.6% OM
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16.0 | 15.9 | solarization: 27 May – 12 June observation: 12 June – 30 June |
UMG 2015 |
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Peru-Tunbridge association; 6.6% OM and 6.1 pH (2012) | 15.0 | 14.4 | solarization: 15 May – 3 June observation: 3 June – 22 June |
Rogers 2016 |
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Pushaw-Boothbay complex; 3.7% OM and 6.2 pH (2014) | 16.4 | 7.7 | solarization: 13 May – 31 May observation: 31 May – 14 June |
Smith 2016 |
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Nicholville very fine sandy loam; 5.0% OM and 5.9 pH (2014) | 16.9 | 6.3 | solarization: 18 May – 1 June observation: 1 June – 15 June |
Experiments were conducted at the University of Maine Rogers Farm and the UMaine Greens Project field (UMG) in 2015, and at Rogers Farm and the University of Maine Smith Farm in 2016. Soil series data are from NRCS [ ] ; † OM = organic matter; ‡ year of soil test shown in parentheses. Weather data are from NOAA [ ] . Dates show periods during which solarization treatments were applied in the field, and periods of observation of weed emergence following plastic removal. |
Field experiments included four treatments, arranged in a randomized complete block design with three replications per site-year (Figure
• Tilled (control) • Tilled + rolled (control) • Tilled + solarized • Tilled + rolled + solarized
Prior to establishment of each experiment, soils were rototilled to 15 cm soil depth, except for the Smith 2016 experiment in which the field was moldboard ploughed followed by cultivation with a Perfecta field cultivator (Unverferth Manufacturing Co., Inc., Kalida, Ohio, USA). In all experiments a 45.4 kg lawn roller was used to simulate cultipacking. This tool was appropriate to the scale of these experiments, but likely firmed soil more consistently than would a standard ring cultipacker. Prior to mulching, all plots were irrigated to approximate field capacity to increase heat conduction [
Plots were 3 m by 3 m with 0.6 m between plots. To secure plastic while keeping plots accessible for measurement during treatment, plastic edges were clipped to 3.3 cm diameter by 3.2 m long pieces of galvanized metal pipe laid in 10 cm deep trenches around plot perimeters. Plastic was removed after approximately two weeks of solarization (Table
Soil temperatures were logged hourly for the duration of solarization treatment using iButton temperature loggers (Maxim Integrated, San Jose, California, USA). One logger per plot was placed in a sealed 5 cm by 5 cm 4-mil plastic bag and buried at 5 cm soil depth. Volumetric soil moisture content was measured and averaged across three locations within each plot using a Delta-T soil moisture meter (HH2 version 4.0, Delta-T Devices Ltd, Cambridge, England) at the start of each experiment and concurrent with each weed censuses (described below).
Weeds were counted once every 2 to 7 days during solarization treatment, and approximately every 7 days for 2 weeks following solarization. Plastic was temporarily removed during census counts. During each census, weed seedlings were counted and pulled from permanent 0.25 m by 0.5 m quadrats during the solarization period, and from a new set of permanent quadrats during the period following solarization. The four weed taxa most abundant in each quadrat were identified and counted; remaining weeds were counted as other broadleaved or other grass-like. Weeds were identified to species level with the following exceptions:
All analyses were performed in R [
To determine whether solarized and rolled treatment effects were significant across site-years, we fit linear mixed effects (LME) models to the weed density data from all four site-years of experiments (Table
To test for treatment effects on weed community composition, permutational multiple analysis of variance (PERMANOVA) models were fit for the period during solarization and the period after solarization, respectively, using Euclidean distances and 999 permutations [
To compare solarization to tarping, experiments (hereafter TARP) were conducted at the University of Maine Rogers Farm (44
Experiments consisted of seven treatments arranged in a randomized complete block design with three replications. Six mulched treatments consisted of factorial combinations of plastic mulch (solarization, tarping) and treatment duration (2, 4, and 6 weeks); the seventh treatment was a nonmulched control. Plots were 1 m by 1 m with 0.6 m between plots, which was considered the minimum size needed to avoid strong edge effects [
Following the methods detailed in section
The nonmulched control treatment was excluded from statistical analysis due to pseudoreplication in the experimental design and because this treatment was not essential to our objective of comparing solarization and tarping efficacy. Data were analyzed in R [
In our spring experiments, soil temperatures were elevated under solarization, with maximum temperatures ranging from 32 to 47
There was no significant difference in weed density between flamed and nonflamed subplots in either solarization treatment (tilled + solarized: t = -0.49,
PERMANOVA models suggested non-significant effects of treatment on weed community composition during (R
Mean cumulative weed density (A) during so- larization and (B) after solarization across experimental site-years and on-farm trials. Means were separated by Fisher’s Protected LSD.
Linear discriminant analyses showing separation of weed communities by treatment along the first two of three linear discriminant (LD) functions (A) during solariza- tion and (B) after solarization. Percent variation explained by each LD function (trace) shown in square brackets.
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Tilled control | 32 ± 2 | 17 ± 1 | <1 | 0 | 0 | 21 ± 8 | 13 ± 3 | ||
Tilled + rolled control | 32 ± 2 | 17 ± 1 | 0 | 0 | 0 | 29 ± 7 | 20 ± 4 | ||
Tilled + solarized | 42 ± 4 | 24 ± 2 | 21 ± 12 | 12 ± 11 | <1 | 21 ± 7 | 13 ± 4 | ||
Tilled + rolled + solarized | 42 ± 3 | 23 ± 3 | 20 ± 11 | 12 ± 12 | <1 | 27 ± 7 | 20 ± 4 | ||
Temperatures were measured at 5 cm soil depth; soil moisture was measured prior to solarization (start) and following plastic removal (end). Summary statistics calculated across four experimental site-years and two on-farm trials |
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quackgrass | 0.12 | -0.03 |
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redroot pigweed | -0.04 | -0.06 |
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common ragweed | -0.15 | |
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shepherd’s-purse
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-0.47 | -0.06 |
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common lambsquarters | -0.16 | 0.02 |
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large crabgrass | 0.04 | -0.07 |
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barnyardgrass | 0.06 | -0.06 |
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hairy galinsoga | -0.06 | -0.01 |
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witchgrass | -0.08 | |
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annual bluegrass
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-0.30 | -0.31 |
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common purslane | -0.13 | |
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common chickweed
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-0.36 | |
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white clover | -0.37 | |
More negative values are associated with control plots; more positive values are associated with solarized plots. † = winter annual species. |
Solarization resulted in higher maximum and average soil temperatures than did tarping (Table
Mean ± SEM density of Portulaca oleracea, other broadleaved weeds, and other grass-like weeds measured at 14 days after plastic termination in the 2017 TARP exper- iment. Data are shown pooled across treatment durations.
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Duration (weeks) |
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2016 | control | 2 | 35 ± 1 | 24 ± 0 | <1 | 0 | 0 | 36 ± 6 | 12 ± 1 | ||
4 | 35 ± 1 | 23 ± 0 | <1 | 0 | 0 | 36 ± 6 | 10 ± 2 | ||||
6 |
nd
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nd | nd | nd | nd | 36 ± 6 | 23 ± 2 | ||||
tarping | 2 | 41 ± 2 | 28 ± 0 | 28 ± 11 | 3 ± 3 | 0 | 35 ± 5 | 18 ± 2 | |||
4 | 41 ± 2 | 28 ± 0 | 71 ± 17 | 8 ± 12 | 0 | 38 ± 2 | 18 ± 2 | ||||
6 | 41 ± 3 | 27 ± 0 | 72 ± 55 | 11 ± 10 | 0 | 36 ± 5 | 17 ± 1 | ||||
solarization | 2 | 46 ± 3 | 31 ± 1 | 48 ± 11 | 30 ± 21 | 4 ± 8 | 33 ± 5 | 18 ± 3 | |||
4 | 46 ± 3 | 31 ± 1 | 101 ± 12 | 64 ± 41 | 8 ± 11 | 33 ± 1 | 16 ± 3 | ||||
6 |
50 ± 1
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31 ± 0
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108 ± 2
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117 ± 2
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49 ± 8
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39 ± 3 | 17 ± 5 | ||||
2017 | control | 2 |
33 ± 4
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23 ± 1
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0
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0
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0
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22 ± 3 | 7 ± 1 | ||
4 |
33 ± 4
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22 ± 1
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0
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0
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0
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22 ± 3 | 10 ± 1 | ||||
6 |
33 ± 4
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21 ± 1
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0
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0
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0
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22 ± 3 | 33 ± 4 | ||||
tarping | 2 |
39 ± 1
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25 ± 0
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16 ± 7
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0
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0
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30 ± 4 | 12 ± 0 | |||
4 |
37 ± 0
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25 ± 0
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12 ± 1
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0
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0
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26 ± 5 | 15 ± 2 | ||||
6 | 39 ± 1 | 24 ± 0 | 31 ± 18 | 1 ± 2 | 0 | 29 ± 4 | 28 ± 6 | ||||
solarization | 2 |
46 ± 0
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29 ± 1
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32 ± 8
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33 ± 6
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5 ± 1
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29 ± 11 | 13 ± 5 | |||
4 | 43 ± 3 | 27 ± 1 | 40 ± 14 | 17 ± 16 | <1 | 32 ± 6 | 14 ± 1 | ||||
6 | 45 ± 1 | 25 ± 1 | 51 ± 36 | 25 ± 6 | 1 ± 2 | 30 ± 2 | 28 ± 11 | ||||
Temperature was logged at 5 cm soil depth; soil moisture was measured prior to solarization (start) and following plastic termination (end). Data averaged across three replicate plots unless otherwise noted: † nd signifies no data; ‡ data from 2 replicates only. |
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2016 | control | 2 | 595 ± 114 | 803 ± 127 |
4 | 803 ± 127 | 635 ± 46 | ||
6 | 635 ± 46 | 680 ± 44 | ||
tarping | 2 | 0 | 261 ± 67 | |
4 | 0 | 640 ± 130 | ||
6 | 0 | 205 ± 69 | ||
solarization | 2 | 5 ± 5 | 141 ± 14 | |
4 | 11 ± 7 | 16 ± 5 | ||
6 | 5 ± 5 | 11 ± 5 | ||
2017 | control | 2 | 56 ± 12 | 403 ± 101 |
4 | 403 ± 101 | 320 ± 47 | ||
6 | 320 ± 47 | 453 ± 107 | ||
tarping | 2 | 0 | 32 ± 9 | |
4 | 0 | 0 | ||
6 | 0 | 27 ± 16 | ||
solarization | 2 | 419 ± 134 | 427 ± 130 | |
4 | 571 ± 50 | 288 ± 41 | ||
6 | 224 ± 61 | 237 ± 80 |
Contrary to expectations (Hypothesis 1), but nevertheless a desirable weed management outcome, springtime soil solarization greatly reduced weed density during two weeks of treatment (Figure
The maximum temperatures and accumulated time under high temperature conditions measured at 5 cm depth during these experiments (Table
The pattern in our weed community data following solarization (Figure
Our decision to employ previously used greenhouse plastic in these experiments may have impacted results. There is a considerable body of research characterizing the effects of plastic optical properties on soil heating [
Solarization, which has long been of interest as a relatively environmentally-friendly alternative to chemical fumigants [
We had expected solarization to result in higher soil temperatures and better weed control outcomes than tarping (Hypothesis 4). Results of our 2016 TARP experiment (Table
We had expected the efficacy of both solarization and tarping to increase with treatment duration, but our data offer weak and inconsistent support for this idea. Treatment duration was not a significant factor in 2017. In 2016, solarization efficacy did increase with treatment duration (Table
Overall, these results suggest tradeoffs between solarization and tarping that should be more thoroughly characterized before either strategy is advocated as a “better” approach for farmers in the Northeast USA and areas of similar climate. Solarization applied as a stale seedbed technique to susceptible species under good conditions may result in greater seedbank depletion than tarping [
Across replicated experiments and on-farm trials, two weeks of springtime soil solarization followed by flaming created a stale seedbed with 78% less subsequent weed density than a control stale seedbed prepared with flaming only. Nonflamed subplots established during one site-year suggested that solarization alone, without flaming, can create an effective stale seedbed. Soil temperatures measured under solarization may have contributed to thermal inactivation of some species of weed seed, and fatal germination of others. Multivariate weed community analyses indicate that solarization may act as an ecological filter shaping weed community composition. We hope future studies of solarization will more thoroughly characterize its impacts on weed seedbanks, and evaluate whether the practice is economically advantageous to growers in our region. Additional experiments compared the efficacy of solarization to tarping with black plastic. Solarization outperformed tarping in one year of study, but the opposite was true the following year. Higher temperatures in our first year experiment, and high density of the relatively heat-tolerant weed
Thanks to Craig Hickman for the excellent question that inspired this project, to Eliot Coleman for his deep knowledge, and to both for collaborating in on-farm trials. We greatly appreciate the excellent work of Liam Kenefic and our other field crew members: Ana Eliza Souza Cunha, Grace Smith, Lucia Helder, and Anthony Codega. Thanks also to Brad Libby, Joe Cannon, Laureen Traclet, Tom Molloy, Tamara Levitsky, Corianne Tatariw, Mark Guzzi, Margaret McCollough, Bryan Brown, Avinash Rude, Brian McGill, Ellen Mallory, Jay Hao, David Hiebeler, our anonymous reviewers, and the many farmers whose questions and insights helped guide this project. This work was made possible by funding from the Maine Food and Agriculture Center and the University of Maine Graduate Student Government. This project was further supported by the USDA National Institute for Food and Agriculture, Hatch Project Number ME0-21606 through the Maine Agricultural and Forest Experiment Station. This is MAFES publication number 3705.
Aerial view showing locations of four spring solarization experiments: (A) Rogers 2015, (B) UMG 2015, (C) Rogers 2016, and (D) Smith 2016.
Illustration of experimental design for spring solarization experiments, including example plot maps for one of four experimental site-years and one of two on-farm trials.
Aerial view showing locations of TARP experiments conducted during the summers of 2016 and 2017 to compare solarization and tarping.