Mulching practices to improve plant growth, water savings, and soil organic matter content during establishment of highbush blueberry

Mulch treatments were applied in winter 2016/17
Mulch treatments were applied in winter 2016/17
'Duke' plants in June 2017, their first growing season
'Duke' plants in June 2017, their first growing season

Since 2011, greater than 80% of the new blueberry acreage in Oregon has been established using weed mat—a porous, black polyethylene ground cover placed in the blueberry row—rather than the traditional Douglas fir sawdust mulch. Positive findings from our 9-year study in organic blueberry, in which the use of black-colored weed mat resulted in about the same yield and fruit quality as sawdust mulch, influenced this trend, because the weed mat was much more economical for weed control. Our research also revealed that weed mat, as an alternative to conventional sawdust mulch, increased irrigation requirements by as much as 50% (Strik et al., 2017a). This is primarily due to increased soil temperature and reduced soil organic matter content compared to sawdust mulch, which is a disadvantage in blueberry production systems. We also observed in an un-replicated trial that blueberries grew particularly well when the plants were mulched with a combination of sawdust mulch covered with weed mat (Strik et al., 2017b). We suspect that adding sawdust under the weed mat helps to mitigate fluctuations in soil temperature, but of course, it also requires additional materials and labor costs.

Little information is available on the impact of the color of weed mat on canopy microclimate. We had originally planned on testing white weed mat, which does not increase soil temperature as much as black, but had issues in past trials with excessive weed growth underneath the white weed mat due to increased light infiltration. Green weed mat color has not yet been tested for effects on growth, yield, and fruit quality in blueberry, but has been shown to maintain lower soil temperatures than black, while preventing most weed growth underneath (Machado and Bryla, unpublished). For this reason, we decided to use green weed mat for this trial.

This study was established in October 2016 at the NWREC with five treatments: Douglas fir sawdust mulch (3” deep layer on the surface of the raised bed in-the-row, replenished over the study as needed); black weed mat placed on the soil surface in the row; green weed mat placed on the soil surface in the row; and both weed mat treatments placed over a sawdust mulch layer of approximately 2 inches in depth.

Non-destructive measurements of canopy growth and canopy density were done in 2017 using digital imaging and hand measurements, and leaf tissue samples were analyzed to assess plant nutrient levels. Soil samples were collected in autumn 2017 to assess soil properties (organic matter, pH, and macro- and micronutrients). Leaves were collected after senescence in 2017 from one plant per plot to determine differences in total plant leaf area, specific leaf weight, and nutrient concentrations. During the dormant season (2017-18), pruning weight will be recorded and the same plant will be carefully removed. The root system of the removed plants will be excavated in 5-cm increments to document the rooting profile in each treatment. Each plant part, including the roots, crown, one-year-old wood, and older wood, will be dried and weighed, and the total root to shoot ratio will be calculated along with a nutrient budget based on nutrient content of each plant part.

Sample of Results from 2017

Soil temperature was monitored from early summer through mid-October. The mulches consisting of weed mat (with or without sawdust) had higher soil temperature than under sawdust mulch. However, the differences between weed mat with and without sawdust was more pronounced in under hotter ambient temperatures.

Significant differences were found in plant height between treatments. Plants grown with the combination of sawdust and weed mat (either color) were taller than plants grown with sawdust alone (37 in vs 30 in). Plants grown with weed mat alone were intermediate, but not significantly different from the other treatments. No differences were found in plant diameter. There was a trend toward greater canopy size (measured by drone imaging) in the Black + Sawdust and Green + Sawdust treatments compared to either color weedmat alone and especially compared to sawdust alone, which had the smallest canopy size. No differences were found in NDVI (normalized difference vegetation index: a measure of plant health and “greenness”) measured in late August or in minimum and maximum temperatures in each treatment plot, though there was a trend toward higher temperatures in the plots with weedmat compared to sawdust alone