Research Shows Sawdust Under Weed Mat is Best Mulch

Oregon State University Extension researchers have found that adding sawdust or organic mulch under weed mat overcomes issues with low soil organic matter that occurs under weed mat alone and can increase yield in blueberry.

Mature blueberry plants with dual weed mat over sawdust mulch. The weed mat has been opened and rolled back to fertilize. (courtesy B. Strik)

In a trial launched in 2016, Bernadine Strik, OSU Extension Berry Specialist and Berry Research Leader at the North Willamette Research and Extension Center, has been addressing concerns of low soil organic matter that arose after a ten-year trial on the effects of different mulches on plant and soil health. In the long-term organic study, when weed mat mulch was applied over bare soil, soil organic matter declined.

While weed mat was the most economical method for weed control and led to increased yield in the Liberty variety of blueberry, Strik and her Senior Faculty Research Assistant Amanda Davis were concerned that a continued decline in organic matter could have a negative effect on yield and other plant-health parameters in long-lived plantings.

“I was worried about that, because if any crop likes good soil organic matter, it is blueberry,” Strik said.

In the ongoing trial, Strik and Davis are studying the effects of three different mulch treatments, including sawdust topped with weed mat, compost plus sawdust topped with weed mat and bare soil topped with weed mat.

“What we found is that while the weed mat was either better or similar to the other mulch treatments, when we added weed mat on top of the organic mulches, our yield actually increased compared to the bare soil with weed mat on top,” Davis said in a presentation at the 2021 Oregon Blueberry Field Day, held July 15 in Aurora.

Mature Duke in the organic research trial being grown with weed mat over sawdust mulch. (courtesy B. Strik)

Further, Davis said, the research showed that the highest organic matter occurred where there was sawdust or compost with sawdust underneath weed mat.

“It is around three percent organic matter with weed mat alone, compared to up to five percent with the other treatments,” Davis said of organic matter. “And we found the yield was highest when we had just the sawdust topped with weed mat.”

“We are really confident that the best mulch for blueberry is the sawdust topped with weed mat,” Strik said. “Even though your mulch costs at establishment will be higher, you’ll get higher returns and you may get a higher yield in the early production years.” Strik recommended this combined mulch of sawdust topped with weed mat for organic as well as conventional production.

Also, Strik said, vole pressure is much lower when using sawdust under weed mat versus weed mat over bare soil. “That can be huge in young plantings,” she said.