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Agroforest Syst (2007) 71:185–193DOI 10.1007/s10457-007-9071-8 A review of suitable companion crops for black walnut Robert Scott Æ William C. Sullivan Received: 9 May 2006 / Accepted: 16 May 2007 / Published online: 21 June 2007 ! Springer Science+Business Media B.V. 2007 Black walnut (Juglans nigra L.) is a temperate tree grown for nuts and wood, but it isallelopathic to certain plants and animals. We com- Black walnut (Juglans nigra L.) has a mixed piled reports of valuable black walnut companion reputation. On the one hand, it is a valuable tree crops which may be grown in the short term crop with beautiful heartwood and delicious nuts. On (<15 years after planting walnut trees), medium term the other hand, it is known to produce allelopathic (15–30 years), and long term (>30 years). There are chemicals that inhibit the growth of other plants many black walnut companion cropping systems for (Willis ). There have been dozens of studies in the short and medium term, but there are few for the recent years on black walnut multi-cropping systems, long term. Companion crops for black walnut serve but the dominant narrative has remained roughly the multiple functions, including nitrogen fixation, added same: black walnut is a long term tree crop, yields, the development of straight walnut stems, and allelopathic in general, which at best can be expected added protection from forest pests such as deer. Black to permit the co-existence of one additional farm walnut polyculture is a viable alternative worthy of yield chosen from a small pool of tolerant species.
further development and implementation.
This paper aims to shift that narrative, to one ofmultiple potential yields, which can recover stand Allelopathy ! Juglans nigra ! Juglone ! establishment costs and add revenue streams up to the Permaculture ! Polyculture day of the timber harvest.
Black walnut is bred for various products (Reid et al. The maroon-black heartwood is the mostvaluable black walnut yield, which is made intofurniture, veneer, and gunstocks. This lucrative commodity was once raised as a ‘‘retirement crop'': Department of Educational Policy Studies, University of farmers would sell an old black walnut stand to fund Illinois at Urbana - Champaign, 360 Education Bldg., their retirement. The nuts of the tree are edible, and 1610 S. 6th St., Champaign, IL 61821, USA black walnut has been described as ‘‘the most e-mail: rscott2@uiuc.edu dependable nut plant for Illinois'' (Meador et al.
). Nut crops can be produced by young trees, but Department of Natural Resources and Environmental yields are influenced by site, tree variety, and Sciences, University of Illinois, 1101 W. Peabody, Room management practices. The market for black walnuts 350, MC-635, Urbana, IL 61801, USAe-mail: wcsulliv@uiuc.edu is not as well developed as that for carpathian walnuts Agroforest Syst (2007) 71:185–193 (J. regia L.). Black walnut sap may be processed into that there is a veritable ‘‘guild'' of species which will a sweet syrup comparable to maple syrup (Matta et al.
live with the walnut forest in all stages of its ), but the sap is more dilute in black walnuts.
development. In this paper, we briefly introduce the Black walnut syrup is novel, but trivial in economic buildup of juglone during the establishment of black terms. The most valuable commercial yield of the walnut plantings, and describe reports of added yields black walnut, its heartwood, is not ready for harvest of food and wood, nitrogen fixation, and tree until decades after stand establishment. If black protection which may be attained in the short and walnut production is going to produce value in the medium term. In doing so, we show that a black short term, additional crop species are required, walnut polyculture is not only possible, but econom- especially species that can produce economic value ically viable as well.
during the first 20 years after establishment of theblack walnut stand. The major barrier to black walnutpolyculture is its reputation as an allelopath.
Spatial and temporal buildup of juglone Black walnut and other members of the family Juglandaceae have long been known to produce an Several factors influence the accumulation of juglone allelopathic compound known as ‘‘juglone'' which in a growing area, including rhizosphere interactions, inhibits the growth of several other species of plant uptake by soil organisms, adsorption of juglone to (Davis ; Willis ). Roots, leaves, and hulls soil particles, and movement in the soil solution.
contain large quantities of harmless hydrojuglone, Living black walnut trees exude juglone into the which may be oxidized to juglone (Segura-Aguilar rhizosphere (Bertin et al. which likely explains et al. ), then transferred to neighboring plants reports that allelopathy can continue after walnut tree primarily as a root exudate in the rhizosphere of the removal, especially if the stumps are not killed black walnut trees (Bertin et al. In susceptible (Piedrahita ). A species of Pseudomonas has plants, juglone interferes with respiration and photo- been identified that can metabolize juglone (Schmidt synthesis (Hejl et al. ). Affected plants turn ) potentially providing a juglone sink, but it is brown, wilt, and die. Most vegetable crops are unlikely that these bacteria prevent plant exposure to vulnerable to juglone toxicity (Crist and Sherf the juglone. Juglone in solution may also be adsorbed ), as are some fruit tree species and all members to soil particles or organic matter (Willis Soil of the plant family Ericaceae (Brooks ). In the moisture regime seems to augment the effects of animal kingdom black walnut hulls are responsible juglone exposure. A study with white pine (Pinus for triggering lamnitis, an often fatal hoof condition strobus L.) suggests that poorly drained soils may in horses (Thomsen et al. which may harm fish exacerbate the effects of juglone toxicity: pines (Radix et al. ) and repel ants and flies (Walker growing near black walnut on a well drained soil ). Common midwest crops which are susceptible grew tall, while pines growing near black walnut on a to the presence of black walnut include corn and poorly drained soil died (Fisher ). Though soybeans (Jose and Gillespie Hejl and Koster juglone accumulation is dynamic, any area planted ), wheat and alfalfa (Bertin et al. with black walnuts may potentially show symptoms What types of crops can be raised in black walnut of allelopathy.
plantations in the midwestern United States? Lists of How many years after planting black walnut do ‘‘compatible'' and ‘‘incompatible'' species are avail- toxic juglone effects appear? Various estimates able in extension publications (Crist and Sherf ; include 12–25 years (Rietveld ; Rink Funt and Martin Baughman and Vogt 15–20 years (Dawson et al. ); or 12–15 years but they often contradict each other (Gordon ; (Beineke ). A high-intensity planting of black Anonymous ), and are not clearly supported by walnut (5.5 m · 5.5 m) interplanted with juglone- scientific literature. Still, this is the most wide- sensitive Alder trees (Alnus glutinosa [L.] Gaertn.) ranging information available on the subject, and it resulted in toxic effects linked to juglone after only has been generated by experienced and professional 5 years (Bohanek and Groninger Plantations growers. Our detailed assessment of extension liter- designed for nut harvesting and other agroforestry ature and of the published scientific literature shows practices such as pasture may be spaced closely Agroforest Syst (2007) 71:185–193 within rows and widely between rows. For instance, aboveground competition for light, probably juglone 60 · 300 spacing gives 242 trees per acre while (Jose and Gillespie The concentration of creating a space between rows in which juglone juglone in the soil, varies as a function of the accumulates less quickly (Baughman and Vogt distance from the rows. In one study, juglone ). Given the caveat that different spacings will decreased in concentration by 80% at a distance of produce different rates of juglone accumulation in the 4.25 m from a row of 10 year old black walnut trees area between the walnut trees, we have designated (Jose and Gillespie When juglone was added three periods of juglone accumulation at 15 year to hydroponically grown corn and soy in the same intervals (see Table concentration found in the alley cropping study, therewere significant inhibitory effects on plant growth(Jose and Gillespie Later work isolated the Short term: multicropping during stand mechanisms by which juglone disrupts corn and soy growth (Hejl and Koster Annual crops may beviable between rows of black walnut for up to 5– Immediately after black walnut seedlings are planted, 10 years, but probably not longer.
the area between the rows may be planted with An interplanting of trees may help produce straight juglone-sensitive crops, but after 10–15 years the black walnut trunks while adding a short to medium juglone and shade will put constraints on what may term yield of non-walnut timber. European Black be grown. In the first few years of growth, Black Alder (Alnus glutinosa L.), was planted simulta- Walnut will grow a deep taproot and put on 12@–18@ neously with Black Walnut as a nitrogen fixing nurse of top growth (30–40 under intensive culture) annually crop with an added yield of harvestable pole wood, in (Rink ). Full sun is available for roughly 5– an Illinois-based study (Bohanek and Groninger 7 years, and it would not be unlikely to see juglone ). The Alder trees would be completely killed affecting sensitive species by the tenth year. Our by the level of juglone released by a 20-year-old research suggests that it is prudent to treat the 10– Black Walnut, so wood harvests must commence 15 year old black walnut stand as a transition period between 5 years and 15 years after establishment. The to a more shady and allelopathic medium term.
authors claim that the alder sale could pay for the cost Studies on short term companion plants for black of establishing the Black Walnut stand if the alders walnut have therefore included juglone-tolerant trees are harvested in well-timed thinnings between 4 years and livestock, pasture and forage, and all sorts of and 19 years after stand establishment (Bohanek and annual produce for the period just after the trees are There is evidence that short term planting of Typical midwest annual crops may be planted livestock forage or pasture in black walnut planta- between widely spaced rows of black walnut seed- tions do not restrict livestock growth, and that lings. Good walnut sites may also be good soybean livestock manures may benefit the adolescent walnut and wheat sites, though shade has been reported to trees. A pine-walnut plantation with annual ryegrass affect these crops after 7 years, even with the rows (Lolium multiflorum Lam.) and cereal rye (Secale spaced 40 feet apart (Burde ). Corn has been cereale L.) between the rows as heifer pasture, raised between rows of black walnut, but it has also produced approximately 80% of the forage produced been demonstrated that after 10 years corn produc- in a control plot without trees, but beef heifer average tivity is reduced by something other than the daily weight gain was equal for both treatments Table 1 Three phases of black walnut stand development Wood, nuts, sap, shade Agroforest Syst (2007) 71:185–193 (Kallenbach et al. This suggests that beef candidates for interplanting with black walnut in the producers likely would not sacrifice livestock pro- first few years, and a clever application of this fact duction when they graze animals under 6–7 year old could set the stage for a second phase of yields during pine and walnut trees compared to open pasture.
the medium term.
After chicken manure was applied as a fertilizer to ayoung (3-year-old) Black Walnut plantation in Mis-souri, total height and leaf nitrogen concentrationsincreased during the summer, suggesting that poultry Medium term: farming amongst mature trees manure is valuable as a fertilizer during establish-ment of young walnut plantations (Ponder et al.
There are several medium term crops that can provide ). In addition to being a suitable environment for valuable functions such as nitrogen fixation, and add poultry, a modular or free-range chicken pasture yields such as wood 15–30 years after stand estab- might also benefit young black walnut trees.
lishment (see Table Species living in the black Young black walnut plantations have been used to walnut stand during this period must be tolerant of grow forage for farm animals to eat off site, and juglone and shade. Even with widely spaced rows, shade (not juglone) seems to be the main issue shade is considerable after 15 years. The woody limiting productivity. Fescue (Festuca arundinacea species listed below might best be planted simulta- Schreb.) grown in 7–8 year old black walnut stands neously with the black walnuts, so that they grow up produced significantly more forage in medium den- with the black walnut and develop straight stems, sity stands (7.3 m between rows) versus low or high while receiving enough sunlight to produce yields density stands (14.6 m and 1.8 m between rows, such as fruit or bolewood. Animal species such as respectively) (Buergler et al. ). A comparison of heifers and hens benefit from partial shade available overall forage yields in fescue fields planted with in the adolescent walnut stand. Most medium term black walnut and honey locust (Gleditisia triacanthos species could also be raised in the short term, but the L.) showed that forage production decreased more available reports focus on older stands.
dramatically under high density Honey Locust than Several soft fruit species are compatible with black under high density Black Walnut (Buergler et al.
walnut, and can make good use of the transitional medium term phase of the walnut stand. A 1951 There are a handful of vegetable crops that are paper summarized 12 years of surveying plant species reportedly tolerant of juglone, including onions found growing within and around black walnut trees (Allium cepa L.) (MacDaniels and Pinnow in five US states (Brooks ). This is the most parsnips (Pastinaca sativa L.) (MacDaniels 1974), methodical survey of naturally occurring black wal- Jerusalem artichokes (Helianthus tuberosum L.) nut compatible plants to date. Brooks reported that (Ross ), sugar beet (Beta vulgaris L.) (Piedrahita black raspberry (Rubus occidentalis L.) grew under ) and certain species of bean (Phaseolus spp.) black walnut canopy whereas blackberry (Rubus (MacDaniels and Pinnow The yields of these fruticosus L.) only grew outside the crown area; crops might not be harmed by juglone, but the currants (Ribes spp.), elderberry (Sambucus canad- increasingly shady conditions cast by the canopy will ensis L.), and wild grapes (Vitis spp.) were common make these annuals less viable in the medium term.
under black walnut canopy, whereas blueberry (Vac- Managing a black walnut stand in the short term, cinium spp.) and other ericaceous plants were never there are at least a few years when the juglone effect observed near black walnut (Brooks Mulberry is negligible, before soil juglone concentrations (Morus spp.) and pawpaw (Asimina triloba [L.] increase while ground-level sunlight availability Dunal) trees were also found to be tolerant of black decreases. Sun-loving juglone-intolerant crops may walnut whereas apples (Malus spp.) and pears (Pyrus be leveraged so long as they are not required to return spp.) were not (Schneiderhan ; Brooks top yields into the medium term. For this reason, One grower has claimed multiple sightings of Amer- annual crops may be the smartest way to leverage ican Persimmons (Diospyros virginiana L.) growing some quick revenue from the young black walnut and producing fruit under black walnut canopy stand. All temperate climate plants are potential Agroforest Syst (2007) 71:185–193 Table 2 Short term, medium term, and long term yields which are possible in the presence of black walnut trees Short term: <15 years Juglone-intolerant field crops Grain, produce, forage Burde Jose and Gillespie Alfalfa (Medicago sativa) Cereal rye (Secale cereale) Kallenbach et al. Fescue (Festuca arundinacea) Buergler et al.
Ryegrass (Lolium multiflorum) Kallenbach et al. Jerusalem artichokes (Helianthus tuberosum) Lima bean (Phaseolus lunatus) MacDaniels and Pinnow Onion (Allium cepa) MacDaniels and Pinnow Parsnips (Pastinaca sativa) MacDaniels and Pinnow Sugar beet (Beta vulgaris) Piedrahita .
Wax bean (Phaseolus vulgaris) MacDaniels and Pinnow Cattle (Bos taurus L.) Kallenbach et al. Chicken (Gallus gallus) Meat, eggs, manure Medium Term: 15–30 years Fescue (Festuca arundinacea) Funt and Martin .
Kentucky Bluegrass (Poa pratensis) Orton and Jenny Brooks Piedrahita ; Funt and Martin .
Red Clover (Trifolium pratense) Timothy (Phleum pratense) MacDaniels and Pinnow Boes .
White clover (Trifolium repens) Piedrahita ; Boes Black Raspberry (Rubus occidentalis) Brooks ; MacDaniels and Pinnow Piedrahita ; De Scisciolo et al. Fuchs .
Currant (Ribes spp.) Brooks ; Anonymous Elderberry (Sambucus canadensis) Brooks ; Anonymous Mulberry (Morus spp.) Brooks ; Mollison .
Pawpaw (Asimina triloba) Brooks ; Anonymous Persimmon (Diospyros virginiana) Autumn olive (Elaeagnus umbellata) Black locust (Robinia pseudoacacia) European alder (Alnus glutinosa) Bohanek and Groninger Russian olive (Elaeagnus angustifolia) Black walnut (Juglans nigra) Eastern white pine (Pinus strobus) Red oak (Quercus rubra) Sugar maple (Acer saccharum) White ash (Fraxinus americana) Long term: >30 years Bamboo (Phyllostachys spp.) Wood, edible shoots (author's observation) Black walnut (Juglans nigra) Ginseng (Panax quinquefolium) Apsley ; Carroll Agroforest Syst (2007) 71:185–193 There are several nurse crops recommended for (Brooks Boes ), though alfalfa (Medica- black walnut growers which are planted at time of go sativa L.) is not (MacDaniels and Pinnow stand establishment, then harvested or killed in the It may benefit black walnut trees to grow Kentucky medium term. Autumn olive (Elaeagnus umbellata blue grass and clover in the earlier part of the Thunb.), Russian olive (Elaeagnus angustifolia L.), medium term, before the shade becomes too dense European alder (Alnus glutinosa (L.) Gaertn.), and to permit profitable growth.
black locust (Robinia pseudoacacia L.) are nitrogenfixers, and eastern white pine (Pinus strobus L.), redoak (Quercus rubra L.), white ash (Fraxinus amer-icana L.), and sugar maple (Acer saccharum Marsh.) are grown for their wood (Burde ). An Illinoisstudy found that autumn olive and black locust both Reports of valuable companion crops for mature improve black walnut growth, which is attributed to walnut stands are scarce. A walnut plantation is shady their role as nitrogen fixers (Ponder et al. and allelopathic after 30 years. A well-developed There are also reports that autumn olive may reduce stand of walnut trees will have long straight trunks leaf spot and anthracnose (Kessler ) because and a closed canopy 20–30 feet above the ground.
associated microarthropods consume the leaf litter Viable understory flora must tolerate shade and carrying the disease (Kessler Autumn olive is juglone. The late leafing-out date of black walnut, shorter than black walnut; it prevents deer from combined with the early leaf-drop that may be damaging walnut trunks without over topping and triggered by anthracnose diseases, can create sunny competing for sunlight. Black locust quickly grows periods in spring and fall (Woeste and Beineke tall, potentially competing with black walnut trees via which suggests that spring ephemerals and late- shading. Black locust can be cropped for bolewood ripening fruits might play a useful role in the long and the resultant nitrogen release in the root zone term. Other productive species might find the summer could benefit adolescent Walnut trees, whereas shade useful, such as mushrooms or livestock, though autumn olive would die off due to shade in the the scarcity of literature suggest that most growers medium term and release N without providing a wood are going to be prepared to sell the wood once the crop. The nurse crops grown for wood help develop black walnut stand has reached maturity.
straight walnut stems, and in the case of European There is scarce scientific literature on the subject alder, can over-top the walnut trees before dying of of full-shade cropping systems under black walnut.
juglone toxicity (Rietveld et al. ). A similar story The presence of black walnut has been called ‘‘a good is apparently the case for the fast-growing, but indicator.of productive sites'' for growing American juglone-intolerant eastern white pine (Fisher ) ginseng (Panax quinquefolium L.) (Apsley ; and sugar maple (De Scisciolo et al. ).
Carroll Shiitake (Lentinula edodes Berk.) and Growing forage for livestock may be one of the Oyster (Pleurotus spp.) mushroom operations require most profitable uses of walnut plantation land in the shady environments such as the mature walnut grove medium term. It is common in extension bulletins can provide, but there are no specific reports of to report that grasses such as Kentucky blue grass culinary mushroom interactions with juglone. Forest (Poa pratensis L.) and tall fescue (Festuca arun- yields such as mushroom logs and pawpaw (Asimina dinacea Schreb.) grow well under black walnut triloba) may require shade to stay moist in the (Funt and Martin ). A comparison of the summer, but pawpaw is more productive when heights of black walnut trees in the lower midwest mature trees receive full sun. Similarly, temperate found that 26 year-old black walnut trees growing bamboo (Phyllostachys sp.) has been observed with Kentucky blue grass had a site index 5 m growing under the high crown of mature black greater than trees growing within tall fescue fields walnuts in Urbana, IL, though it seems to grow tallest (Ares and Brauer ). There is also potential for in the sunny edges. Phyllostachys can provide trellis leguminous forage production: red clover (Trifolium material, edible shoots, and even a ‘‘wood'' crop in pratense L.) and white clover (Trifolium repens L.) sunny positions, but its slow growth and sun appear to be compatible with black walnut trees requirements may disqualify it from long term walnut Agroforest Syst (2007) 71:185–193 polyculture. There is no mention of bamboo in the to be the least inhibitory of six species of wood chips black walnut literature.
tested for allelochemical response (Rathinasabapathi Livestock operations may make use of the walnut stand as a source of shade for their animals, but there Black walnut is not universally detrimental to is no known benefit to the walnut trees. The other cropping systems. Soil type and moisture application of poultry litter to a 35 year old stand regime have been implicated in differential juglone of black walnuts in Missouri failed to produce toxicity levels detected in soils (Fisher Zitzer significant differences in nut yield or diameter growth and Dawson For some species black walnut (Ponder et al. Furthermore, the livestock will may function as a friend rather than foe due to require food that can only be grown in sunny reduced competition with susceptible species (Brooks locations. The shortage of literature on long-term ). Earthworms may be scarce under black walnut black walnut polyculture suggests that the value of a (MacDaniels and Pinnow ) but arthropods may mature black walnut stand is enough for most be more abundant and diverse (Summers and Lus- growers without additional yields.
senhop ). Elevated soil pH (Brooks ) andsoil organic matter levels under black walnut havebeen attributed to the retardation of decay organisms (Summers and Lussenhop It could be thatsome plants take up juglone from the soil (Thijs et al.
A forester with the singular objective of producing ) and could thus function as ‘‘buffer'' species for black walnut wood as fast as possible will find that intensively planting trees in protective plastic tubes, This assessment suggests the need to de-stigmatize applying herbicide to understory vegetation, and juglone and recast it as a resource in need of applying chemical fertilizers in the rows during stand management. Just like shade, low pH soil, or a dry establishment is the quickest route (Bendfeldt et al.
moisture regime, juglone will favor certain species ; Ponder and Jones Cutter et al. ). But and disadvantage others. The prospect of growing the highest intensity route also carries the highest black walnut polycultures in the short and medium input cost, by ruling out opportunities for multi- term, merits further research. The logic of short-term cropping or providing comparable functions with gains is a major argument against tree crops in the flora such as nitrogen fixing nurse crops.
midwest, and the potential for multiple yields in an Black walnut could be re-cast as an ecological ecosystem mimicking polyculture could become its resource if its beneficial interactions were empha- counterargument. Research in the long term sized as often as its detrimental interactions. Black (>30 years) is especially sparse in terms of polycul- walnut's allelopathic chemical juglone is lethal to tural alternatives, likely owing to the practice of some popular flora, but the remainder of the plant fastest-possible returns: selling the black walnut stand kingdom may effectively receive a selective advan- once sufficient stem diameter is achieved. For black tage due reduced competition when grown with black walnuts to become an investment that people protect walnut. Additionally, black walnut is not likely to as carefully as their retirement funds, a new gener- attract insect pests, as the leaves have been observed ation of research and production based on compati- to be amongst the least popular for forest insect pests bility and diversity may be required.
(Shields et al. which holds consistent with The negative reputation of walnut allelopathy their reputation as folk insect repellant (Walker dates back to ancient Greece (Willis ). The ). Though black walnut trees can inhibit some discovery of juglone in the 20th century, and its plants from growing, it has never been demonstrated identification as a causal agent in black walnut to ‘‘ruin the soil'' as is often rumored, and to the allelopathy, helped frame an era of research in which contrary it appears that soil processes such as the toxic effects were put in focus. Since 1980, there nitrification occur unfettered in the soil with juglone have been several papers reporting successful poly- (Thevathasan et al. ). Black walnut wood chip culture production methods with black walnut, mulch is often suspected to be antagonistic to plant including mixed stands, alley-cropping and silvopas- growth, but a recent study found black walnut chips ture. There now exists a lengthy list of companion Agroforest Syst (2007) 71:185–193 species that may be grown with black walnut stands, Davis EF (1928) The toxic principle of Juglans nigra as adding value to the system by making use of its identified with synthetic juglone and its toxic effects ontomato and alfalfa plants. Am J Bot 15:620 unique conditions. Though there is not a single Dawson JO, Knowlton S, Sun SH (1981) The effect of juglone agroforestry system for black walnut growing, a concentration on the growth of Frankia in vitro. U.S.D.A.
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