MSU Extension

Beginning in 1995, a group of scientists at Michigan State University set out to answer how orchard floor management could affect other horticultural practices, soil quality, and pest management in newly planted and established tart cherry orchards. Tart cherry growers and industry representatives provided consultation for the work through the Northwest Michigan Integrated Fruit Systems Think Tank.

Applying research results

Orchards are grown on many soil types and under different weather conditions. These factors, market demands, and a host of other considerations must be weighed when determining the best orchard floor and nutrient management options for a particular farm. Furthermore, many practices have both advantages and disadvantages; there is no ”best” system that can meet all objectives. We hope that this research helps growers consider various management options and weigh some of the trade-offs associated with selected practices.

Picture 1. Mixed species ground clover.

Effects on yield

How did the different systems affect yield?

• The use of ground covers with no herbicides for weed control had surprisingly little effect on yield. There was some suppression of tree growth where cover crops were grown on the entire orchard floor with no herbicide strip, but over five years, yields were not significantly reduced. These studies give good evidence that weed control under the trees does not need to be overly aggressive.
• Hay or straw mulch, applied 6 to 8 inches deep, produced better tree growth and improved yield. There were three negatives to the heavy mulch system:

1) Two-spotted mite populations were higher than all treatments with broad-leafed plants growing under the trees. But even with more mites, the trees produced heavier yields.

2) Cherries were softer in two of the seven years.

3) The system can be expensive if mulch is not obtained free of charge. Even at $1 per small bale (a value equal to cost of standing hay free for the harvesting), costs exceeded all other systems.

• Systems that used a side-delivery mower to mow row middles and blow the trimmings under the trees were also among the top yielding systems in the trials.

Picture 2. Newly planted mixed species ground cover.

What was most profitable?

The three most profitable systems included two fertigation treatments, plus the Conventional treatment.

• Conventional plus ½ rate fertigation gave high yields at low cost ($760/10 ac/yr)
• Conventional gave slightly lower yields, also at low cost ($850/10 ac/yr)
• Cover Crop + Fertigation gave comparable yields to Conventional, but at slightly higher cost ($1190/ 10 ac/yr)

Above, Figure 2 shows how high costs (dark color) knocked the high-yielding mulch and compost systems off the profitability pedestal in favor of the lower cost systems.

While growth and yield may present the most compelling reasons for adopting new management strategies, profitability also takes into account the cost of inputs necessary to implement each orchard floor management system.

• Profitability sometimes involves trade-offs with other objectives, such as reduced nitrate leaching. Below, Figure 3 shows that while conventional ground-applied N in spring is a profitable system, it also resulted in five times more nitrate leaching than the next highest treatment measured.
• Conventional + fertigation at 1/2 N rate gave high profitability and minimal nitrate leaching. As shown in Figure 3, it looked best in both dimensions among the five treatments where leaching was measured.

Ground cover management influences nitrate leaching

Under Northwest Michigan conditions, most total N leaching occured from October through April. Low soil moisture during the summer minimized leaching potential. The conventional and cover crop treatments in Figure 4 received identical nitrogen rates and timing. The only difference was the amount of vegetation under the trees.

• Vegetation growing under the trees reduced N leaching:

1) Vegetation provides carbon and soil organic matter to support increased populations of microorganisms that utilize N, reducing the amount of N in a leachable nitrate form.

2) Plants take up N, reducing the leachable nitrate N in the soil.

• Spring and fall are the critical times to have vegetation growing under the trees to minimize nitrate leaching.

Fertigation helps to reduce leaching

• Nitrate leaching is significantly reduced when nitrogen is applied via fertigation:

1) Less total N is applied

2) N is used at much lower rates per application

• Yields and growth that are comparable to fall and/or spring broadcast application can be obtained by fertigating at 50 percent of the total N rate split in four applications.

Is simazine leaching a problem?

Simazine leaching was very low in all treatments. The highest level, found in the conventional treatment, was less than 0.0002 lbs/A. Simazine levels were so low that there was no significant difference between treatments with and without simazine applied, due to residuals from prior years.

Soil quality - Improving habitat for beneficial microbes

Soil quality can be defined as the ability of a soil to resist degradation and respond to management. All orchard sites should be assessed regularly. Soil organic matter is one of many indicators of soil quality (see list on this page). Through the process of mineralization, soil microbes decompose organic matter releasing nutrients to the tree. Adding carbon under the trees is one way to boost soil organic matter. Our work clearly indicated:

• The highest population densities of the microbes involved in making nutrients available were located in the orchard surface litter layer (Figure 5).
• Active soil carbon and nitrogen were significantly lower under a conventional fertilizer and weed management system than in the other systems.
• There were fewer beneficial nematodes, more plant parasitic nematodes (Figure 6), and more nitrate leaching (Figure 4) associated with the lower quality conventional soils.
• In-row soil population densities of beneficial nematodes, mycorrhizae and oligocheates (microearthworms) were greater under an organic production system.

Picture 3. Use of ground covers was associated with increased biotic diversity.

Ground cover management in young orchards

Managing ground cover in young orchards is very different from managing bearing orchards. Young trees will benefit from adding mulch or compost, but remember:

• Trees in young orchards can be severely stunted by competition with ground cover plants for moisture and nutrients.
• Seasonal weed control with herbicides decreased both microbial activity and soil organic matter.
• Mulching around young trees will add nutrients, carbon, and organic matter into the system, and will reduce moisture loss.

Weed control in established orchards

Two of the concerns with growing ground cover plants in the tree row are: 1) plant competition with trees for moisture during midsummer, 2) difficulty seeing tree trunks during harvest. These concerns can be addressed by promoting a good population of plants under the trees in fall and spring and by providing good weed control from early June through harvest (in Northwest Michigan) with the use of a contact herbicide (such as glyphosate or paraquat) or by use of a mulching system. A third concern is that vegetation provides habitat for voles.

Mite management

High mite populations were consistently associated with vegetation-free (herbicide) strips in conventional systems and systems using thick mulch. High mite populations in the mulch system were not detrimental to yield. Combining mixed species ground cover plants and no herbicides supported low pest mite populations.

• Season-long populations of beneficial mites were associated with red clover.
• Beneficial mite populations were higher on red clover than on white clover.
• Compost biosystems consistently had the lowest mite populations.

Using legumes in ground cover

In using legumes in a ground cover, consider:

• Legumes provide nitrogen to the orchard ecosystem.
• Deep-rooted legumes (alfalfa) help break up hard pans and aerate soil to a greater depth
• Legumes did not harbor high plant parasitic nematode populations in these studies.
• Legumes generally do well when potassium is relatively high. Consider periodic applications of potassium with a broadcast spreader that fertilizes the row middles.
• To optimize soil nitrogen, do not mow legumes until near flowering.
• Red clover, white clover and alfalfa were relatively easy to establish.
• Over time, legumes in bearing cherries tend to be replaced by grass:

1) Shade hastens the decline of legumes such as alfalfa.

2) Frequent mowing will favor the short plants such as Dutch white clover but is hard on the larger legumes.

3) Legumes require periodic reseeding to rejuvenate ground cover plantings.