The benefits of a woodchip mulch layer over the root system of a tree are now widely recognized. These include conservation of soil moisture to reduce the detrimental effects of drought and weed suppression, and, as the mulch layer is degraded, the nutrients released provide a valuable organic fertilizer. In this article, we’ll look at ongoing research in mulch formulation, namely the use of a single-species or pure mulch to enhance transplant survival and protect against soil-borne diseases.

Soil-moisture stress

Soil-moisture stress is recognized as a major cause of tree decline within U.S. landscapes. If newly planted trees are not irrigated, their survival relies heavily on precipitation. If a transplant does not receive sufficient precipitation, especially during the period of new root growth, internal water deficits increase due to water transpiration and non-absorption of water from the soil.

Physical loss of the root system during lifting from the nursery bed can result in as little as 5% of a tree’s root system being moved with a tree. Following leafing out in spring, the ability of the root system to supply leaves with water can be severely restricted. Water deficits are therefore regarded as one of the main causes of failure of newly planted trees in urban landscapes.

With hotter, drier summers predicted throughout the U.S. due to climate change, techniques to prevent or reduce moisture stress of landscape trees are now of importance. Mulching to reduce soil-moisture stress, suppress weeds and fertilize plants has been used in forestry, agricultural, fruit and ornamental crop-production systems for decades.

A mulch layer across the soil surface under the canopy of a tree has been shown to minimize soil temperature and moisture fluctuations, stimulate root growth, provide essential soil nutrients, reduce soil erosion, suppress soil- and root-borne pathogens and enhance soil biological activity, as well as improve soil aeration.

In addition, mulches can reduce damage to the tree trunk caused by mower and trimmer activities, and can act as a natural buffer to prevent de-icing salts from percolating into the soil around the root zone.

Presently, a number of inorganic (crushed stone, crushed brick, gravel, polyethylene films) and organic mulches (shredded branches, leaves bark, woodchips, sawdust, pine straw and mixes of the above) exist for landscape use. Based on available research, the use of organic rather than inorganic mulches is suggested for improving the growth of newly planted trees and/or enhancing the health of established trees.

Pure or single-species mulches

Although woodchip mulches are widely applied to trees, few studies exist focusing on the effectiveness of woodchip mulches derived solely from one tree species – defined in this article as a “pure mulch.”

For example, are mulches derived solely from oak better than ones derived solely from apple or maple for enhancing tree-transplant establishment or improving tree vigor? Likewise, would an oak transplant perform better when a mulch derived solely from oak woodchips is used compared to a mulch from another tree species? Or is there one species of tree from which a pure mulch is ideal for all other species? Finally, the influence of mulches on enhancing resilience to pest and disease attack has received little study.

Several mulching trials have been undertaken during the past 15 years at the Bartlett Tree Research Laboratories, both in the U.S. and the UK, to answer these questions.

Container experiments

The first such experiments performed in the UK in 2009 used bare-rooted stock of beech (Fagus sylvatica) and hawthorn (Crataegus monogyna), as beech is regarded as transplant sensitive, while hawthorn is regarded as transplant tolerant. Each bare-rooted tree was planted into 7.5-liter (2-gallon) pots containing John Innes No. 2 compost medium.

Pure mulches were then prepared from European beech (Fagus sylvatica), hawthorn (Crataegus monogyna), silver birch (Betula pendula), wild cherry (Prunus avium), evergreen oak (Quercus ilex) and English oak (Q. robur). All mulches were made during December when the trees were dormant and when, with the exception of evergreen oak, no foliage was present.

Mulches were applied in early February, when trees were dormant. Each pure mulch was applied to a depth of 12-15 cm (4.72-5.9 inches), using 10 trees per pure mulch. Tree survival and growth were recorded in late September, i.e., toward the end of the growing season. No fertilizers were used throughout the experiment, although irrigation was applied as required.

Results showed that each pure mulch had a marked influence on survival and growth by the end of the growing season. In the case of beech, survival rates of non-mulched control trees was 10%. However, use of a pure mulch increased survival rates by 20-70%, depending on the type of pure mulch used. The pure mulch providing the highest survival rate was that derived from hawthorn. (Figure 1) Not only were survival rates improved, but also there was a marked difference in the appearance of the surviving control beech tree (Photo 1) compared to a hawthorn-mulched beech tree. (Photo 2)

Field trials

Based on the results of the container-pot experiments, several field trials have been instigated using larger 1-1.5-meter (4.9-foot) trees of a range of species, including spruce (Photo 3), beech, sycamore and red maple. At the time of planting, all trees were root pruned by 45-55% of total root volume to produce a root:shoot ratio of 0:33, a ratio associated with transplant stress in trees. In addition, other experiments using smaller, 2- to 3-year-old trees looked at the effect on root growth of mulching just half of the root system. (Photo 4) Finally, the effect of different types of pure mulches on one tree were evaluated by mulching four individual quarters around the root zone. (Photo 5)

Results showed that the type of pure mulch can have a marked effect on growth above ground. For example, Photo 6 shows the canopy of sycamore (Platanus occidentalis) with no mulch compared to Photo 7, where a pure mulch derived solely from apple (Malus Floribunda) was used.

Mulching for disease control

Few studies exist focusing on the effect of mulches derived solely from one tree species, or on their potential to manage soil-borne diseases, such as Phytophthora and Armillaria pathogens, or above-ground diseases such as apple cedar rust. Consequently, early research focused on a range of pure mulches to reduce the development and impact of pathogen severity caused by Phytophthora cactorum, and P. criticola. Table 1 shows data on containerized white flowering horse chestnut (Aesculus hippocastanum) infected with Phytophthora cactorum and P. criticola.

Application of a pure mulch significantly influenced root-rot lesion severity of both Phytophthora pathogens. In the case of P. cactorum, root-lesion severity was reduced by 53% (hawthorn pure mulch), 45% (cherry pure mulch, silver birch pure mulch), 61% (English oak pure mulch, beech pure mulch) and 39% (evergreen oak pure mulch) compared to non-mulched controls.

In the case of P. criticola, root-lesion severity was reduced by 44% (hawthorn pure mulch), 36% (cherry pure mulch), 33% (silver birch pure mulch), 53% (English oak pure mulch), 39% (evergreen oak pure mulch) and 61% (beech pure mulch) respectively compared to non-mulched controls. Such a result shows how mulches can provide a simple and effective cultural means of managing Phytophthora. How mulching influences severity of other soil-borne pathogens is currently under investigation at the Bartlett Tree Research Laboratories.

More recent trials have shown that application of a pure mulch made solely from willow (Salix alba) can significantly influence the severity of a naturally occurring outbreak of apple cedar rust. Photo 8 shows apple cedar rust severity on non-mulched controls compared to Photo 9, where a mulch made solely from willow was used.

Why the differences between mulches?

It is well recognized that trees contain a wealth of chemicals involved in tree defense, growth, flowering, seed set, etc. We suspect that as a pure mulch degrades, many of these chemicals are released into the soil and taken up by the roots, in turn exerting specific effects on tree biology. As the chemicals contained within a pure mulch differ from tree species to species, this may account for the differences in transplant survival and disease management recorded between mulches.

In support of this, a mulch derived solely from cypress trees has been shown to reduce the growth of several plants, including hydrangeas, spirea and viburnums. Since cypress trees are known for their resistance to decay fungi due to an inherently high phenolic content within their wood, it was suggested that leaching of phenolics into the soil would in turn inhibit the root growth of surrounding trees.

Research has shown mulches made solely from Eucalyptus grandis were high in organic oils and acids that in turn were toxic to germinating seedlings of several plants. Another example is the tree-of-heaven, or Ailanthus altissima, which contains high concentrations of Ailanthone, an allelotoxin known for its potent post-emergence herbicidal activity.

Other chemicals, on the other hand, are very effective at stimulating root growth. For example, hawthorn, apple, pear and cherry woods are high in sugars such as sucrose and sorbital. Applying sugars around the base of transplanted trees has been shown to promote root growth and mycorrhizal associations. Other studies have shown that fresh and composted mulch made from Eucalyptus cladocalyx enhanced transplant performance of Platanus racemose, while fresh pine bark mulch has been shown to promote establishment of English oak (Quercus robur).

Guidelines for applying mulches

1. Mulch should be applied from the drip line to the trunk. If this is not practical, minimum mulch-circle radii should be 0.3 m (1 foot) for small trees, 1 m (39 inches) for medium trees and 3 m (9.8 feet) for large trees.
2. When applying mulch, it is useful to kill or remove existing ground cover, or at least to mow the grass very short and remove clippings. Mulch should be applied directly to the soil surface; do not use landscape fabric to separate the mulch from the soil.
3. A mulch layer should be 5-10 cm (2-4 inches) thick, depending on the tree species and type of mulch applied.
4. To avoid root disruption, mulch should not be removed. Additional mulch should be added on an annual basis to maintain 5-10 cm.
5. Mulch should not be placed against the trunk. Mulch will retain too much moisture against the trunk, which may result in disease.

Conclusion

Results of our research and that of others show that pure mulches offer several benefits for trees. Survival rates of difficult-to-transplant tree species, such as beech, can be significantly improved by using an appropriate pure mulch. In addition, pure mulches can help in the suppression of soil-/root-borne diseases, such as Phytophthora, as well as above-ground diseases, such as apple cedar rust, with limited chemical input.

These benefits have positive implications not only for professionals involved in the care and maintenance of trees growing in the urban landscape, but also for agricultural, forestry, orchard and horticultural crop production. Importantly, the use of pure mulches requires very little capital investment and only small adjustments to standard management-aftercare procedures.

Glynn C. Percival, Ph.D., is the senior arboricultural researcher at the Bartlett Tree Research Laboratory based in Charlotte, North Carolina, a division of Bartlett Tree Experts, a 49-year TCIA member company.
Initial results of some of the research reported here were included in an article that originally ran in the July 2010 issue of The Horticulturist, the journal of the Chartered Institute of Horticulture.

Selected references

Buckstrup, M.J. and Bassuk, N.L. (2000) Transplanting success of balled-and-burlapped versus bare-root trees in the urban landscape. Journal of Arboriculture. 26(6): pp. 298-308.
Case, L.T. and Mathers, H.M. (2006) Herbicide-treated mulches for weed control in nursery container crops. Journal of Environmental Horticulture. 24(2): pp. 81-90.
Chalker-Scott, L. (2007). Impact of mulches on landscape plants and the environment – A review. Journal of Environmental Horticulture. 25:239-249.
Downer, A.J. and Faber, B. (2005) Effect of Eucalyptus cladocalyx mulch on establishment of California sycamore (Platanus racemosa). Journal of applied horticulture. 7(2): pp. 90-94.
Duryea, M.L., English, R.J and Hermansen, L.A. (1999) A comparison of landscape mulches: chemical, allelopathic and decomposition properties. Journal of Arboriculture. 25(2): pp.88-97.
Gilman, E.F. and Garbosky, J. (2004) Mulch and planting depth affect live oak (Quercus virginiana Mill.) establishment. Journal of Arboriculture. 30(5): pp. 311-317.
Gklavakis, Evangelos (2006). The use of pure mulches to improve root vigor and aid transplant success of urban trees. MSc Thesis. University of Reading, Reading, Berkshire, UK.
Hoitink, H. A. J., A. G. Stone, and D. Y. Han. 1997. Suppression of plant diseases by composts. HortScience 32:184-187.
Iles, J.K. and Dosmann, M.S. (1999) Effect of organic and mineral mulches on soil properties and growth of Fairview Flame® Red Maple trees. Journal of Arboriculture. 25(3): pp. 163-167.
Jin, Z., Chung, B.Y., Iiyama, K. and Watanabe, S. (2002) Changes of chemical components of leaf litter of ginkgo biloba during mulching. Journal of Arboriculture. 28(4): pp. 171-177.
Percival, G.C., Fraser, G.A. and Barnes, S. (2004). Soil injections of carbohydrates improve fine root growth of established urban trees. Arboricultural Journal. 28: pp. 95-103.
Percival, G.C.; Gklavakis, E; Noviss, K. (2009). The influence of pure mulches on survival, growth and vitality of containerised and field planted trees. Journal of Environmental Horticulture. 27(4): pp. 200-206.
Struve, D.K., Burchfield, L. and Maupin, C. (2000) Survival and growth of transplanted large- and small-caliper red oaks. Journal of Arboriculture. 26(3): pp.162-169.
Watson, W.T. (2005). Influence of tree size in transplant establishment and growth. HortTechnology. 15(1): pp. 118-122.