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Alternatives to Sawdust Mulch in Blueberries

Bernadine Strik, Ross Penhallegon, Joseph DeFrancesco, and Juliet Mann, Oregon State University

With the increasing cost of sawdust, we were interested in testing the efficacy of possible alternatives for surface mulching in blueberries. Mulch treatments were compared in a 1990 established planting at the North Willamette Research & Extension Center (NWREC) and at two grower cooperator sites in Lane County.

Up to 10 mulches, sawdust, mint straw, filbert chips, high density plastic, grass seed straw compost, bailed and broken straw, grass seed straw (perennial rye ‘Saturn’) at two depths, leaf mulch, composted yard debris, and crumb tire, were compared to an un-mulched control, depending on the year and site. Mulches were applied at a 2″ (grower sites) or 3″ (NWREC) depth, and were replenished, if necessary during the 3 year study.

In 1992, mulches did not cause a significant difference in soil temperature at a 2″ depth. However, seasonal average soil temperature at 2″ was affected by type of surface mulch. Crumb tire had a higher temperature (103øF) than all other mulches at the grower sites. Lowest temperatures at 2″ were found in the high density plastic (80øF; a white-colored product that may have reflected solar radiation compared to the black colored crumb tire) and yard debris (79øF). Temperature at 2″ in the sawdust was 85øF. The differences in soil temperature at these sites and at NWREC were not great, although significant, and may not be of any biological importance. Differences were even smaller at the 6″ depth during the growing season (69 to 72øF).

Crumb tire also had the greatest depth to moisture indicating that this is not a good mulch for moderating/conserving soil moisture. Nor did we find any blueberry roots up in the plastic or crumb tire mulches. At NWREC, depth to moisture tended to be highest in the crumb tire and the 8″ straw plots, especially in 1994.

The unmulched plots had significantly more weeds than any of the other treatments in 1993 at NWREC. Weed infestation in the composted yard debris was significantly greater than in the sawdust and crumb tire mulch treatments. Also, weed species composition was more diverse in this treatment; it is believed that a wider variety of weeds was introduced with the yard debris. The predominant weed in the straw mulch plots was perennial ryegrass, seed of which was introduced with the straw.

Percent leaf nitrogen was significantly less with the sawdust mulch than other treatments (fertilizer rate was not adjusted for treatment). No other significant differences appeared among mulch treatments when analyzing for soil pH, percent organic matter, and other soil and plant nutrients. No plant-pathogenic nematodes were found in the soil under any of the mulch treatments.

Total seasonal yield was collected per plot at NWREC in 1993 and 1994. No significant treatment effects were observed on yield in 1993 or 1994. The crumb tire plots tended to have the higher yield in 1994 (30 lb per plot compared to 22 for sawdust), but not in 1993. Grass seed straw treated plots produced yields equal to that of sawdust. Control or un-mulched plots tended to have the highest yield in 1993 but not in 1994. There was no treatment effect on date of 50% bloom, as we may have expected, particularly with mulches that increased temperature, such as crumb tire. There was little treatment effect on berry size in 1993, but in 1994, sawdust mulched plots had larger berries than control plots.

This study showed that there are many alternatives available to blueberry growers for sawdust which has become rather expensive. In this study, the control plots also performed well, supporting the findings of commercial growers who have good producing fields that are not surface mulched. However, mulches can help conserve moisture and control annual weeds.

Perennial ryegrass straw was equal to sawdust in its effects on blueberry growth, yield, berry size, and soil temperature. This may be an alternative for commercial growers who prefer using a surface mulch. The amount of grass seed straw required would be about 3.8 tons/acre/year or per 2 years. As weeds (grass seeds) were a problem with this product, grass seed straw used for a mulch in commercial fields should be weed-free.