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Mulching – overlooked and underutilized

Effect of depth of organic mulches on trunk cross-sectional area (TCSA) of young pecan trees. Four data points represent mean for all mulches: leaves, pine nuggets, pine straw, grass clippings, and chipped limbs. Zero depth treatment was unmulched and herbicide-treated. Regression equation was y = 33.2 + 1.2(D), r2 = 0.15, P = 0.0006, n = 75. Each point is mean of 20 trees plus standard errors (indicated by vertical bars).

It shouldn’t be surprising that pecan trees grow better when mulched, as mulching mimics the natural state of trees growing in a forest where the forest floor is covered with a layer of decaying organic matter. Roots developing underneath this layer are growing in a favorable environment, where temperature and moisture are more uniform, soil compaction is less, organic matter is higher and beneficial mycorrhizal fungi that aid root absorption of nutrients proliferate. Roots grow liberally all the way to the soil surface and mulch-soil interface.

Mulching of old trees in bearing orchards may not be feasible as the mulch would interfere with efficient harvesting of the nuts. On young trees, however, mulch can be applied soon after trees are planted, and it will deteriorate while trees are growing and be gone before harvest is an issue. In the meantime, substantial growth increases from the mulch effects will be attained, and early yields will be magnified. The growth increase from mulching will be greatest when trees are unirrigated, but is still substantial even when compared to the industry-standard practice of weed free and irrigated.

Foshee et al. (1996) mulched young unirrigated pecan trees with either leaves, pine nuggets, pine straw, grass clippings, or chipped limbs at depths of 0, 4, 8 or 12 inches in a 10-feet-by-10-feet area around the trees for 3 years. Mulch treatments were compared to an unmulched weed-free herbicide treatment (the industry standard), and to common bermudagrass sod. Trunk cross-sectional area of the mulched trees after 3 years was larger than from unmulched weed-free trees and from trees in sod. Trunk size of mulched trees increased linearly as mulch depth increased up to 12 inches deep (Fig. 1). All mulch types resulted in similar increases in trunk cross-sectional area. Mean trunk cross-sectional area for mulched trees increased 14-fold compared to an increase of 8-fold for the unmulched trees and the sod in this 3-year study. Our data indicate a 60 percent to 70 percent growth increase from mulch compared to H and sod, respectively.

Now a 60-percent increase in growth compared to the industry standard of weed-free herbicide treatment is not to be taken lightly. Consider that, in pecans, early growth correlates positively with early production (Patterson and Goff, 1994). In that study, a growth improvement of 151 percent (over 7 years) was associated with a 5-fold increase in yield through the first 7 years from young ‘Desirable’ trees. In the mulch study above, tree growth over 3 years was improved 60 percent over 3 years, and likely would be much more than that if the study had measured improvement over 7 years. We would expect, even though the study was not continued to measure yield, that yields in early years would be substantially improved, perhaps doubled or more. This presumption of high yields from mulched trees has been strengthened as I have observed yields from trees in Richard Merritt’s orchards near Weston, Georgia, where young trees for many years have been routinely mulched with the pecan shells from his shelling operation. Also, trees I have planted myself are routinely mulched, and growth, even without irrigation, has been impressive.

Smith et al. (2000) mulched seedling pecan trees with wood chips in either a 3.3-feet-by-3.3-feet or 6.6-feet-by-6.6-feet square to a depth of 30 cm (11.8 inches). Mulch treatments were in factorial combination with 2 nitrogen rates applied as either a single application at budbreak or as a split application at budbreak and 3 weeks later. Tree height was positively related to mulch width each year of the 3-year study, and trunk diameter was positively related to mulch with during the second and third years of the experiment. Leaf phosphorus and potassium concentration during 2 years and leaf nitrogen during one year of the study were positively related to mulch width. Trees receiving the higher nitrogen rate were taller during 2 of the 3 years, but leaf nitrogen concentration was not affected by nitrogen rate. No differences in the parameters measured were observed whether nitrogen was applied as a single or as a split application. Irrigation was applied with traveling gun whenever tensiometers at the 30 cm depth (11.8 inches) in the treatments without mulch averaged-40kPa.    It is important to note that in the Smith study, control trees were irrigated and kept weed free with herbicides, the industry standard. When mulch was applied in addition to the weed control and water, tree growth over 3 years increased 36 percent from the mulch effects. As noted above, resulting early yield would likely be substantially improved from mulch, much more than 36 percent.

Is mulching feasible? Young pecan trees are especially suited to mulch compared to most other plants because the trees are widely spaced so that, on a per-acre basis, treating a comparatively small area to the relatively few plants is more cost-effective and efficient.

There are many sources of mulch that would be suitable. Powerline trimming crews generate a lot of wood chips and often look for a place to dump it. Yard waste, leaves and grass clippings mostly, take up a lot of volume in landfills, and makes excellent mulch. Trimmings from hedging can be ground up into chips, and used as mulch. Tub grinders can create mulch from limbs and unusable small trees where orchards are thinned or land is cleared. In one orchard I had, I had mulch applied to young trees, where the provider supplied the mulch and applied it for me at a cost of less than $3 per tree, about the same as 1 pound of pecans is worth, so yield improvement wouldn’t have to be much to make it worthwhile. We planted one orchard by planting a fast-growing biomass plant called Sunn hemp (Crotalaria juncea L.). This is a legume that when grown as a summer annual can produce over 5,000 pounds of biomass and over 100 pounds of nitrogen per acre in a period of 8 to 12 weeks. The cover crop was grown the year prior to planting the pecan trees, then was cut and swept and blown around the newly planted pecan trees, thus the mulch was “grown in place” and improved the soil as well as providing the mulch.

Apply mulch liberally. The linear increase in growth depicted in Fig. 1 would suggest improvement up to 12 inches deep around pecan trees on well-drained soils. Most of the increase captured when mulch was 8 inches deep, so 8 to 12 inches is a good rule of thumb for mulch depth. A 6-feet-by-6-feet area, with more even better, should be treated around each tree. Some studies have been conducted with mulch only about 3 inches deep, with correspondingly lesser effect as Fig. 1 would suggest.

Don’t worry about nitrogen tie-up. Green mulch can “tie-up” nitrogen initially, before eventually breaking down and supplying nitrogen and other nutrients. This concern is not important, as nitrogen is easily monitored with leaf samples and small applications of supplemental nitrogen if necessary can quickly overcome it. Green mulch materials can go through a heat, with potential harm to the trunks of young trees. We used a tree guard in our mulch studies to keep the mulch from direct contact with the tree trunks, and have had no problem. Mike Smith allowed the mulch to remain in a pile until it had gone through the first initial heat before applying to the trees.

Erosion breaks. When pecan orchards are planted on sloping land, and weed-free strips are maintained, water can run down the weed-free strip and create erosion problems. A pile of mulch 12 inches deep can be angled in such a way as to divert the water coming down the slope over into the grassy middle, providing an effective erosion break, while also providing the benefits of the mulch itself.

Author Photo

Bill Goff

Bill Goff is an extension and research horticulturist at Auburn University.