Hope against Replant Syndrome

Understand plant communities in order to avoid the pitfalls of replant syndrome and to develop a generational succession plan for your perennial crops

Perennial crops, both woody and non-woody, are defined as plants that survive many growing seasons to yield crops for many years in a row — some for a few years (strawberries, raspberries, etc.) and some for a century or more (walnuts, pecans, etc.). 

But just because a crop is a perennial doesn’t mean it’s eternal! Many newer growers seem to overlook this in their farm enterprise planning, and they end up in a bind when their plants get older and begin to go into decline. Just as planting corn after corn after corn eventually causes declining yields, declining soil health and increased pest and disease management costs, planting a monocrop of apples or cherries after a 25-year monocrop of apples leads to agronomic problems. This is the phenomenon known as “replant syndrome” or “replant disease”: it’s quite difficult to re-establish a perennial crop in the same place as the previous perennial crop. 

What Is Replant Syndrome?

Replant syndrome of fruit and nut trees, cane fruits and vines can cause total crop failure in replacement plantings. Milder cases lead to reduced tree vigor and crop yields that seem to straggle along at low levels forever. Both situations can result in significant economic losses. 

Replant syndrome is not clearly defined. It is a complex of hostile soil biology combined with cumulative mineral imbalance (caused by continued removal of crops), along with tree genetics that are susceptible to certain decay organisms, especially fungi and nematodes. I.e., RS is not always a single thing; this is why I use the term replant “syndrome” instead of replant “disease.” 

RS can be a big deal for a grower. I have seen peach plantings completely fail when planted in to a “retired” cherry orchard (both are Prunus species), and likewise with pears after apples (both Rosacae “cousins”) and avocados after avocados (which are genetically susceptible to phytopthera and other root rots). The chemical approach to the problem is “solved” (i.e., the symptoms are suppressed) through aggressive chemical fumigation. Not only is this expensive and highly toxic, it basically erases the soil life and restarts the chemical dependency bandwagon. Once you transition away from “rescue chemistry,” don’t go back if you can at all help it — reach out to someone here before you do! 

Replant Syndrome in History

An ecological understanding can help us put RS into perspective and can give us some indications about how to make the transition to the next main crop. 

Most perennial plants do not naturally grow in monocultures. Some surely do — sugar cane, beechnuts, fir (Christmas trees), hazelnuts and lowbush blueberries commonly form nearly pure stands. But most perennial plants are established, multiply, and are regenerated within an ecological plant community. 

An ecological community is defined as a group of species that are commonly found growing together. Sometimes they are clumped, sometimes they’re mixed together, sometimes they’re in vertical relation to one another (tall tree over short tree over shrub over cane over ground level). Ecological communities may be animal or plant groupings with similar nutrient and rainfall requirements. 

Some plant communities contain species that may support one other, inhibit others, or depend on similar environmental processes. For example, plants that are adapted to extreme temperatures and prolonged drought will be found growing together, while assemblages of plants that survive periodic inundation and lack of calcium will be found together. 

Our vineyard and orchard crops are actually part of ecological communities. In deep history, these plant communities survived and thrived without human interaction. Indigenous peoples from all continents began interacting with, modifying and cultivating plant communities, even taking on their regional plant community as part of their cultural identity. In the past few thousand years, beginning with grapes, olives and walnuts, individual plant varieties were removed from their ecological context and put into “orchards” and “vineyards.” And within the last 100 years (only the blink of an eye), perennial crops have been grown as chemically bathed, high-density monocrops. 

Why do modern growers struggle with replant syndrome when these plants have existed since before the birth of humanity? How is it that these plant communities somehow rejuvenated themselves for thousands, perhaps millions, of years without any human intervention, purchased inputs or equipment? Rejuvenating these systems used to cost nothing and required no human intervention. 

Overcoming Replant Syndrome

Let’s take a look at three common plant communities and see what they can teach us about how to overcome replant syndrome. 

Most biological and chemical processes in a region are driven by the longest-lived species of that area. Walnuts are a good place to start understanding this because most folks are familiar with the fact that Juglundacae (walnut, pecan, hickory) produce chemical juglones, which are a targeted herbicide. If you are a plant and cannot tolerate juglones, you will not be able to live with or near walnuts. Walnut or pecan trees can live hundreds of years, and their exudates and surface litter will determine what kind of biology will grow and thrive there. Oaks don’t exude toxins like juglones, but they have their own suite of chemical uniqueness (lots of tannins, etc.) that drives the biology around them, as do pines, which tend to acidify their surroundings and can leach iron and deposit it into impenetrable layers belowground. 

The walnut plant community includes the Prunus genus (cherries and plums), grapes (Vitus) hazelnut (Corylus), raspberries (Rubus) and gooseberries (Rbes), among others. The red pine (from Minnesota eastward to the Atlantic, and south down the Appalachians) and longleaf pine (mid-central to southeast Atlantic) communities include beech, cherries, blueberries, hazelnuts, serviceberries, lingonberries, grapes, raspberries, strawberries and more. The fact that these plant communities have persisted almost forever without inputs, while constantly changing form, is proof that we can ecologically rejuvenate our orchards and vineyards at a fairly low cost, forever, with no fear of replant syndrome.

Let’s take a cherry orchard in Michigan. This cherry orchard has had 25-30 years to develop the perfect suite of cherry pathogens. Once trees start to die or are removed, the population of cherry root decomposing fungi explodes (part of the cause of RS). Ideally, before decline begins to accelerate, the grower can choose the next main crop — perhaps one with a quick turnaround like strawberries or raspberries. Instead of planting the new crop in the same row as the removed crop, the new crop can go in the alley between tree rows. 

Since raspberries and cherries are part of both the walnut plant community and the pine plant community, the grower can decide whether to steer the site in the walnut direction or the pine direction. The decision might be made based on which species will have a better timber market or whether nut processing infrastructure is (or will be) available. 

An additional strategy during this time of transition is to plant cover crops to help lessen pathogen loads. Research has shown that incorporating brassica seed meal or planting brassica cover crops helps to mitigate replant syndrome. Tillage radish/daikon and mustard greens can be planted to help knock back pathogens. Tillage radish/daikon is especially effective against nematodes, and its roots are very antifungal.

For the sake of the growers from Georgia to Maine, let’s say that when it’s time for the cherries to come out we decide to replant with raspberries and pine (Korean pine, Siberian pine, Digger pine or Italian pine produce pine nuts in those regions). The decaying cherry/peach debris doesn’t bother either the raspberries or the pine. There will be no replant syndrome. 

After 10-15 years of raspberries, the grower can go back into cherries or peaches between the pines, with grazing underneath, or could move into hazelnuts. After 25-30 years, the pines can be thinned for pulp, allowing more light to the grass beneath. Alternatively, the alleys between rows of pine can be filled with a block planting of either hazelnut or blueberry. 

I’ve met Acres U.S.A. subscribers who grow pine for timber that have experienced regeneration of groundcover after timber harvest. Thinning harvests tends to keep the understory more humid because of shade effects, which tends to favor regeneration of blueberries. Clearcutting of red pine in many places favors the hazelnuts, as additional sunlight can dry out the blueberries enough to make them not competitive on the site. 

The commercial lowbush blueberry industry in Maine is one of the finest examples of a modern agricultural system that closely mimics nature. Eastcentral Maine has tens of thousands of acres of blueberries grown intermixed with pines. Raspberries naturally spring up in and around the pines and blueberries, as do cherries (mostly wild pin cherries, chokecherries and black cherry), along with serviceberries and wild strawberries. Many growers, especially folks who have bought derelict blueberry ground, have added retail raspberries and strawberries to their operation. Young evergreens are managed as Christmas trees, wreaths and greens. Blueberries are often managed with fire when chipping with an orchard mower is not practical (because of rocks); this closely mimics “pine barrens” ecology. 

By understanding the plant community of your main woody crop, growers can begin planning for tree succession when they are young farmers. Perhaps this means planting a polyculture with pine, fir and cherries with a perennial rotation in the alleys between rows: strawberries for two years, followed by raspberries for six, followed by blueberries for 20. Strawberries and raspberries produce immediately, followed within five to seven years by the cherries and Christmas trees. 

In 20 years, the kids have moved out of the house and 50 percent of the pines need to be thinned for pulp or cabin logs. After the timber harvest, the alleys, which by now have all been converted to blueberries, can be burned down with a flame weeder and the pre-retirement farmer can stay in blueberries. Or perhaps the blueberries can be mowed, pasture established, and stocker cattle brought in for the farmer’s retirement years. 

A perennial pine-barrens/pine-savanna ecosystem has been established on the bones of a rundown cherry (or peach) orchard. Agricultural production never stopped. The ecosystem can recover from a century of toxic-chemistry agriculture. The newly minted grandparents can hand over the system to a new generation, who can immediately go in and plant cherries or peaches with no fear of replant syndrome. 

Mark Shepard is a land designer and consultant and is the author of Restoration Agriculture, Water for Any Farm and the Water for Any Farm Technical Manual.