What Difference Does Mulch Make?

We have been leading research trials to determine the effects of various types of in-row mulches on blueberry plant growth, yield, fruiting season, and fruit quality in establishing and mature blueberry fields as well as the impact on voles and weed management costs. Here we summarize our findings and provide links to published research papers for further reading.

Harvesting ‘Duke’ in the fourth growing season (2020) in the weed mat color trial at the North Willamette Research & Extension Center, OSU (photo B. Strik, OSU)

Historically growers have mulched the in-row area, often with sawdust to help conserve soil moisture, control annual weeds and improve soil organic matter over time as compared to bare soil. As sawdust became less available and more expensive, growers looked to alternative options. Black weed mat (porous, woven, plastic ground cover) was attracting attention in the early 2000s and was used for the first time by some growers in the region.

In 2006, we started our first trial with weed mat. While this was done in certified organic blueberry, our results are applicable to all growers. The mulches we studied were Douglas-fir sawdust, a thin layer of yard-debris compost topped with sawdust and black weed mat. We evaluated these mulches in ‘Duke’ and ‘Liberty’ from establishment (Oct. 2006) through maturity (2016). Mulch type had no effect on cumulative yield (year 2 through 10) of ‘Duke,’ but in ‘Liberty’ was 11 percent greater with weed mat than with sawdust. There was no effect on fruiting season, berry weight, Brix or firmness. We did observe a higher presence of vole runs under weed mat than in the other mulches. Learn more about yield and quality effects at: https://journals.ashs.org/hortsci/view/journals/hortsci/52/9/article-p1201.xml?rskey=7SvtZm

Amanda Davis and Pat Jones, Faculty Research Assistants, lifting the crown and roots of a plant on March 1, 2019 (showing growth after two seasons). Plant had sawdust topped with black weed mat mulch. (photo B. Strik, OSU)

While soil pH remained within the ideal range for blueberry during the entire study (4.5 – 5.5), by the end of this trial weed mat led to reduced soil organic matter as compared to sawdust mulch (2.9 percent compared to 3.3 percent). This was very likely due to the higher soil temperature measured under weed mat (9ºF warmer at 2 inches deep). Weed mat increased irrigation requirement by 50 percent to maintain a similar soil water content as compared to sawdust. Learn more about mulch effects on soil properties and soil and plant nutrients at: https://journals.ashs.org/hortsci/view/journals/hortsci/54/10/article-p1777.xml?rskey=7SvtZm

Weed mat offered advantages for weed control in our organic study, reducing cumulative costs by 50 percent compared to sawdust including costs of replenishing the sawdust three times and replacing the weed mat once from 2006-2014. Learn more about weed management costs at: https://doi.org/10.17660/ActaHortic.2017.1180.47

With advantages of reduced weed management costs, similar or improved yields and similar fruit quality, weed mat looked very promising. However, longer-term effects on reduced soil organic matter, possible negative impacts of higher soil temperature (especially in hotter regions), a higher irrigation requirement and increased presence of voles were concerning.

In 2006, we had the foresight to look at an alternative mulch treatment within the organic study. We evaluated ten cultivars in a yard-debris compost heavy treatment (amendment and as part of the mulch) as compared to a sawdust topped with weed mat mulch. All were drip irrigated. After ten years it was clear all cultivars performed best with the sawdust/weed mat combination. We also observed many fewer vole runs and noticed the sawdust did not decompose much under the weed mat. This showed great promise as an alternative mulch. Learn more at: https://journals.ashs.org/hortsci/view/journals/hortsci/52/6/article-p844.xml?rskey=7SvtZm

‘Duke’ in its fourth growing season (June 24, 2020) prior to first harvest. Mulched with sawdust topped with black weed mat. (photo B. Strik, OSU)

In October 2016, we established a new trial comparing sawdust, green weed mat, black weed mat, green weed mat over sawdust and black weed mat over sawdust. We initially wanted to compare black to white weed mat but could not find a white product with similar density and water infiltration rate as black weed mat. ‘Duke’ was grown on raised beds and was drip irrigated and fertigated in a conventional production system. Plants were pruned to produce a limited crop in year 2 (2017). Our objectives were to look at mulch effects on plant root and top growth, plant nutrient uptake and yield and fruit quality. Our collaborator (David Bryla, USDA/ARS) led data collection on soil and canopy temperature using sensors and a drone and irrigation requirement. This study will be finished at the end of the 2020 growing season. Unfortunately, the data collected on irrigation requirement were unreliable. We will summarize our other key findings here.

There was little difference between black and green weed mat. Soil temperature was higher at several depths under weed mat alone as compared to sawdust topped with weed mat and was coolest under sawdust alone. Weed mat also increased canopy temperature. Temperature differences affected plant growth in the first growing season. Mulch had no effect on rooting depth in year 1 or 2, but plants grown with sawdust alone or sawdust topped with weed mat had a wider root system than those with weed mat alone in both years. After the first two growing seasons, the most growth (total plant dry weight) occurred in sawdust topped with weed mat mulch and the least in weed mat alone. When plants were dug in winter, the presence of vole runs was rated. There were fewer runs in the sawdust topped with weed mat mulches than in weed mat alone. Learn more about this part of the study at: https://journals.ashs.org/hortsci/view/journals/hortsci/55/8/article-p1280.xml?rskey=7SvtZm

We dug up whole plants the first two winters and calculated nutrient uptake. In the first growing season, plants took up 11 pounds of nitrogen (N) per acre (with black weed mat) to 15 lbs/acre (with black weed mat over sawdust, due to more growth). In the second growing season there was no effect of mulch and N uptake averaged 29 lbs/acre. Plants took up 8 percent to 12 percent of the fertilizer N applied in year one and 42 percent in year two – plants become more efficient at taking up fertigated N as the root system gets larger. Uptake of other nutrients was unaffected by mulch and, depending on the year, ranged from 1.2–3.8 lbs/acre of P, 3.6–7.1 lbs/acre of K, 1.9–4.4 lbs/acre of Ca, and 0.9–1.3 lbs/acre of Mg. Since we did not apply any fertilizer other than N (nutrients were sufficient in soil and leaf nutrient analyses), each of these other nutrients were derived from the soil or decomposing roots. Clearly blueberry plants have a low nutrient requirement during establishment as compared to many other crops. Learn more about this study at: https://journals.ashs.org/hortsci/view/journals/hortsci/aop/article-10.21273-HORTSCI15271-20/article-10.21273-HORTSCI15271-20.xml

We collected our last year of yield and fruit quality data this summer – fortunately on-going research is considered an essential function during this pandemic and we were able to manage this trial and have pickers wear masks. In 2018, the second growing season, mulch affected yield but there were few differences in berry quality or fruiting season. Plants mulched with black weed mat alone had a significantly lower yield than those mulched with sawdust topped with black weed mat – all of the other mulch treatments were intermediate (Table 1). Berries were larger in plants mulched with sawdust than with black weed mat alone with all other mulches intermediate. In 2019-20, the third and fourth growing seasons, there was no longer an effect of mulch type on yield or berry weight. There were no mulch effects on berry firmness in any year and effects on Brix were inconsistent. These results are similar to what we found in our earlier trial – no difference between sawdust and black weed mat on long term yield or berry quality in organic ‘Duke.’ While there were differences among mulches in the second growing season, when the mulches were well exposed to the sun, these differences disappeared when the canopies got larger. We found no difference between green and black weed mat mulches. We will publish these results soon.

Compiling our three trials on mulch types and impact on growth, we have come to the following conclusions:

• Weed mat mulch reduces weed management costs compared to sawdust mulch.

• When comparing weed mat alone to sawdust alone, yield and fruit quality were either the same or improved in short and long-term studies.

• Using weed mat as a mulch may significantly increase irrigation requirement compared to sawdust mulch alone. Adding a layer of sawdust under weed mat decreases soil temperature and likely reduces irrigation requirement compared to weed mat alone (however, we have no data on this).

• Weed mat alone decreases soil organic matter, which is likely a disadvantage in long-term plantings. Organic matter can be added under weed mat when it is replaced, but improvements in soil organic matter would then take time.

• While adding a layer of sawdust under weed mat at planting establishment increases costs, the sawdust does not decompose very quickly (value). Topping sawdust with weed mat offers benefits of cooling soil relative to weed mat alone (preserving soil organic matter) and likely reduces presence of voles. It increased yield in the second growing season compared to weed mat alone in our trial.

• When using weed mat as a mulch, especially on raised beds, remember to drip irrigate and fertigate under the mulch. When granular fertilizers need to be applied, open the weed mat to do so.

We appreciate research support provided by the Oregon Blueberry Commission and the Agricultural Research Foundation and in-kind support from Fall Creek Farm & Nursery and Bird Gard LLC.