Using a Buckwheat Cover Crop for MaximumWeed Suppression after Early Vegetables

Thomas Bjorkman1,2,4 and Joseph W. Shail, Jr.1,3

ADDITIONAL INDEX WORDS.

SUMMARY. Establishment of a weed-suppressive cover crop after vegetables harvestedearly in the season is important in the northeastern United States because of theshort growing season. Buckwheat (Fagopyrum esculentum) is an effective cover cropin vegetable production because of its short growing season, ability to outcompetemany weeds, resistance to damage by insects and disease, and requirement for onlymoderate soil fertility. In two separate 3-year field experiments, we determined thebest tillage techniques and the optimal timing for use of buckwheat as a cover cropafter early vegetables in the northeastern United States. Incorporating crop residuewith a disk was necessary and provided sufficient tillage to obtain a weed-suppressive buckwheat stand. Buckwheat growth was stunted when direct seededwith a no-till drill immediately after pea (Pisum sativum) harvest because of poorsoil penetration by buckwheat roots. Planting buckwheat after incorporating thepea crop was successful; waiting 1 week to plant was optimal, whereas a 2-week waitproduced a weaker stand. We determined that optimal timing for sowing buckwheatin central New York was late June to early August. Generalizing to other geo-graphical regions in the United States, we calculated that a minimum accumulationof 700 growing degree days is necessary to reach 1 to 1.5 tons/acre of buckwheatdry matter at the appropriate growth stage for incorporation (6 weeks after sowing).

Many vegetable growers pre-fer to stagger harvest overthe growing season. In the

northeastern United States, however,the growing season is too short tofollow early-harvested cool-season cropswith a second vegetable crop, butlong enough that summer weeds canbe problematic in fallow fields. Covercropping can provide effective weedcontrol, but the practice must besimple to implement. In the north-eastern United States, there are fewoptions available fitting the require-ment for a rapid-growing summercover crop. The short growing sea-son, disease resistance, and low cost ofbuckwheat make it an ideal option for

weed control (Bjorkman et al., 2008;Clark, 2007; Magdoff and van Es,2000; Sarrantonio, 1994).

Vegetable growers consideringimplementation of buckwheat as partof their weed management and soilconservation practices are confrontedwith several questions. First, is no-tillage (NT) planting effective and ifnot, how much tillage is necessary?Second, how long after incorporatingthe crop residue must one wait to sowbuckwheat? Third, what are the early-and late-season limits for seedingbuckwheat? Fourth, what is the drymatter yield of the buckwheat covercrop? The current research addressesthese questions and offers produc-tion advice on the implementation ofbuckwheat as a cover crop option invegetable production.

Growers need inexpensive, simple,and effective methods for using covercrops for soil conservation and weed

management. Traditionally, growersare advised to wait 2 weeks beforeplanting a cover crop following vege-table harvest (Magdoff and van Es,2000). Bjorkman (2010) recommen-ded sowing buckwheat, to be har-vested for grain, in early July in thenortheastern United States. We soughtto determine whether the buckwheatcover crop could be planted less than 2weeks following pea harvest to testwhether a quicker planting of the covercrop might suppress weeds even better.We also examined different plantingtreatments to evaluate the extent towhich tillage might be reduced. Fi-nally, we varied the date of buck-wheat planting to determine whenduring the summer a successful standcan be established.

The incorporation of fresh or-ganic matter can result in the presenceof phytotoxins or increased microbialactivity detrimental to germinatingseeds and young seedlings (Cochranet al., 1977). The emergence of sweetcorn (Zea mays) and lambsquarters(Chenopodium album) are negativelyaffected by crimson clover (Trifoliumincarnatum) residue (Dyck andLiebman, 1994). Phytotoxins and mi-crobes and/or their by-products mayresult in seedling mortality or loss ofvigor. Therefore, planting a cover cropimmediately after incorporating freshcrop residue is not recommended, anda 2-week wait is the common rec-ommendation (Magdoff and van Es,2000). However, that recommenda-tion precludes many cover crops andseverely reduces the possibility ofdouble-cropping vegetables. It would,therefore, be valuable to identify al-ternative options to the conventionalrecommendations. The ultimate goalof reducing wait time is a better useof cover crops in vegetable systems,and better soil health by reducing thetime that soil is bare while growingconditions are good, particularly inwet temperate climates like the GreatLakes region and much of the Atlantic

UnitsTo convert U.S. to SI,multiply by U.S. unit SI unit

To convert SI to U.S.,multiply by

0.4047 acre(s) ha 2.47110.3048 ft m 3.28080.0929 ft2 m2 10.76392.54 inch(es) cm 0.39371.1209 lb/acre kg�ha–1 0.89222.2417 ton/acre Mg�ha–1 0.4461(�F – 32) O 1.8 �F �C (�C · 1.8) + 32

This research was funded by the USDA NortheastSustainable Agriculture Research and Education (NE-SARE) program.

The authors are indebted to Cheryl D. Galvani forskillful editorial assistance in preparing this manu-script. Appreciation is extended to Stephen Reinersand Ryan White, Cornell University, and CarolMacNeil, Cornell Cooperative Extension, for helpfuldiscussions and review of the manuscript; JoshuaScandrett, University of Tasmania, for assistance infield experiments; and Jessica Runnells of the NortheastRegional Climate Center, Cornell University, for theproduction of the buckwheat planting date map.

1Department of Horticulture, Cornell UniversityNew York State Agricultural Experiment Station,630 W. North Street, Geneva, NY 14456

2Associate Professor of Horticulture

3Research Support Specialist

4Corresponding author. E-mail: tnb1@cornell.edu.

• October 2013 23(5) 575

Coast. Our results indicate that the2-week wait can be reduced undercertain conditions and that 1 weekmay in fact be optimal.

Incorporated residue of succu-lent vegetables in the warm growingseason can decompose more quicklythan more mature plant tissue, orthan during the cooler months (Parrand Papendick, 1978). Vegetablecrop residue may be high in nitrogen(N) and low in phenolics immediatelyfollowing incorporation. Incorpo-rated pea residue, in particular, con-tributes to high soil carbon (C) and Nlevels as well as increased overall mi-crobial biomass and microbial enzy-matic activity (Fauci and Dick, 1994).In young pea residue, the C to N ratio(C/N) is less than 20 (Copas, 2010),which represents a N content thatpermits substantially higher decom-position rates than when C/N isgreater than 20 (Nicolardot et al.,2001). High-N residues decomposefaster than low-N residues (Bruunet al., 2006; Lupwayi et al., 2004;Nicolardot et al., 2001; Sarrantonio,2003). Legume residue with a C/Nless than 20 can provide net N min-eralization within 10–20 d, whereasthe equivalent amount of N in moremature residue (C/N = 25–50) cantake many months to be available (Bruunet al., 2006; Sarrantonio, 2003). Infact, soil microbial activity can peak inthe first week after incorporation ofgreen pea residue (Lupwayi et al.,2004). Furthermore, buckwheat is lessaffected by N tie-up than most othercover crops, and may grow even whenN availability is reduced by freshly in-corporated residue (Stone, 1906). Nev-ertheless, the buckwheat seed could besensitive to soilborne pathogens stimu-lated by the crop residue (Bjorkman,2001).

Materials and methodsFIELD HISTORY. Field composi-

tion is Honeoye silt loam, and fieldlocation is the Cornell UniversityNew York State Agricultural Experi-ment Station Vegetable Research Farmin Geneva, NY (lat. 42�52#N, long.77�02#W). The fields had a historyof moderate compaction and relativelydegraded aggregates. To simulate plant-ing the cover crop in a vegetable rota-tion, we grew peas, then flail mowed atharvest maturity with a tractor-mountedmower (Standard 12 Multicrop Shred-der; Loftness Manufacturing, Hector,

MN) to mimic soil compaction en-countered in commercial machineharvest. Peas were sown with a no-tilldrill (model 1560; John Deere, Mo-line, IL) at 100 lb/acre on 19 Apr.2005, 25 Apr. 2006, and 30 Apr.2007.

FIELD TREATMENTS FOR TILLAGE

AFTER VEGETABLES. Treatment plotsof 10 · 200 ft were assigned in acompletely randomized design withfour replications and the followingtreatments: fallow, no-tillage (NT),disk (D), chisel and disk (C), diskand wait 1 week (D1) or 2 weeks(D2) to plant the buckwheat. Buck-wheat planting dates were based ontime of pea maturation and incorpo-ration. The buckwheat planting datesfor NT, C, and D were 5 July 2005,13 July 2006, and 18 July 2007,respectively, whereas D1 and D2 weresown 1 and 2 weeks later, respectively.Buckwheat seed [Cover Crop Seed(variety not stated); The Birkett Mills,Penn Yan, NY] was sown at 55 lb/acrewith a no-till drill.

Establishment and growth ofbuckwheat cover were documentedby collecting stand counts at 1 and4 weeks after seeding and biomass at 4and 6 weeks after seeding. At 1 and4 weeks after seeding, stand count wasmeasured by counting the number ofseedlings in a 1-m2 plot placed ran-domly within the planting area. Buck-wheat groundcover was measured usingthe beaded-string method (Morrisonet al., 1995), with a 25-ft string havingtwo marks per foot strung diagonallyacross the width of the plot. At 4 and6 weeks after seeding, buckwheat bio-mass was determined by harvestingall aboveground biomass in 1-m2 plot,separating buckwheat from weeds,drying both for at least 72 h at 70 �Cin a forced-air oven, and weighingboth. The effectiveness of weed suppres-sion by buckwheat was determined bycomparing weed growth in buckwheatplots with weed growth in buckwheat-free subplots. One-square-meter sub-plots within each treatment weresprayed with glyphosate (RoundupWeatherMax�; Monsanto, St. Louis,MO) shortly after the buckwheat ger-minated (at 3–4 d) but before theweeds emerged (at 7–8 d). Weed dryweights in the 1-m2 subplots withineach treatment were determined.

Growing degree days (GDD50)were calculated based on daily temper-atures measured at a weather station at

the field site on the Cornell UniversityNew York State Agricultural Experi-ment Station Vegetable ResearchFarm, Geneva, NY. The planting datemap was generated by the NortheastRegional Climate Center (CornellUniversity, Ithaca, NY) using temper-ature data from 1981 to 2010 for allmonitoring stations in the region.The formula identified the latest plant-ing date in summer for which greaterthan 700 GDD50 would accumulatein the subsequent 6 weeks. The dailynormal GDD50 for 1981–2010 wassummed for 42 d beginning after thetermination of the growing season(31 Oct.), going backward in 1-d in-crements until GDD50 reached 700,or 1 June. 31 Oct. was selected be-cause it is known to be too late in theseason for sufficient heat units toaccumulate.

FIELD TREATMENTS FOR PLANTING

DATE TRIAL. Fields left fallow overwinter were plowed in the spring. In-dividual plots measured 10 · 200 ft.The plots were planted sequentiallywithin each of the four blocks, har-rowing the ground of each strip im-mediately before planting buckwheat.Each strip was planted with a 10-ftdrill, at 50 lb/acre. Plantings weredone about every 10 d, weather per-mitting, beginning as soon as theground could be worked (typicallymid-May) and ending just beforethe average first frost (25 Sept.), fora total of 7 to 10 plantings.

Statistical analyses were per-formed with JMP (version 9; SAS In-stitute, Cary, NC). The buckwheat andweed response to tillage were analyzedusing a General Linear Model withtreatments nested within year to esti-mate treatment means and to testdesigned contrasts. The planting dateresults were analyzed as a one-wayanalysis of variance (ANOVA) to de-termine the least significant differenceindependently for each year. The dataclosely fit a normal distribution, anddata were analyzed accordingly.

ResultsTILLAGE AFTER VEGETABLES. If

buckwheat was planted directly afterpea incorporation with a no-till drill,weed suppression was not satisfactoryrelative to the tilled treatments; but itwas superior to the fallow treatment,in which weeds quickly established(Fig. 1). Buckwheat emergence wassimilar in all treatments, although NT

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resulted in lighter green seedlings (Fig.1), forecasting their subsequent weakgrowth. Incorporating crop residuewith a disk provided sufficient tillage;chisel plowing to fracture the plowlayer was not necessary (Fig. 1). Whenbuckwheat was planted 2 weeks fol-lowing disking, germinated weed seed-lings were already large enough tocompete with the buckwheat seed-lings (Fig. 1). Weed competition wasless problematic when buckwheatwas planted 1 week following peaincorporation and disking.

In the NT treatments, there wasno loss of plants during vegetativegrowth (Table 1) because of self-thinning or root rot and no reductionin seedling emergence (Table 2). TheNT buckwheat produced only half asmuch dry matter at harvest as thetilled treatments (Table 3). The tilledtreatments resulted in � 95% weedcontrol, whereas, the NT treatmentgave only� 65% weed control (Table 4).The poorer NT performance was dueto slower buckwheat growth and notto a reduced stand.

The conditions needed for chiselplowing to make a difference (e.g.,ponding after rainfall) did not occur,therefore, this treatment had no effectin the present trial. Fracturing the panlayer by chisel plowing did not im-prove stand establishment (Table 1)nor seedling survival rate (Table 2),as it would have if poor water per-colation caused seed rot (Bjorkman,2001). Chisel plowing also had noeffect improving buckwheat drymatter production (Table 3) as shal-low compaction did not prevent ade-quate root development, and did notreduce weed populations (Table 4).We cannot exclude the possibility thatchisel plowing would offer improvedbenefit if rain events were sufficient tocause standing water or in heaviersoils.

POSTINCORPORATION INTERVAL.Delaying the buckwheat planting by1 week after incorporation of the peacrop improved stand (Table 1) andbiomass (Table 3) compared with im-mediate planting after incorporationof the pea crop. Increasing the intervalbetween pea incorporation and seed-ing buckwheat to the typical 2 weeksreduced buckwheat stand and growth.Weed control was not diminished byshortening the wait time.

PLANTING DATE. For optimal weedsuppression, buckwheat must quickly

provide ground coverage. We used70% soil coverage at 3 weeks afterseeding as the criterion for successbecause this level suppressed earlyweed growth in preliminary trials.In replicated trials from 2006 to2008, 60% to 70% groundcover at20 d was first achieved with plantings

established mid-June (Fig. 2) when400 GDD50 had accumulated sinceseeding (Fig. 3).

Plant vigor is dependent on warmsoil and air temperatures in addition toplant genetics. Vigorous early growth,as indicated by plant height at flower-ing, occurs with planting dates between

Fig. 1. Early establishment of buckwheat seedlings and weeds after peaincorporation following different treatments in 2005. Treatments are indicated oneach photo. Photos were taken 7 d after seeding buckwheat cover crop in therespective treatments. Fallow treatments planted 1 week (D1) or 2 weeks (D2) afterincorporation by disking looked similar to the fallow treatments plantedimmediately after disking (D) or chisel plowing and disking (C) at the same timepoint (data not shown). In the no-tillage treatment, established weeds continued togrow; buckwheat seedlings came up quickly but were noticeably paler than in thetilled treatments. In the treatment in which buckwheat was seeded 2 weeks afterdisking (D2), weed seedlings emerged before or at the same time as buckwheat.

Table 1. Optimum treatment in establishment of a buckwheat stand 1 week aftersowing was determined using test of contrasts, pooled over 3 years.

Incorporation Estimated stand(1000 plants/acre)y t-ratio PMethod Contrastz

Tillage NT vs. others 9.8 0.607 0.54Plow pan D vs. C 15.6 0.765 0.44Wait D vs D1 & D2 –61.3 –3.45 0.0007Std waitx D1 vs. D2 145.9 7.13 <0.0001zNT = no-tillage, D = disk, C = chisel and disk, D1 = disk and 1-week wait, D2 = disk and 2-week wait. Treatment inunderlined bold indicates the preferred treatment at statistical significance of P £ 0.05.yMean = 486,000 plants/acre; 1 plant/acre = 2.4711 plants/ha.xStandard (i.e., recommended) wait.

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late June to early August, when at least700 GDD50 are accumulated over 4 to6 weeks before flowering (Figs. 4 and 5).

Six weeks after planting, buck-wheat dry biomass can be expected toaverage 1.5 tons/acre (Fig. 6). Theoptimal time to terminate the buck-wheat cover crop is when vegetativegrowth is mostly complete and plantsare in full flower, but seeds are not yetviable. In buckwheat the optimal timeis typically 6 weeks after plantingduring a warm growing season. Buck-wheat yields were considerably higherin 2008 following a long-term (3 year)alfalfa (Medicago sativa) stand preced-ing pea planting (Fig. 6) than in 2006and 2007 following fallow. Conversely,buckwheat growth was inhibited byexcessive moisture at establishment,which occurred in early July 2006

and 2007 (Fig. 6). In each year, bio-mass accumulation increased linearlywith GDD50 beginning at 450 GDD50

(Fig. 7), achieving maximum biomassat 700 GDD50. The slope (biomassper degree day) was positively corre-lated to soil fertility, whereas droughtor flood produced less biomass. Buck-wheat requires at least 700 GDD50 in6 weeks after planting to producesufficient biomass of 1.5 tons/acre(Fig. 7) and justify its use as a covercrop. In New York State, buckwheatmust be planted by early August toachieve this minimum heat accumu-lation. Optimal buckwheat plantingdate for weed suppression was aboutearly June to early July in all 3 years(Fig. 8). Weeds grew very little underthe established buckwheat canopy, andwere sometimes completely absent.The summer weeds normally presentin the buckwheat-free subplots were

Table 2. Optimum treatment in buckwheat seedling survival at 4 weeks aftersowing was determined using test of contrasts, pooled over 3 years.

Incorporation Estimated stand(1000 plants/acre)y t-ratio PMethod Contrastz

Tillage NT vs. others 8.7 1.13 0.25Plow pan D vs. C 14.7 0.71 0.47Wait D vs. D1 & D2 –7.3 –0.40 0.68Std waitx D1 vs. D2 47.1 2.26 0.02zNT = no-tillage, D = disk, C = chisel and disk, D1 = disk and 1-week wait, D2 = disk and 2-week wait. Treatment inunderlined bold indicates the preferred treatment at statistical significance of P £ 0.05.yMean = 450,000 plants/acre; 1 plant/acre = 2.4711 plants/ha.xStandard (i.e., recommended) wait.

Table 3. Optimum treatment in buckwheat dry matter production at 6 weeksafter sowing, when buckwheat cover crop is typically ended, was determinedusing test of contrasts, pooled over 3 years.

Incorporation Estimated dry matter(tons/acre)y t-ratio PMethod Contrastz

Tillage NT vs. others –0.57 –14.67 <0.0001Plow pan D vs. C –0.03 –0.70 0.48Wait D vs. D1 & D2 –0.44 –10.40 <0.0001Std waitx D1 vs. D2 0.24 5.01 <0.0001zNT = no-tillage, D = disk, C = chisel and disk, D1 = disk and 1-week wait, D2 = disk and 2-week wait. Treatment(s)in underlined bold indicate the preferred treatment(s) at statistical significance of P £ 0.05.yMean = 0.84 ton/acre; 1 ton/acre = 2.2417 Mg�ha–1.xStandard (i.e., recommended) wait.

Table 4. Optimum treatment in weed biomass reduction in a buckwheat standrelative to that in a buckwheat-free subplot within the buckwheat plot wasdetermined using test of contrasts, pooled over 3 years.

Incorporation Estimated weed biomassreduction (%)y t-ratio PMethod Contrastz

Tillage NT vs. others –30 –8.11 <0.0001Plow pan D vs. C –4 –0.90 0.36Wait D vs. D1 & D2 –6 –1.63 0.10Std waitx D1 vs. D2 6 1.37 0.17zNT = no-tillage, D = disk, C = chisel and disk, D1 = disk and 1-week wait, D2 = disk and 2-week wait. Treatment(s)in underlined bold indicate the preferred treatment(s) at statistical significance of P £ 0.05.yMean weed biomass in buckwheat stand was 74% lower than in the buckwheat-free subplot control.xStandard (i.e., recommended) wait.

Fig. 2. Effect of planting date on earlybuckwheat groundcover. Groundcoverwas measured with the beaded-stringmethod 20 d after sowing. Rapid soilcover is an important mechanism ofweed suppression, and 60% to 70%early cover was sufficient to outcompetemost weeds. Planting dates between 15June and 20 Aug. resulted in sufficientgroundcover to suppress weeds.

Fig. 3. Accumulated heat units, orgrowing degree days (GDD50), asa predictor of buckwheatestablishment. Groundcover wasmeasured with the beaded-stringmethod. Degree days are a usefulmeasure if growth is temperature-limited at similar growth stages.

Fig. 4. Effect of planting date onbuckwheat plant height at flowering.Plant height at flowering is a measureof growth close to the time when thecover crop should be ended. Floweringoccurred 27 to 40 d after planting,depending on the temperature; 1 inch =2.54 cm.

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powell amaranth (Amaranthus powellii)and giant foxtail (Setaria faberi).

DiscussionTillage is necessary for optimal

weed suppression when buckwheat isused as a cover crop following early-season vegetables. Because of thefine root structure of buckwheat, no-tillage seeding into a pea crop residueis not sufficient, because the soil istoo hard for buckwheat establishment.Disking provides tillage for establish-ment of buckwheat and subsequentweed control. Plowing is excessivetillage under normal conditions andcan be omitted to preserve soil con-dition and reduce cost.

Traditionally, waiting 2 weeksfollowing early vegetable incorpora-tion before planting a cover crop isrecommended (Magdoff and van Es,2000). However, in that short time,perennial weeds become establishedand cannot be suppressed by a sub-sequent cover crop. There is a trade-off between allowing weeds to growand waiting for inhibitors from thecrop residue to subside. With amplerain before buckwheat emerges, weedsgrow sooner, and the balance shifts tofavor earlier planting of a cover crop. Itcould be argued to cultivate weedsbefore sowing buckwheat, but vegeta-ble production already includes moretillage steps than is good for soilhealth, so the best scenario would beto minimize tillage and therefore re-duce the cost of establishing a covercrop. We tested the most conservativetillage for the buckwheat cover crop,but growers may choose additionalcultivation for weed management ifweather prevents timely planting orweed pressure is extreme. An addi-tional benefit to a shorter wait timefor sowing the cover crop would be tomaximize the growing time for the fallcrop, especially in regions such as thenortheastern United States with shortgrowing seasons (125–175 d). In thehope of realizing the numerous bene-fits of decreasing the 2-week period,we sought to determine whether thatwait could be shortened. Our resultsindicate that a 1-week wait followingincorporation of early vegetables issufficient and superior to a 2-weekwait. Delaying the buckwheat plant-ing by 1 week after incorporationof the pea crop yielded improvedbuckwheat stands and biomass com-pared with planting immediately after

incorporation of the pea crop orwaiting 2 weeks postincorporation.In addition, shortening the postin-corporation time did not affect weedcontrol in buckwheat.

Buckwheat suppresses weeds andproduces a useful amount of biomassonly if the temperature is warm enoughfor it to grow rapidly. Although therewas considerable variation among theyears examined (2006–08), a consistentpattern emerged. When the weatherwas cool in the spring, early buckwheatgrowth was too slow to outcompeteweeds, but subsequent growth pro-duced abundant biomass. In themiddle of summer, when the heataccumulation was >700 GDD50, bothweed suppression and growth wereexcellent. When planted later in theseason, buckwheat seedling establish-ment was strong, but growth ceasedbefore much biomass could accumu-late. This pattern was used to generatea map for the northeastern UnitedStates showing the estimated last plant-ing date for a buckwheat cover crop(Fig. 9). The dates vary over 3 months(1 June to 1 Sept.) across the region,indicating that planting-date recom-mendations must be location-specific.

ConclusionsBuckwheat is a successful cover

crop for weed suppression after earlypeas. Growers should be aware thattillage is required when using buck-wheat for weed control, and thatdisking provides sufficient tillage. Ourresults indicate that a 1-week waitfollowing residue incorporation ofearly vegetables is sufficient, and is infact superior to a 2-week wait. Planting

Fig. 5. Effect of accumulated heatunits, or growing degree days(GDD50), on buckwheat plant heightat flowering. The lines represent theregression line for each year; 1 inch =2.54 cm.

Fig. 6. Effect of planting date onbuckwheat biomass at the time ofincorporation (6 weeks after sowing).Biomass was determined by harvestingall aboveground buckwheat in 1-m2

(10.8 ft2) plot, drying, and measuringdry weight. The least significantdifferences for the sequentialbuckwheat biomass 6 weeks afterplanting are: 0.23 ton/acre (2006),0.04 ton/acre (2007), and 0.13 ton/acre (2008); 1 ton/acre = 2.2417Mg�haL1.

Fig. 7. Effect of accumulated heatunits, or growing degree days(GDD50), on buckwheat biomass at thetime of incorporation (6 weeks aftersowing). Biomass was determined byharvesting all aboveground buckwheatin 1-m2 (10.8 ft2) plot, drying, andmeasuring dry weight; 1 ton/acre =2.2417 Mg�haL1.

Fig. 8. Effect of buckwheat plantingdate on weed biomass in buckwheatstand at 6 weeks after planting. Weedbiomass was determined by measuringweed dry weight at 6 weeks, the timea buckwheat cover crop wouldnormally be ended; 1 ton/acre =2.2417 Mg�haL1.

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buckwheat 1 week after incorporationof pea crop residue improved buck-wheat stands and biomass in compar-ison with planting immediately orwaiting 2 weeks following incorpora-tion. The potential for rapid growthis an advantageous characteristic ofbuckwheat in the northeastern UnitedStates, yet temperatures must be highenough for buckwheat to thrive andproduce an adequate amount of bio-mass to suppress weeds. A heat accu-mulation of ‡700 GDD50 yieldedexcellent buckwheat growth and sub-sequent weed suppression, and there-fore can be used as a guide whenplanting buckwheat as a cover crop.In the northeastern United States,optimal buckwheat planting datesfor weed management typically rangefrom about early June to early July.

Literature citedBjorkman, T. 2001. Causes of poor standestablishment after heavy rains, p. 134–137. In: S.S. Ham, Y.S. Choi, N.S. Kim,and C.H. Park (eds.). Advances in

buckwheat research. Proc. Eighth Intl.Symp. on Buckwheat. International Buck-wheat Research Assn., Chunchon, Korea.

Bjorkman, T. 2010. Buckwheat production:Planting. 7 July 2010. Cornell AgronomyFact Sheet Series No. 50. <http://nmsp.cals.cornell.edu/publications/factsheets/factsheet50.pdf>.

Bjorkman, T., R.R. Bellinder, R.R. Hahn,and J.W. Shail. 2008. Buckwheat covercrop handbook. A precise tool for weedmanagement on northeastern farms. NewYork State Agr. Expt. Sta. Bul., CornellUniversity, Geneva, NY.

Bruun, S., J. Luxhøi, J. Magid, A.de Neergaard, and L.S. Jensen. 2006. Anitrogen mineralization model based onrelationships for gross mineralization andimmobilization. Soil Biol. Biochem.38:2712–2721.

Clark, A. (ed.). 2007. Managing covercrops profitably. 3rd ed. Sustainable Ag-riculture Network, Beltsville, MD.

Cochran, V.L., L.F. Elliott, and R.I.Papendick. 1977. The production of phy-totoxins from surface crop residues. SoilSci. Soc. Amer. J. 41:903–908.

Copas, M.E. 2010. Evaluation of covercropping strategies designed for imple-mentation into the intensively managedvegetable crop system of central Wisconsin.PhD Thesis. University of Wisconsin—Madison.

Dyck, E. and M. Liebman. 1994. Soilfertility management as a factor in weedcontrol: The effect of crimson cloverresidue, synthetic nitrogen fertilizer, andtheir interaction on emergence and earlygrowth of lambsquarters and sweet corn.Plant Soil 167:227–237.

Fauci, M. and R.P. Dick. 1994. Soilmicrobial dynamics: Short-and long-termeffects of inorganic and organic nitrogen.Soil Sci. Soc. Amer. J. 58:801–806.

Lupwayi, N.Z., G.W. Clayton, J.T.O’Donovan, K.N. Harker, T.K. Turkington,and W.A. Rice. 2004. Soil microbiologicalproperties during decomposition of cropresidues under conventional and zerotillage. Can. J. Soil Sci. 84:411–419.

Magdoff, F. and H. van Es. 2000. Build-ing soils for better crops. 2nd ed. Sustain-able Agriculture Network Publications,Burlington, VT.

Morrison, J.E., Jr., J. Lemunyon, andH.C. Bugusch. Jr. 1995. Sources of var-iation of nine devices when measuringpercent residue ground cover. Trans.Amer. Soc. Agr. Eng. 38:521–529.

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Parr, J.F. and R.I. Papendick. 1978. Fac-tors affecting the decomposition of cropresidues by microorganisms, p. 101–129.In: Oschwald, W.R. (ed.). Crop residuemanagement systems. ASA Spec. Publ.31. American Society of Agronomy, Mad-ison, WI.

Sarrantonio, M. 1994. Northeast cover crophandbook. Rodale Institute, Emmaus, PA.

Sarrantonio, M. 2003. Soil response tosurface-applied residues of varying carbon-nitrogen ratios. Biol. Fertil. Soils 37:175–183.

Stone, J.L. 1906. Buckwheat. CornellUniv. Agr. Expt. Sta. Bul. 238.

Fig. 9. Predicted last planting date for a buckwheat cover crop in the northeasternUnited States, assuming a minimum of 700 growing degree days in 6 weeks fora successful stand. The analysis was performed by the Northeast Regional ClimateCenter, Cornell University, Ithaca, NY. The map was generated from climate datacollected from 1981 to 2010. Model estimates at the extreme dates may beinaccurate if other conditions limit buckwheat growth.

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