The last issue focused on using living mulches. The use of killed cover crops is another way to control vegetative competition. Everyone knows that undesirable weed growth can destroy tree plantings. Mulches have been used for ages to control this unwanted weed growth. While mulches work well for small plantings, for large plantings, the logistics of making it actually happen are daunting. It can take over 10 tons of straw to mulch one acre of plantings (about the same area in a mile of a single tree row). The transport and spreading of this mulch, as well as periodic replacement, is extremely labor intensive. The use of synthetic fiber mulches as a substitute for these organic mulches has potential but the convenience and labor savings is replaced by quite high costs. Killed cover crops have the potential to reduce the labor and cost associated with using mulches in tree plantings.
This short summary will discuss one project in Minnesota. In Minnesota, hybrid poplar plantations for pulpwood have been promoted as an alternative cover for marginal farmlands. The main key to success for these plantations has been effective weed control. Sites without good weed control have generally failed. For this reason, clean cultivation with herbicides has been utilized as a primary means of weed control. This is estimated to cost more than $140 an acre for the first two years of maintenance (at the time of the project, now fuel costs would push this somewhat higher). For this project, we decided to use a killed cover crop of sorghum-Sudan grass as a potential alternative to the costly tillage and herbicide applications. We estimate this site treatment to cost $33 (once again, this is at the time of the project). This is a significant reduction in cost over conventional tillage and herbicides.
We conducted this planting in June 2001. The sorghum-Sudan was broadcast and tilled in. This was chosen due to availability of equipment. No further treatments were given to this site. Because sorghum-Sudan naturally winter kills in Minnesota, no chemical application was needed. The following spring, the dead stalks were cut with a sicklebar mower. Originally, we had hoped that winter weather would compress the stalks into a mat (thus reducing costs); however, as this did not occur, we mowed and windrowed the stalks. Ideally, this would be done after a killing frost in fall. We planted four rows of hybrid poplars cuttings (locally called "sticks") in spring of 2002. Two rows were maintained by conventional tillage and some hand hoeing. Two rows were planted into the windrowed sorghum-Sudan. In August, survival and height growth were measured.
One general comment is that few weeds grew through the sorghum-Sudan windrow. Approximately 1 weed per 8 feet of row was seen (ocular estimate). While even fewer were in the conventionally treated rows (we were quite diligent about weed control), this is certainly below any threshold where important impacts would occur on tree growth. This low level of weed infestation may be partly due to a recent history of these plots being in row crops; however, this can not be tested with this dataset as we have no sites in areas that were not in row crops.
We saw a higher percentage of hybrid poplar cuttings successfully rooting in the sorghum-Sudan windrows (85%) than in the conventional tillage (51%). This is probably partially due to abnormally low soil moisture conditions during and immediately after planting. Growth was comparable between both treatments (for trees that successfully rooted) with a mean of 1 1/3 feet of height by August.
While these results look encouraging, this deserves more testing before implementation on a broader scale. Killed cover crops are a success story with many horticultural crops. We believe that with further testing and efforts with a broader array of cover crops, that this method can reduce the costs and environmental impact that results from clean cultivation in tree plantings. This may not work under all conditions but it appears to have great potential in short-rotation woody crops.
For more specifics, please see Demchik, M.C. and Krause, N. 2007. Growing Your Mulch on-Site: Using Site Produced Sorghum/Sudan as a Mulch for Hybrid Poplar. Northern Journal of Applied Forestry. 21(1):77-78.
By Mike Demchik, College of Natural Resources, University of Wisconsin Stevens Point.