This project was done at the Morris Farm, a community-owned farm in Wiscasset, and involved a comparison of hemlock bark (readily available in Maine as a sawmill by-product) with the more usual straw, as a mulch for raspberries. It was begun by Chris Bailey, who was the farm manager until the fall of 1998. Following his departure, the project was overseen first by Rick Harding, his successor, and then by Tom Hoerth, who submitted the final report.
The experiment was conducted on a ½-acre patch of Killarney cv. raspberries. The soil here is a Buxton silt loam, with a pH, thanks to liming, of 6.7. The preceding cover crop of oats, hairy vetch, and rape was plowed under in May, 1998, and the mulch spread—bark to a depth of 4” in some places, and straw to a depth of 4-6” in others. The raspberries were planted into the mulch, and tended according to organic practice. The parameters of interest were weed control and crop vigor, as indicated by the growth of primocanes and floricanes. It had also been proposed to examine the effects of mulch, and its removal over the winter, on the incidence of Phytophthora, but as it turned out this disease was completely absent, thus no comparison of causal influences was possible.
The summer of 1999 was exceptionally dry, which necessitated frequent irrigation. While irrigating Tom noticed that the ground under the straw mulch dried out more quickly than that under the bark. He also reported fewer weeds in the bark-mulched areas, as compared with the straw.
On August 19-20, 1999 primocane and floricane heights and densities were measured in the two treatments.
Results: Mean heights were almost identical. Floricanes measured 25.5”, whether mulched with straw or bark. Primocanes measured 19.4” in the areas mulched with bark, and 18.9” in the straw mulch. Densities, however, were very different. Floricanes in the bark-mulched areas averaged 0.71/ft, but only 0.60/ft in the straw-mulched plots. Primocanes averaged 4.4/ft in the bark, vs. 3.1/ft in the straw. Thus we see that bark mulch can increase raspberry cane densities without stunting them.
Tom also made a cost comparison of the two mulches; he found that while the bark was more expensive and required more labor for application in the first year, it decayed more slowly than the straw, and more effectively suppressed the growth of weeds. The need to replace the straw in the second and third seasons, together with the increased labor cost of weeding in the straw-mulched plots, made the latter, in the long run, much more expensive. Tom reckons his total cost for the straw mulch system, for 100 feet of plantings, at $220 over three years, compared to $157 for the hemlock bark system.