Jan 8, 2010
Keeping Under Cover: The Ideal Look of an Orchard Floor

In 25 years of studying the matter, Ian Merwin has seen all kinds of systems for managing groundcover vegetation in orchards. In some European orchards managed for cider apples, the grass grows like a hayfield. In others, sheep keep down the vegetation.

Some orchards are clean tilled and some are managed with mowed grass floors with bare strips carved out with herbicides.

After all his research, Merwin knows exactly how he’d go about it if he were establishing a new fruit orchard for himself.

The Cornell University researcher has reached several conclusions about orchard floor management, and he tells you what they are, usually after describing the precise details of the experiments that led him to add each one to his list.

Here’s the “short list, “ with details later:

Use grass alleys, preferably with fine-leaf fescue turfgrass

Use narrow weed control strips in the tree rows.

Use post-emergence herbicides.

Suppress groundcover vegetation in May and June.

Avoid replant disease by maintaining soil health and planting disease-resistant rootstocks.

Here’s the longer explanation.

Avoid replant disease

First of all, in establishing a new orchard, he would work the ground properly but he wouldn’t fumigate, even in a replant situation. Replant disease, he said, has “a quagmire of causes, “ mostly a buildup of “a very long list “ of pathogens. For Northeastern orchards, the economic benefits do not usually justify fumigation.

“We tried brassicas and we tried marigolds as cover crops, and we fumigated with Telone and we applied compost, and always with variable results. Sometimes it helped, sometimes it didn’t. It took me 12 years of research to realize we needed a fundamentally different approach. “

In 1999, a grad student in Merwin’s program, Dorcas Isutsa, went to the USDA apple germplasm collection in Geneva, N.Y., looking for genetic sources of apple replant disease resistance.

“After testing some 1,400 Malus genotypes, including 12 different species, we found genetic resistance very close to home, in the new Cornell-Geneva rootstocks “ he said.

Two of 12 rootstock clones they evaluated, G30 and CG6210, “really stood out “ in subsequent experiments involving graduate student Michelle Leinfelder, and G41 and G5935 have also shown good resistance in on-farm trials.

In an effort to find out why, Merwin, a soil microbiologist at Cornell (Janice Thies), a research associate (Angelika St. Laurant) and another grad student (Shengrui Yao) used molecular RNA fingerprinting methods to study the multitude of soil microbes around tree roots in orchards. They found that some apple genotypes can resist or tolerate pathogen attack by “inducing “ unique, protective communities of microflora around their roots.

Apple replant disease is tough to study, because each orchard is different and there are too many different kinds of soil microbes to isolate and identify them one by one. To find robust disease resistance, Merwin’s strategy was to collect soil from 16 New York orchards with a history of replant problems.

“We collected tons of soil and we made a composite of them, using a big cement mixer,” he said.

Then they planted different apple seedlings or rootstocks in pots containing the soil, which was treated by fumigation or compost or both. They also replanted an old orchard, planting trees in a grid and keeping careful track of old tree locations.

It all convinced Merwin that the most successful strategy for managing replant problems is to choose a resistant rootstock. Resistant ones grew well anywhere, even on the old tree sites, and grew just as well in fumigated or non-fumigated soil.

Narrow weed control strips

Once the soil was prepared, he’d seed a fine-leaf fescue grass in August and get a new sod growing. Then the next spring, he’d spray a strip 2 feet wide using the herbicide glyphosate (Roundup). He would not use any kind of pre-emergent soil residual herbicide then, or ever.

Why glyphosate? Why 2 feet? Why not pre-emergence herbicides? How about mulch?

There’s a lot of research history in the answers.

Merwin began studying orchard floor management while earning his Ph.D. in the 1980s, working with Warren Stiles. Beginning in 1991, he received grants from USDA and EPA to study long-term sustainability issues like pesticide and fertilizer leaching at a Cornell research orchard on the slopes above Cayuga Lake. The idea was to find out the long-term effects of various orchard soil and groundcover management practices on soil health in the orchard and water quality in the lake.

What practices lead to soil erosion or leaching of fertilizer or contamination by pesticides?

Before planting the trees (Empire on M.9/111) in the test orchard, a grid of below-ground drain tile lines was put in so scientists could measure and test percolating water as well as surface runoff. Experiments started in 1992 are still taking place, with ongoing support from USDA-SARE and other programs. This kind of long-term data gives Merwin some confidence in the advice he gives.

The ground covers he studied in this experiment included:

Method 1. Wood chip mulch made from composted hardwood bark from New York sawmills, reapplied every three years. After 12 years, he said, the decaying bark has developed a thick layer of humus just above the mineral soil, “like a vein of coal, “ full of tree roots.

The bark mulch tied up a lot of nitrogen in the early years, but after eight years, Merwin said, the trees in mulch really began to outgrow the others.

“They didn’t produce more fruit, they just got bigger,” he said. “The soil became almost too fertile for growing apples, and we began to see some excess runoff and leaching of nitrogen and phosphorus from the mulch treatment.”

Over 17 years, he said, organic matter content doubled from 4.5 percent to 9 percent from the addition of 4 inches of bark mulch every three years. The low-nitrogen mulch (less than 1 percent N) eventually broke down and was later able to generate 100 pounds of nitrogen per acre per year – more than the trees needed or could take up.

“We used to think that adding organic matter was always a good thing, but now we know at some point it may begin to leach nutrients. Nonetheless, all things considered, this was the second best way to manage an orchard floor, “ he said.

Method 2. A combination of glyphosate and two residual herbicides (Karmex and Solicam) applied in May each year to maintain a 6-foot wide weed-free strip in the tree-row all year round.

This system has the least weed competition, and you might think it would support the most tree growth and fruit production, but Merwin found that trees in this system were not as productive over the long term. There was substantial erosion and soil deterioration in the continuously bare soil. Nutrient and pesticide losses were also greater in this system than in the other three, especially from runoff during the early years at this orchard. This system also had fairly high mortality of surface feeder roots, because the soil became too hot and too dry during summer months near the surface.

“Malling apple rootstocks like relatively cool soil, “ he said.

Method 3. A fine-leaf fescue within the tree row, mowed monthly at a short height. Trees that grew in complete sod were stunted by competition from the grass during the early years of this treatment, and slower coming into fruit production than the other treatments.

“Trees can’t compete well with herbaceous ground covers for nitrogen or water, “ he said.

What Merwin found surprising, however, was that after the first decade these trees adapted to the grass competition, sending roots deeper beneath the sod, and they eventually became as productive as those in the herbicide or mulch strips.

“After 17 years, they caught up and produced almost as much fruit. The trees finally found a way to get enough nutrients. Apple trees are survivors, and can adapt to weed competition over time. “

Other researchers have found the same thing in other fruits. Over the years, trees adapt to the grass floor cover and may “actually become more efficient,” producing greater fruit yields per unit of tree biomass with somewhat lower pruning costs than more vigorous trees.

Method 4. Two applications of glyphosate to suppress weeds from May to August during the growing season, in a 6-foot wide strip beneath trees.

Over the past 17 years, this treatment has usually produced the most yields and optimal tree growth, with the lowest annual maintenance costs. This was somewhat surprising, because the tree rows in this system are fairly “weedy “ during the dormant season – with about 30 percent to 50 percent weed cover consisting of plants like dandelion, yellow rocket and other brassicas, common groundsel, ground ivy, crabgrass and a thin crust of moss that is resistant to glyphosate and covers the entire ground surface during the cool, wet winter months.

Apparently this groundcover competition does not harm the apple trees, because it occurs when trees are not actively growing or carrying the crop.

“I think of dormant-season weeds as a free cover crop provided by nature, “ Merwin said.

They use adjustable width or “bat-wing “ mowers to trim weeds beneath the trees. This prevents weed interference with harvesting or pruning operations, and discourages meadow voles.

Other research in this long-term study has shown that soil microbial communities in the tree rows (based on molecular fingerprint studies) become very different in each groundcover system over time. Nutrient “budgets “ can also be developed based upon the observed differences among management systems, so that fertilizer efficiency is maximized and nutrient losses are minimized in the orchard.

Weed-control area and timing effects

Another complex experiment from 1991-1996 compared different strip widths and timing of herbicide weed control. The trees were Gala on M.26, with drip irrigation in a clay loam soil.

The weed-free areas beneath trees varied from none (a mowed sod) to 2, 4 or 6 feet wide.

“We were expecting a big area effect, but we found no effect from strip width: Trees grew and yielded the same in 2-foot strips as in 6-foot strips of weed control. We think this was because the trees were drip irrigated and there were several dry summers during this experiment, so the drip lines concentrated apple roots where the water was.”

It was an easy call to make. Keep the strips narrow and maintain wide-grassed alleys to work on.

The herbicide timing effects were also interesting. Merwin tested nine different monthly combinations of herbicide weed suppression, using paraquat to suppress weeds during 0 (a mowed sod), 1, 2 or 3 months from May to August during the growing season.

“We saw a big effect of timing, “ he said. “Suppressing weeds in May and June gave us the most bang for the buck, and the May-only plots did almost as well as those with May and June weed control. We got virtually no benefit from weed control later in the season. This makes sense when you think about apple trees. There is a lot happening during May and June … bloom, fruit set and thinning, shoot and root growth and leaf area development. So, weed competition during those early summer months is especially stressful for the orchard.”

In choosing between glyphosate and paraquat as burndown herbicides to use in May and/or June, Merwin likes glyphosate because it translocates and kills weeds completely ¬– as opposed to merely burning them down – and because it is safer and easier to handle.

He also recommends painting trunks on young trees to partially protect the young green tissue from herbicides and also using a shielded boom sprayer to further protect trees from herbicide drift during application.

In another experiment, Merwin tried a system now called the Swiss sandwich method. In that system, vegetation is left along the center tree line, and strips are tilled on both sides of the tree row. The method is enticing to organic growers because it eliminates chemicals for weed control and keeps flowering ground covers in place under the trees for potential habitat predator/prey relationships to develop.

“This system was nowhere near as good for trees as mulch or herbicide treatments in our past tests,” Merwin said. “Deep cultivation damaged tree roots and reduced tree growth and yields compared with other systems. Mechanical tillage isn’t good for tree roots or for soil. It destroys soil organic matter and does not provide effective weed control. Some newer cultivators – for example, the Wonder Weeder developed by a Washington grower ¬– are somewhat easier on roots and soil, because they do not disturb the soil or roots as deeply.

“These new cultivators make vegetation ‘sandwich’ systems more feasible, but there is inevitably more soil disturbance and weed competition with trees in these systems, compared with no-till.”

So, that’s the package. Use grass alleys. Narrow the strips and narrow the timing of herbicides. Use post-emergence herbicides only. Use drip irrigation. Use replant disease-resistant rootstocks.

Merwin is quite pleased with the results he’s gotten and the fact that he can prove their worth ¬– documented with published scientific experiments conducted over nearly 19 years.





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