Integrating Living Mulches into Strawberry Production

By Steve Poppe1, Emily Hoover2, Emily Tepe2, Andy Petran3, Nate Dalman1, Anna Mahoney1, and Esther Jordan1

1West Central Research and Outreach Center, University of Minnesota; 2Department of Horticultural Science, University of Minnesota; 3Twin Cities Berry Company

February 2020

Living MulchLocally grown strawberries are a high value crop in Minnesota, and the potential for a longer production season is now possible with new day-neutral cultivars grown in an annual system. The system can be incorporated into an annual horticultural crop rotation, which boosts biodiversity and economic diversity on local farms. However, even in varied production systems, weed control remains a constant battle for fruit and vegetable farmers. In our recent survey of over 200 farmers, weed control was ranked as the top concern. Many farmers fall back on herbicides or tillage to manage weeds, which reduces environmental sustainability and profit potential. 

We have been refining a reduced-input annual strawberry production system in Minnesota since 2014. This research has taken place at the University of Minnesota (U of MN) West Central Research and Outreach Center (WCROC), as well as several farmer-cooperator sites.  Our past research has focused on optimizing a system for day-neutral strawberry production in the Upper Midwest. This research has resulted in recommendations for planting layout, plastic mulches for in-row weed control, low tunnel construction, and fertility in systems that produce marketable fruit from July through October. We have used landscape fabric for weed control between the rows which effectively controls weeds but doesn’t benefit the soil and results in excessive waste.

In 2019, we set out to make the system more sustainable by demonstrating a between-row living mulch in order to suppress early-season weeds without competing with the strawberry plants, as well as to contribute to soil health. Research has been conducted using various living mulch species in different horticultural crops with mixed results. Recent research on winter genotypes of camelina, canola, and rye show these plants have the qualities that might work well as living mulch for weed suppression.  At the end of the season, when the living mulches are tilled under, they will add valuable organic matter to the soil.

Design

In the spring of 2019, we established two experimental trials working with a farmer-cooperator, Twin Cities Berry Company and another trial at WCROC. The WCROC site was planted/trialed inside a deer fence exclosure on silty clay loam soil where the previous cropping history is fallow ground. The Twin Cities Berry Company is located in Farmington, MN, has a sandy loam soil, fallow ground and is certified organic. We used our existing equipment to shape raised beds, lay drip irrigation tape, and secure white-on-black plastic mulch over the beds. We planted dormant, day-neutral strawberry transplants through the plastic mulch and connect the drip irrigation tape to existing irrigation systems on the planting sites. 

At each location, we established annual strawberry rows with the day-neutral cultivar Albion and Cabrillo with 4 between-row treatments: (1) winter camelina, (2) winter canola, (3) winter rye, and (4) landscape fabric. Each treatment was replicated 4 times. The plots were maintained with organic production methodologies. We performed an analysis of variance to determine if there are significant differences among treatments. Soil test data was taken at the beginning and end of the growing season which will indicate the effect of the living mulches on soil organic matter percentages.

Results

The two locations (WCROC, Morris and Twin City Berry Company, Farmington) had very different outcomes in terms of how well the living mulch germinated and became established. 

At Farmington, germination was poor, so the mulches never became fully established and therefore did not offer any weed control. At Morris, germination and establishment were good for all the living mulches. 

The weather around the time of seeding is critical in establishment. Heavy rain and cold temperatures in spring delayed seeding and made the soil difficult to work. These poor conditions likely contributed to low germination rates at Farmington.

Living mulches and weed suppression

Here is a breakdown of how each of the living mulches performed over the growing season at the WCROC in terms of establishment and weed control. All crops were planted by hand broadcasting at the same time in early summer after the strawberry rows had been planted and only watered twice to aid in germination and promote establishment.

Living Mulch

Winter camelina

Winter camelina is a crop that is typically planted in the fall where it then germinates and forms a small rosette and then lies dormant in the soil over the winter before it grows and flowers. With this information, we believed that over the summer camelina would remain in the rosette stage. The camelina seed was provided by the USDA North Central Research Lab in Morris, MN and was recommended for this particular study by weed Scientist Dr. Frank Forcella.

We seeded the camelina at 10lb/acre and had very good germination at WCROC, which led to good establishment. Weed suppression was successful for the first month while the plant remained a rosette. However, likely due to the cool, wet June, the camelina started to flower by early July which caused a decline in weed suppression. The rosette leaves started to deteriorate which exposed soil, allowing weed seeds to germinate. By the end of August, the camelina plants had very little biomass left. Overall, camelina was unsuccessful at controlling weeds throughout the season.

Winter canola

Winter Canola is similar to camelina in its growth and development stages in the way that it requires a dormancy period in order to flower. The variety we used for this project was Torrington Winter Canola. Again, it was seeded at 10lb/acre and there was very good germination and establishment. 

The canola grew quickly into large rosettes, which led to excellent weed control. In fact, the rosettes grew so large that they had to be mowed back four times throughout the whole season to keep them from growing up and over the strawberry rows. This made the canola a relatively high maintenance mulch compared to the others. However, it provided excellent weed control over the entire growing season since its large rosettes left no exposed soil for weeds to grow.

Winter rye

Winter Rye, (Secale cereale) was the final mulch we experimented with, which also needs a dormancy period. Ryman rye was seeded at 60lbs/acre and had good germination and establishment at WCROC.  As a grass, winter rye doesn’t have a rosette stage like broadleaf plants, but it does stay relatively short. It too is a biennial and requires a period of cold temperatures before flowering. It grows quickly meaning it would likely choke out most weeds. Its height is easily managed by mowing, and it makes a good surface for walking. Plus, the seed is readily available and inexpensive. 

Soil Results

Initial soils samples before treatment application had oxidizable carbon levels at 623ppm, extractable nitrate at 5.8ppm, extractable ammonium at 1.5ppm, mineralizable ammonium at 2.0ppm and total ammonium at 3.5ppm. Oxidizable carbon indicates how much organic matter is in the soil that is easily degradable within 2-5 years depending on soil conditions. Both plant and microbial carbon is detected, so typically over a growing season the levels increase due to increased microbial activity. Extractable nitrate is the amount of nitrate present in the soil at the time of sampling. Nitrate is the form of nitrogen typically preferred by plants but is very volatile and easily released into the atmosphere or leached out via water. Extractable ammonium is the amount of ammonium present in the soil at the time of sampling. Ammonium is a form of nitrogen that plants are able to use and is more stable in the soil than nitrate but still subject to leaching. Mineralizable ammonium is the measured amount of organic matter present in the soil that can be broken down and used by a plant within one growing season. The ammonium is not readily available for the plant at the time of soil sampling but will be in the near future. Total ammonium is the combination of the extractable and mineralizable ammonium levels to get a better look at what is happening in the soil over the whole growing season.

In comparison to the initial soil samples before treatments were applied, the fabric treatment increased the oxidizable carbon levels and mineralizable ammonium by 211ppm and 0.9ppm respectively. Fabric decreased extractable nitrate, 3.2ppm, and ammonium, 1.4ppm, levels of the soil throughout the growing season which caused a decrease in total ammonium by 0.5ppm. Therefore, this treatment is depleting the soil of nutrients that will be available for the following year’s strawberry crop meaning more fertilizer will be required to achieve peak production. Strawberries are a heavy nitrogen user and reduced ammonium and nitrate levels will lead to deprived plants.

The rye treatment showed similar results to the fabric; an increase in oxidizable carbon, 206ppm, and mineralizable ammonium, 0.7ppm, but a decrease in extractable nitrate, 3.8ppm, and ammonium, 1.3ppm, levels leading to a decrease in total ammonium by 0.6ppm. This shows that this particular living mulch is using up the available nutrients in the soil but not releasing them back into the soil within one growing season. Since the nitrogen is tied up in the biomass of the rye, it will likely not be readily available for the following year’s strawberry crop and more fertilizer will be required to maintain plant health.

Camelina increased the oxidizable carbon, 206ppm, mineralizable ammonium, 1.5ppm, and total ammonium, 0.3ppm, levels but decreased the extractable nitrate, 2.0ppm, and ammonium, 1.1ppm, indicating that this mulch has the potential to increase soil nitrogen levels. Even though extractable ammonium was decreased, it is possible that the decomposing biomass readily decomposes and releases the ammonium that the plant used while growing, especially since camelina senesced early in the growing season. However, the decrease in soil nitrate could be problematic for the next strawberry crop.

Finally, the canola treatment showed an increase in oxidizable carbon, 240ppm, extractable nitrate, 1.1ppm, mineralizable ammonium, 1.4ppm, and overall ammonium, 1.2ppm. The only decrease was in extractable ammonium, 0.3ppm. These results indicate that this living mulch does indeed create a more nutrient rich soil within just one growing season. By increasing the nitrogen levels, the next strawberry crop should benefit from this and possibly require less fertilizer over the season.