Cooperative Extension Publications

Bulletin #2179, Wood Chip Mulching in Wild Blueberry Fields (PDF for printing)

Prepared by Lily Calderwood, Ph.D., Wild Blueberry Specialist and Associate Professor of Horticulture, University of Maine Cooperative Extension, and Jordan Parks, Wild Blueberry Research Technician, University of Maine Cooperative Extension.

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Wood Chip Mulching in Wild Blueberry Fields

Introduction

Most wild blueberry plants are cultivated in the state of Maine, Nova Scotia, and the Québec Province. In these areas, blueberry plants grow in well-drained sandy soils of glacial till and moraines. This soil type has a low water holding capacity which can cause complications to the plants during drought conditions. As climate change progresses, the state of Maine will likely experience both an increase in the severity of drought spells and increased precipitation across the region (Fernandez et al., 2020). This fact sheet will focus on meteorological and agricultural drought. Meteorological drought occurs when there is a decrease or absence in precipitation; an agricultural drought occurs when a crop’s root zone becomes dry during the growing season even though deeper soil layers may be saturated, causing significant crop damage (Heim, 2002). The main factors contributing to drought are increased temperature and wind, low relative humidity, lack of cloud cover, and low precipitation (Condra, 1944).

Seasonal droughts in Maine generally occur in June, July, and August which are critical growing months for wild blueberries in prune and crop cycles. Wild blueberry fields have experienced drought in six (2016 to 2018, 2020, 2022, and 2025) of the last ten years. In 2020 and 2025, growers experienced yield losses from 0.5 to 97.0%, with a state average loss of 44%, due to drought and increased temperatures (Schattman et al., 2021). As blueberry plants become water-stressed, they also start to lose their ability to take up nutrients without water. This plant requires one inch per week of water allowing it to function properly. Few growers use irrigation with well or pond water sources, and on occasion, water is trucked in to irrigate fields during critical times. One technique that can be used to reduce the effects of drought is to cover entire fields with wood chip mulch.

Main Recommendation

When whole field mulching, the research to date suggests that a soft wood chip mulch thickness of 1.0–1.5 inches to be the most successful when soil, pest, and plant parameters are considered.

Research on Wood Chip Mulching

Beginning in the 1980s, wood chip mulching has been used in wild blueberry fields to cover bare patches in fields to stimulate rhizome spread, increase water-holding capacity, and provide weed control. The Natural Resources Conservation Service (NRCS) developed a cost-share program to help farmers cover these bare spots with two to four inches of mulch for erosion management based on research conducted at the University of Maine (DeGomez and Smagula, 1990). Given the success of bare-spot mulching, growers became interested in applying mulch to entire fields. Recently, the University of Maine began studying mulch in this context, and those results are summarized below. This was a study that took place in Hope and Orland, Maine, from 2022-2025 where mulch was applied across whole fields with a no mulch control, 0.5, 1.0, 1.5, and 2.0 inch thick mulch treatments.

Benefits of Whole Field Mulching

1. Increased Soil Moisture. Four years of soil moisture data from mulched treatments shows clearly higher soil moisture in all mulch depths at the Hope location where there is very good wild blueberry cover and the soil is heavier. During the 2025 drought conditions the more mulch applied, the higher the soil moisture at this location. At the Orland site which was de-rocked and land leveled right before mulch application in 2022, results are more mixed largely because of the ledge and thin soil present at this site which makes it much more difficult to get good quality soil moisture readings. During 2025 drought conditions, 1-2 inches of mulch buffered the impact of drought by 4% soil moisture only in July before severe drought occurred in early August. Irrigation is required to save a crop from severe drought. See Figure 1.
2. Cooler Soil. Over four years of data collection (2022-2025), as mulch thickness increased from 0.5 to 2.0 inches soil temperature decreased by about 2°F. Cooler soil during the spring may slow down plant development just enough to avoid late spring frost events during bloom and cooler soils during hot summer months may reduce stress on wild blueberry plants.
3. Pest Suppression. Mulch decreases leaf spot diseases and the number of tip midge pupae in the year of application and the year after. Incidence of these two pests re-occurs once mulch becomes the ground surface. Previously, it was found that having the soil surface covered with mulch smothered small annual weeds but not grasses or broadleaf perennials (Drummond et al., 2009). From 2023- 2025, the presence of all weeds decreased as mulch thickness increased as seen in Figure 2 (Calderwood et al., 2025).
4. More Chlorophyll. Mulch clearly increases the level of chlorophyll in leaves and increases the number of buds per stem in the crop year (Calderwood et al., 2024; Gumbrewicz and Calderwood, 2022).
5. No Nutrient Immobilization. There was concern that microbes in the soil that release plant available nitrogen (NH4) may be refocusing their breakdown efforts on the newly applied wood chips, which would reduce the amount of nitrogen available for the crop. We have NOT observed any nitrogen immobilization or “tie up” over four years (2022-2025) of foliar nutrient testing. This is likely because the mulch is not incorporated into the soil at all and therefore not directly in contact with microbial organisms. We are now studying fertilizer rates when applied as a top dress after mulch application where immobilization is much more likely.

Drawbacks to Thick Mulch Applications

In the very wet season of 2023, we found more mummy berry disease in the area mulched with two inches. This is not a surprise because we would expect mulch to hold water and mummy berry to thrive in wet conditions. Stem height was found to decrease where 1.5 and 2.0 inches of mulch was applied however the number of stems per unit area (stand density) was not affected by mulch thickness (Calderwood et al., 2025).

To summarize the whole field mulching research to date, wood chip mulch at thicknesses between 1.0 and 1.5 inches are the most successful when soil, pest, and plant parameters are considered.

Best Management Practices

Types of Fields that Benefit from Mulching the Most

There are wild blueberry fields in all different stages across the state of Maine. Some have become overgrown while others are healthy, some are disturbed from recent land improvement, and some are established stands of commercial blueberry. Fields recovering from de-rocking and land leveling in addition to those that are especially drought prone are excellent candidates for mulch applications.

Recovery Fields – Spot Mulching

These are fields that have been recently de-rocked, a practice supported by NRCS. Typically, sections or spots of these fields without blueberry cover are mulched. Field management activities in these fields should occur in the following order: de-rock (after harvest, before prune in fall year 1), option to mulch and then mow at this time, year of recovery (prune year 2), sulfur, herbicide, and fertilizer application (crop year 3 fall or spring), option to mulch at this time (crop year 3 fall after harvest, before prune).  Mulch pieces can get caught in harvesters if the plant height is still low, and therefore, we strongly recommend harvesting in year 3 (2 years after land improvement). In summary, we recommend applying mulch right after rocks are removed to smother weeds and aid in blueberry recovery immediately. If you choose, you can wait to apply mulch until weed control has occurred in year three after de-rocking. A visual representation of this schedule is in Table 1.

In many cases mowing after mulch is applied will not cut blueberry stems low enough for optimal production (within 1 inch of the soil). Therefore, your plants will have more branches in the first crop year after de-rocking. After the first harvest in year 3, the mulch will have settled enough to get a lower prune moving forward.

It is important not to over mow or apply any herbicide to a de-rocked field for at least one season. This disturbance causes wild blueberry plants to be stressed and mowing and herbicide application can set the crop back further. It is also very important not to fertilize a recently disturbed field because any fertilizer will feed the weeds while you are giving the field a break from weed control activities. Ideally, the field would have a low soil pH before starting this land improvement process to reduce weed pressure as the wild blueberry crop recovers.

Drought Prone Fields – Whole Field Mulching

These are established wild blueberry fields that are well managed. All wild blueberry fields are prone to drought due to well-draining soil properties. Some fields are especially sandy, and those fields in particular would benefit from whole field mulching. In these fields that are well established, mulch should be applied either in the early spring of the prune year before any blueberry leaves emerge or in the crop year after harvest and after pruning. Here, mowing before mulch application ensures a quality prune but growers may consider a smoothing/chopping pass with a mower after mulch is applied to even out and break up the mulch application, reducing the risk of mulch pieces ending up as contaminants in processing facilities.

Critical Suggestions

1. Field owners selling fruit to an IQF processor are strongly encouraged to check with their processor before investing time and money into mulching activities to make sure the processor will accept fruit from mulched fields. Even if you are spot mulching, material can cover many acres of a field.
2. Ideal timing to apply mulch is after de-rocking, before pruning. After mulch application, NRCS comes to document the depth. Then the farmer mows the field. Mowing the field after mulch application is a critical step to chopping it up more and working it down to the ground for a uniformly smooth surface.
3. Harvesters must be careful not to dip rakes into the mulch layer, but this is no different than raking in a field with no mulch layer. We recommend that the raking head should not drop below 5 inches off the ground because the majority of fruit lives between 5 and 10 inches off the ground. Once berries have been harvested, mulch pieces have been found to float and be removed in a float tank, but this has not been consistent over the years. Some growers were able to remove mulch pieces with a laser sorter which removed almost all mulch pieces. Furthermore, the pieces that made it past the laser sorter were easily removed. An individual quick freeze (IQF) processor determined that more mulch got caught in the fruit when plants were too short and when conditions were wet.

Mulch Cost

The NRCS has a cost-share program that will pay for most if not all mulch material, equipment, and labor costs associated with the application. The itemized cost of mulch prices is shown in Table 2.

What Type of Mulch Should be Used?

It is important to choose a wood chip mulch and not a bark mulch. Bark mulch will get caught in harvesters, and the operator will be constantly removing it from the rake or reel. Mulch can come in a variety of particle sizes, and it has been found to be beneficial in sawdust form all the way to chip size (Gumbrewicz and Calderwood, 2022) as long as the depth is a minimum of 1.0 inches (Calderwood et al., 2024).

How Should Mulch Be Applied?

The application of mulch should be done during the prune cycle when the blueberry plants have not yet emerged. The most efficient way to apply mulch to whole fields is with a manure spreader (Image 3).  Another way to spread this material is with a tractor operator who fills the bucket with mulch and “feathers” the mulch on the ground while a ground crew rakes the mulch to the desired thickness (Image 4). These are not the only options for spreading mulch. If you have an idea on how to utilize the equipment you have, by all means use those techniques.

Calculating the Amount of Mulch Needed

1. Calculate the square footage of your field.
2. Determine what mulch depth you would like to apply.
3. Calculate how many cubic yards of material to purchase. There are mulch depth calculators online.
4. A useful equation you can use is:
   • Area to Cover (square feet) x [Depth of Mulch (inches) ÷ 324]
5. Round your answer up or down for the supplier and your needs.

Limitations

The mulch data collected for this fact sheet were from fields that were mowed instead of burned after the crop cycle. We do not yet know the effects of burning with a wood chip mulch. Anecdotally, growers who burn two seasons after mulch application must be careful that the fire doesn’t continue to burn in areas of thick mulch. Overall, growers who burn have not had problems.

Additional Questions

For questions or more information about Maine wild blueberry production, contact Dr. Lily Calderwood, UMaine Extension Wild Blueberry Specialist and Associate Professor of Horticulture, via email at: lily.calderwood@maine.edu

Or, visit the University of Maine Cooperative Extension’s Maine Wild Blueberries website for wild blueberry bulletins, videos, tools, and more.

References

• Calderwood, L., Annis, S., Fanning, P., Schattman, R., and Zhang, Y.-J. (2024). 2023 Wild Blueberry Research and Extension Report.
• Condra, G. E. (1944). Drought, its effects and measures of control in Nebraska.
• DeGomez, T., and Smagula, J. (1990). Mulching to improve plant cover. University of Maine Cooperative Extension Fact Sheet (228).
• Drummond, F., Smagula, J., Annis, S., and Yarborough, D. (2009). Organic wild blueberry production. MAFES Bull, 852, 43.
• Fernandez, I. J., Birkel, S., Simonson, J., Lyon, B., Pershing, A., Stancioff, E., and Jacobson, G. L. (2020). Maine’s climate future: 2020 update.
• Gumbrewicz, R., and Calderwood, L. (2022). Comparison of wood mulch particle sizes for wild blueberry management in a changing climate. International journal of fruit science, 22(1), 551-567.
• Heim, R. R. (2002, 01 Aug. 2002). A Review of Twentieth-Century Drought Indices Used in the United States. Bulletin of the American Meteorological Society, 83(8), 1149-1166.
• Schattman, R., Goossen, C., and Calderwood, L. (2021). The 2020 Maine drought and agriculture report. University of Maine: Orono, ME, USA, 1-30.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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