Weeds in Organic Farms

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An ASABE Meeting Presentation

Paper Number: 12-1337536

Innovative Systems for Weed Control in Small ScaleOrganic Production

John Wilhoit, Extension Associate Professor, University of Kentucky,jw [email protected]

Tim Stombaugh, Extension Associate Professor, University of Kentucky

William Pomeroy, Graduate Student, University of Kentucky

Mark Williams, Associate Professor, University of Kentucky

Written for presentation at the

2012 ASABE Annual International MeetingSponsored by ASABEHilton AnatoleDallas, Texas

July 29 August 1, 2012

Abstract.The use of herbicides is not allowed in organic vegetable production, so weeds must becontrolled by mechanical means, usually by the use of mulches or by cultivation, or a combination ofboth, as well as through cultural methods. Plastic film mulches, which are very effective atcontrolling weeds within the vegetable bed, are used extensively in both conventional and organicproduction, but weed control between the rows of plastic mulch is still very challenging, especially fororganic growers. Mulching with round bales of hay and straw is a method used by some organicgrowers to control weeds. An offset bale unroller offers a simple concept for modifying a standardimplement for unrolling hay bales into a new configuration that can straddle a row of plastic andunroll the bale in the space between rows for mulching. It greatly reduces the labor requirements forthis practice. Precision cultivation is another important method used for weed control in organicvegetable production. A power steering retrofit for an older, inexpensive cultivating tractor, makesprecision cultivation considerably easier, both for bare ground and plasticulture cultivatingapplications. A study looking at GPS-based automatic guidance system applications in plasticulturevegetable production is giving an indication of some of the potential benefits, but the high cost of thetechnology isa significant challenge for smaller scale vegetable growers. With the reduction in costsfor various automation technologies, it will be important to conduct further investigations intoapplications of these technologies to benefit weed control for smaller-scale organic production.

Keywords.Organic production, Cultivation, Automatic Guidance, Mechanization, Mulch


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Introduction

Weed control is especially challenging for organic vegetable growers because the use ofherbicides is prohibited in organic production. Weeds must be controlled by mechanicalsuppression, usually with cultivation, mulches, or a combination of both, as well as through

cultural methods intended to reduce weed populations.

Plasticulture, the term used for the practice of growing crops with trickle irrigation on bedscovered with plastic film mulches, is very effective at controlling weeds within the row and alsohas strong moisture conservation benefits; therefore this practice is used extensively in bothconventional and organic vegetable production. Weed control between the rows of plastic canstill be a significant challenge, however. Weeds can hinder access for crop maintenance andharvesting activities, and the crop growth can be adversely affected from shading if the weedsget large enough (Law et al., 2006). Dealing with retrieval and disposal of plastic mulches at theend of the growing season is a major disadvantage of their use, as is the expense. For organicproducers, there is also some concern that plastic mulches, while allowable under the rules fororganic certification, may leach chemicals into the soil.

Organic vegetable growers almost always must employ some mechanical means of controllingweeds, whether between the rows of plastic, or in the row itself in bare ground productionsystems (without the use of plastic film mulch). Mechanical cultivation can be done using avariety of tractor-mounted tools, usually in combination with a certain amount of manualweeding as well, especially for in-row weed control.

Mulching is used to control weeds mechanically by impeding germination of weed seed byblocking light and keeping seed from contacting soil, and smothering out growing weeds.Mulching with organic materials such as hay and straw has additional benefits. The mulch canhelp reduce the need for washing produce, it helps keep soil from splashing onto beds when itrains, reducing certain disease pressures, and the decomposed mulch incorporates readily atthe end of the season, adding organic material to the soil (Waterpenny Farm, 2012). The weedcontrol effectiveness of hay and straw mulches depends on the amount of mulch used. Thehigh cost of purchasing enough material to get sufficient weed suppression, and the effortrequired to apply the materials in such quantities, are disadvantages of the practice(Schonbeck, 2009). Another disadvantage of using hay and straw mulches is that it hinders theuse of tractor-mounted tools for cultivating weeds, once the mulch is applied.

A third approach to controlling weeds between rows of plastic (in addition to mechanicalcultivation and mulching) is to keep them mowed. Many growers will seed cover crops betweenrows of plastic with the specific intent of reducing weed pressures through allelopathic effects(like rye) or by out-competing the weeds (buckwheat or millet). The cover crop and remainingweeds are controlled by repeated mowing using tractor-mounted rotary or flail mowers, or evenlawnmowers. Controlling weeds right near the edge of the plastic without damaging it is a bigchallenge with this approach, as it is with mechanical cultivation.

All of these approaches are very labor intensive unless larger-scale, sophisticated equipment isused. Such equipment is often used in industrial-scale organic vegetable production becausethe economics of scale reduce the per acre costs of the mechanization. For smaller-scaleorganic producers, the costs of such expensive equipment cannot be justified, so the laborrequirements for the various operations required for weed control can be excessive. As a result,many smaller-scale organic operations either get overrun with weeds, reducing efficiency foreverything else throughout the production season, or they have to spend so much for labor tokeep weeds under control that profits are reduced significantly. Organic producers need cost-


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effective alternatives for equipment and methods that can improve the labor efficiency ofoperations required for weed control.

The Specialty Crops Mechanization program at the University of Kentucky has searching forinnovative approaches to controlling weeds in smaller-scale organic production. Three differenttechnologies are being explored as possibilities to help the industry. First, we developed an

offset bale unroller that allows round bales of hay and straw to be unrolled for mulching betweenrows of plastic. It has been used for several seasons in watermelon production. Second, wedeveloped a power steering retrofit for an older, inexpensive cultivating tractor which makesprecision cultivation considerably easier. Third, we have been conducting investigationscomparing lower and higher cost GPS-based automatic guidance systems for variousplasticulture applications. The objective of this paper is to describe these three technologies,show how they can be applied, and report preliminary results from evaluations of these systemsfor applicability to smaller-scale organic production systems.

Innovative Organic Weed Control Methods

Offset Bale Unroller

Hay and straw make good mulches for suppressing weed growth in vegetable production, but itis very labor intensive to break apart and spread square bales. Using round bales for mulchingcan reduce the labor requirements considerably because there is so much more hay in eachbale than in small square bales, and the round bales can be rolled out to peel off layers that are

just about right for mulching between row of plastic. It is still very labor intensive to roll the balesout manually. Commercially available tractor three-point hitch mounted bale unrollers withhydraulically-actuated arms can be used to clamp on the center point of round bales and unrollthem in layers by driving forward with the bale against the ground. These implements carry thebale along the centerline of the tractor, so they cannot be used to unroll bales between rows ofplastic because there is not enough space to drive the tractor between the rows. In order to

make the make it possible to use a bale unroller to roll out round bales for mulching betweenrows of plastic, we modified a bale unroller by extending the toolbar and adding another mastand lower hitch point, so the bale is offset a sufficient distance that the tractor can straddle therow of plastic and unroll the bale in the space between adjacent rows of plastic (Figure 1). Wealso added a hydraulic top link so the length of the top link can be increased as the bale getssmaller, allowing the arms gripping the bale to be pivoted downward to keep the bale on theground as it gets smaller. This keeps the toolbar higher to prevent damage to the plants or theplastic covering the bed. We used this offset bale unroller to apply different hay and strawmulch treatments to watermelon plots at the University of Kentucky Horticulture Research Farmduring the 2009 2010 seasons, and also demonstrated its use on several different farms in2012.


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Figure 1. Offset bale unroller unrolling a round bale of straw between rows of plastic mulch.

The modifications to the bale unroller were pretty straight forward and can be made to anystandard bale unroller provided that the clamping arms are open at the end where they pivot onthe toolbar, allowing the additional length of toolbar to be welded on. We used a Worksaverbrand bale unroller for this reason. The offset bale unroller is intended for use with smaller

round bales (4 ft width) because they are an appropriate width for mulching between plasticrows on typical bed spacing, and they are considerably lighter than the larger bales, so they canbe carried by smaller tractors (around 35 hp minimum). The added-on mast and lower hitchpoint were configured so that the offset bale unroller can be used with either Category I or IIthree-point hitch systems. In our on-farm trials, we discovered that if a smaller tractor and threepoint hitch configuration did not allow the bale to be lifted high enough off the ground fortransport (without unrolling it), the arms could be used to clamp on the bale below and to therear of the bale center point so that it wedged against the toolbar. The bale could then be liftedand transported (without unrolling) to position it at the edge of the field, and then the clampingarms could repositioned in the center of the bale for the unrolling operation. The offset baleunroller uses two hydraulic cylinders, one for the clamping arms, and one for the hydraulic toplink, so the tractor used to operate it must have two double-acting remote hydraulic outlets.

Prior to the on-farm trials this past season, we added a double control valve so the bale unrollercould be operated by tractors with a single remote outlet.

Using round bales to mulch between rows of plastic is most appropriate for vining crops likewatermelon and winter squashes, because after the vines have run into the space between therows it is no longer possible to do mechanical cultivating with a tractor, but the mulch willcontinue to help suppress weeds. An added benefit of the mulch is that it keeps fruit off thebare soil, keeping them cleaner and reducing the chances for rot. There are additional benefitsrelated to disease and fruit cleanliness because of the reduction in the splashing of bare soil,


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and some market grower operations like to mulch all of their vegetable crops. WaterpennyFarm in Springfield, VA uses more than 300 round bales of mixed grass hay a year to mulch allof their 12 plus acres of vegetable crops grown on plastic (Waterpenny Farm, 2012).

There are considerations related to the baled material used for mulching. Straw is consideredto be effective for mulching, but it tends to be a lot more expensive than mixed grass hay. In

our experiences, fescue-based mixed grass hay tends to roll out in better matted layers thanother types of grasses, while freshly-baled wheat straw tends to not be very matted at all, sothat a third to half of the bale may fall off when the netting or strings are first cut off the bale.There seems to be better matting (and therefore better unrolling) with year-old bales of wheatstraw. Old or spoiled hay that is no longer good for animal feed can be a low-cost alternative formulching with hay (Stout, 1998). However, in our experiences, the large rotted spot at thebottom old round bales that have been stored on the ground outdoors can make them unrollpoorly, as large amounts of hay are left in clumps until the bale is rolled out past the depth of therot spot.

The possibility of introducing weed seed has been a concern with using hay mulch, and strawfrom harvested grain can introduce grain seed that can also be a weed problem (Stall, 2008;Relf and McDaniel), 2004). We conducted studies mulching watermelon plots with different hay

and straw treatments in 2009 and 2010 partly with this concern in mind, to investigate theeffects of different factors on weed control, including the type, quantity, and age of the mulchingmaterial used (Wilhoit and Coolong, 2010). Treatments included new mixed grass hay, newwheat straw, year-old mixed grass hay, and year-old wheat straw. The treatments were appliedin two thicknesses, one rolling the bales out once (approximately 4 in. thickness) and the otherrolling bales out twice (approximately 8 thickness). The treatments were applied to plots ofwatermelon seedlings growing on plastic-covered raised beds that had recently beenmechanically cultivated to leave the bare ground between the rows of plastic clean of weedsright before the vines starting running, a common practice used by growers (Schonbeck, 1996).Weed control effectiveness was assessed at the end of the growing season by collecting all ofthe aboveground weed material from fixed areas in the plots and oven-drying the material todetermine biomass dry matter. In our trials, all of the mulch treatments provided significantly

better weed control than the control (no mulchl) both years. The results for the new versus oldhay and straw were inconsistent. The double thickness of mulch gave significantly better weedcontrol, but the single thickness was still quite effective, giving an indication that a singlethickness of mulch is sufficient. The mixed grass hay in general had fewer weeds than thewheat straw, primarily because there was a lot of wheat grass that sprouted in the plotsmulched with wheat. The weed control effectiveness of the mulch treatments decreasedconsiderably through the season, but was considerably better than no mulch, as shown inFigure 2, and it seemed to be sufficiently effective for good watermelon production. Introducedweed seed did not seem to be a problem with the mixed grass hay that we used, but we did seesome indication of the potential for a weed problem from grain seed in the straw, as mentionedabove.

There is one particular hazard associated with mulching with hay that growers need to be awareof and avoid. Herbicides with the active ingredient amino pyralid (ex. Forefront, Milestone) aresometimes used on pastures for controlling broadleaf weeds. These herbicides are extremelypersistent, and hay from fields treated with these herbicides can kill vegetable crops if used formulch. Anyone considering using hay for mulching between rows of plastic needs to beabsolutely sure that the hay is from fields that have not been treated with these herbicides.


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Figure 2. Weed control in mulched plots (left side) versus unmulched control (right side).

Small Tractor Power Steering RetrofitThe Farmall 140 and other similar tractors have the tractor frame offset from the center so thatthe driver can easily look down directly over the crop row that the tractor is straddling whiledriving (Figure 3). This feature combined with belly-mounted cultivating tools made thesetractors very popular for cultivating row crops, and thousands of them were in use on tobaccofarms in Kentucky for many years. As tobacco production has decreased significantly in recentyears and market grower vegetable operations have increased, these older tractors havebecome popular as a low-cost cultivating option for some vegetable growers. While theconfiguration and excellent over-the-row visibility make these tractors well-suited for cultivation,they are difficult to use for the precision cultivation often required for vegetables because thesteering mechanisms have so much play in them.


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Figure 3. Farmall 140 Cultivating Tractor

There are two main applications for cultivating vegetables, cultivating between the rows of

plastic mulch, and cultivating rows of plants growing in bare ground systems (without plasticmulch). Cultivating between rows of plastic requires good precision because of the need tocultivate the soil right next to the side of the plastic bed without catching and ripping up theplastic. In bare ground vegetable production, there is no weed control from the plastic mulch, socultivation especially close to the plant is required to give the needed level of control. In order toimprove the effectiveness of cultivating vegetables with a Farmall 140, as well as to make iteasier on the driver, engineers and technicians in the Biosystems and Agricultural EngineeringDepartment at the University of Kentucky set out to retrofit an older tractor with power steering.

The retrofit adds a double-ended steering cylinder attached to the front axle with mountingsaddles (Figure 3). The steering cylinder is controlled by a steering motor that replaces theoriginal steering shaft. Initially, an automotive power steering pump was added to the front of

the engine to provide the hydraulic power for the system. It was difficult to fit an adequate beltdrive for the power steering motor in the space under the hood, however, so hydraulic powerwas instead obtained by tapping directly into the tractor hydraulic system. With the addition ofan external relief valve, this arrangement proved satisfactory for the power steering system(Figure 4).


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Figure 4. Steering motor and hydraulic lines for Farmall 140 power steering retrofit.

The power steering on the Farmall 140 has worked very well, greatly improving the operation ofthe tractor for precision cultivation of vegetables. The first Farmall 140 that was retrofitted forpower steering has been used primarily with belly-mounted cultivation tools for cultivating twinrows of vegetables grown in a bare ground system. The equipment for the belly mountedcultivators uses a special frame with three diamond toolbars (Figure 5). Four side knifecultivators are mounted on the middle toolbar, for cultivating between the twin rows. Mountedon the back toolbar are spring hoe weeders designed to oscillate vigorously just beneath thesurface of the soil, to disturb the soil and efficiently mulch and weed up close to the establishedplants in the vegetable rows. Because of the space requirements for the spring hoe weeders,this configuration could be used for double rows planted a minimum of 16 in. apart. Note thatonly one set of spring shoes is shown in Figure 4. The spring hoe weeders have a gap of onlyabout 1.5 in. for the vegetable plants to pass through, so the requirements for precise drivingare very stringent. The Farmall with the new power steering performs quite well for thiscultivation application. It can be driven fairly easily with sufficient precision at a good speed.


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Figure 5. Belly-mounted cultivators on the Farmall 140.

A second Farmall 140 has been retrofitted for power steering using the same system as the firstone. This tractor is configured for cultivating between rows of plastic mulch. The tractorstraddles the row and has belly-mounted knives and discs for weeding the edge of the plastic(Figure 6). An implement mounted on the three-point hitch with knives for cultivating themiddles of the space between the rows rides on gauge wheels that control the depth. The belly-mounted knives dig the soil covering the edges of the plastic to uproot weeds right up next tothe plastic row, and the angled discs throw soil back over the edges of the plastic to keep theplastic pulled tight over the raised bed. The weeds right next to the plastic are difficult to controlby any means, so being able to cultivate in this way is very beneficial for vegetable growers.The Farmall with power steering also performs well in this cultivation application.


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Figure 6. Farmall 140 set up for cultivating between rows of plastic.

GPS-based Automatic Guidance Systems for Plasticulture Applications

GPS-based automatic tractor guidance systems are being used to great advantage in large-scale grain operations, but the cost of the technology is hard to justify for smaller scalevegetable production. With plasticulture systems of vegetable porduction using plastic mulch,

there are important benefits to the level of precision possible with GPS-base automaticguidance systems, not only for cultivation as previously described, but also for planting andplastic lifting (part of the disposal process) operations. The Biosystems and AgriculturalEngineering Department at the University of Kentucky has extensive experience and researchcapabilities with GPS-automatic tractor guidance systems so, taking advantage of thosecapabilities, a study was initiated in 2011 exploring the applications of GPS-based automaticguidance systems in plasticulture production. Of particular interest in the study is whether thebenefits of autosteer could justify the costs in smaller scale vegetable production systemstypical of market grower and organic operations.

Field tests comparing the performance of two different autosteer systems, one a high end andthe other a smaller-scale economy system, were conducted in June of 2011 at the University ofKentucky Horticulture Research. The higher-end hydraulically-integrated steering system wasimplemented on a larger (90 hp) tractor pulling a high-end single-row plastic layer that can layplastic in raised beds up to 8 in high. The lower-cost mechanically-assisted steering system wasinstalled on a smaller tractor (40 hp) pulling a lower-cost single-row plastic layer that can layplastic in raised beds up to 4 in. high (Figure 7). Both systems used a local RTK (Real TimeKinematic) base station to provide differential correction to the guidance system. Tests wereconducted with both systems laying plastic on 7-ft centers both with and without the tractorautosteer engaged, in flat plots as well as following the contour on sloped plots. Plots were 250


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ft long, a distance typical for plasticulture vegetable production and long enough to give theautosteer systems a chance to converge to the desired paths. Each of the systems collectedGPS data from the tractor as well as from an additional receiver mounted on the implement at apoint directly above the center of the plastic-covered bed. Guidance was based on the tractor-mounted GPS antenna.

Figure 7. Economy version of GPS-based automatic guidance laying plastic on the contour.

Processing of the data for assessing the accuracy of the placement of the plastic-covered bedshas not been completed, so comparisons between the two systems and between manual andautomatic guidance have not been made yet. But various observations were made about theperformance of the systems during the field tests. Both systems appeared to be quite accuratemost of the time. There were occasional glitches with both systems when the guidance systemwould swerve at the beginning of row as it was adjusting to find the next row (AB line) to follow.Both systems did a good job of guiding the tractor in laying down a second row of plastic on thecontour (next to the first row which was driven manually), but it was obvious that implement driftwill be an issue in accurate guidance for subsequent operations such as cultivation or plasticlifting on the same rows. The mechanically-assisted steering system required more attentionfrom the tractor operator than the integrated system, as the motor turning the steering wheel

has to be manually engaged and disengaged. The ability of the tractor driver to look back at thebed forming and plastic laying operation was a distinct advantage of the use of automaticguidance systems. This ability would make it easier for vegetable growers to make the requiredadjustments for the proper functioning of the plastic layer (forming the raised bed, placing thedrip tape, stretching the plastic, and covering the edges of the plastic with soil to hold it down),and it could help them avoid problems. On one occasion during the tests, the tractor driverobserved the end of the drip tape coming off the spool and was able to stop the tractor within afew feet of the spot, allowing a new roll to be spliced on and avoiding considerable problemscaused when this happens unobserved during conventional plastic laying operations.


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Even though we have not yet analyzed results and have not yet done any formal testing lookingat the accuracy requirements for subsequent operations on the same rows of plastic inplasticulture systems, it has already become fairly obvious that the cost of GPS-basedautomatic guidance systems will be difficult to justify in smaller-scale vegetable productionsystems. This is because the potential benefits seem insignificant in terms of actual reductions

in labor requirements, which is the primary area that a grower can realize cost savings. Thehigher end autosteer system carries a price tag of approximately $25,000 including thehydraulically integrated steering system on the tractor, GPS equipment, and required software,etc (but not including the RTK base station). The economy system, at approximately $10,000,would be considerably cheaper, but both systems require a base station for differentialcorrection, an additional cost of approximately $15,000. In smaller-scale vegetable productionsystems, growers could realize significantly more reduction in labor requirements investing thatkind of money in additional tractors or implements than they could ever hope to realize fromGPS-based automatic guidance technology.

Another possibility considered was using GPS-based automatic guidance without an RTK basestation and instead utilizing a differential correction subscription that would lower the cost

significantly, but the accuracy with such systems (+/- 4 to 6) is not sufficient for precisioncultivation applications. On a stand-alone basis, the additional cost of GPS-based automaticguidance may be difficult to justify even with larger-scale organic production systems. But thereare definitely some benefits that can be realized with its use in precision cultivation andplasticulture applications. For larger-scale diversified farming operations that already have thetechnology for use in grain production, it could make good sense to use it to their advantage inorganic vegetable production as well.

Conclusions and Recommendations for Further Work

The use of herbicides is not allowed in organic vegetable production, so weed control is very

challenging. Mulching with round bales of hay and straw is a method used by some organicgrowers to control weeds. An offset bale unroller offers a simple concept for modifying astandard implement for unrolling hay bales into a new configuration that can straddle a row ofplastic and unroll the bale in the space between rows for mulching. It greatly reduces the laborrequirements for this practice. Precision cultivation is another important method used for weedcontrol in organic vegetable production. A power steering retrofit for an older, inexpensivecultivating tractor, the Farmall 140, makes precision cultivation considerably easier, both forbare ground and plasticulture cultivating applications. Because of the great weed control andmoisture conservation benefits, the practice of using of plastic mulch and trickle irrigation(plasticulture) is used extensively in both conventional and organic vegetable. A study lookingat GPS-based automatic guidance system applications in plasticulture vegetable production isgiving an indication of some of the potential benefits, but the high cost of the technology still

remains a significant challenge for smaller scale vegetable growers.

With the advances in various automation technologies, and the corresponding decreases in thecosts for the technology, there may be new opportunities for marrying older concepts ofmechanical guidance, such as guiding off a raised bed or crop row or following a furrow, withsensors for electro-mechanical control of cultivating tools, to give the kind of accuracy requiredfor precision cultivation at a lower cost. There may be possibilities for integrating such systemswith lower accuracy GPS technology to give the required accuracy at a lower cost. Finally, eventhough the costs of GPS-based automatic guidance are unlikely to be justified by labor savings


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in current smaller-scale vegetable production systems, there may be possibilities for actualautonomous tractor operation in the future which would save considerable labor. So, it will beimportant to conduct further investigations into applications of GPS-based and other automationtechnologies for smaller-scale vegetable production systems.

Acknowledgements

References

Law, Derek M., A. Brent Rowell, John C. Snyder, and Mark A. Williams. 2006. Weed ControlEfficacy of Organic Mulches in Two Organically Managed Bell Pepper ProductionSystems. HortTechnology. Vol. 16(2):225-232.

Shonbeck, Mark. 2009. An Organic Weed Control Toolbox. eOrganic article posted oneXtension website. http://www.extension.org/article/18532.

Shonbeck, Mark. 1996. Mulching for Weed Control in Annual Vegetable Crops. VirginiaAssociation for Biological Farming. Information Sheet No. 9.

Stout, Ruth. 1998. Gardening Without Work: For the Aging, the Busy, and the Indolent. The

Lyons Press; 1stedition. 224 p.Waterpenny Farm. 2012. Personal communication, Erik Plaksin, Waterpenny Farm.

Wilhoit, J., and T. Coolong. 2010. Weed Control Effectiveness of Hay and Straw Mulches.Between Plastic-covered Beds. In: 2010 Fruit and Vegetable Research Report. PR-

608. Ed. T. Coolong, J. Snyder, and C. Smigell. p. 47.

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