Bark up the right tree

Bark is an important component of the substrate mix of many container nurseries. Pine bark in particular is popular in the southeastern U.S. Growers like to use bark because it promotes strong root anchorage for perennials, shrubs or other plants in containers.

There are several advantages to using bark in nursery mixes. Brian Cantin, grower advisor at Berger, cites improved drainage, a good quality to price ratio, and nutrient containment as strong points of using bark in your nursery’s growing media. Also, bark’s rate of decomposition is quite slow. This makes it an ideal element for growing long-term crops. Bark adds weight to containers, which leads to better pot stability.

However, the flip side of that advantage is the first disadvantage of using bark as a substrate. While its weight is helpful for preventing container blowover and improving stability, it also means higher shipping costs. Also, the chemical characteristics depend on decomposition rate. Cantin cautions that if bark is not managed properly, these characteristics can be variable from load to load. Another concern is that if the bark is allowed to dry under 35 percent water content, wettability becomes an issue.

Types of bark

In his writings on the subject, Ted Bilderback, nursery crops extension specialist at North Carolina State University, says pine bark is as variable as “soil” as a description of potting substrate component.

Jack pine (Pinus banksiana) and red pine (Pinus resinosa) are commonly used in the northern U.S. and Canada. In the south, loblolly pine (Pinus taeda) is common, and Douglas fir bark is popular in the Pacific Northwest.

Bark from hardwood trees is not as commonly used, as it typically contains toxic compounds, its high cellulose content causes it to decompose quickly, and it needs large amounts of nitrogen to decompose.

Softwood trees — especially conifers — are generally preferable due to lower amounts of toxic compounds, slower decomposition time, and smaller amount of nitrogen needed.

There are three types of bark typically used in substrate mixes: fresh, aged, and composted. Though there isn’t a universally-accepted definition of what exactly constitutes “fresh” or “aged,” a good general rule is that fresh bark is material that is sold shortly after removal from a tree, grinding, and screening to an appropriate particle size. Aged bark refers to material that goes through the same preparation process, but then also sits in undisturbed piles several months before use.

Fresh bark may contain phenolic compounds, which can inhibit plant growth, says Cantin. It offers rapid decomposition, which causes nitrogen depletion in the medium. It’s often very coarse, and water is less easily available compared to aged or composted bark.

Fresh bark has little uniformity of particle size, often containing too few fine particles for adequate water retention. Growers often add sand or peat moss to a fresh pine bark mix in order to increase moisture content in the substrate.

One of the disadvantages of fresh bark is nitrogen immobilization. Aging and composting are two techniques to combat nitrogen immobilization through breaking down toxic compounds.

Aged bark production is fairly simple. Take fresh bark and just add time. Specifically, six months to a year. Cantin says that is the necessary time to age bark for substrates. Any less and the toxic compounds may not be fully degraded.

There are several advantages to using aged bark in your potting mixes, including low nitrogen tie-down, smaller particle size than fresh bark, and toxic compounds are degraded.

Composted bark is more complicated, requiring the addition of a nitrogen source, applying water, monitoring the water content, and turning the piles for aeration to bring oxygen. Composted bark brings the same smaller particle size and degradation of toxic compounds of aged bark, with even lower nitrogen tie-down and the added benefit of sterility. The high temperatures that arise during the composting process kill weed seeds, pathogens and insects.

Of course, all this extra preparation work adds to the cost. A grower must decide whether to learn to manage fresh pine bark potting mixes, and the more frequent irrigation they require, or develop a bark handling area at the nursery. In his writings, Bilderback says a bark handling area would preferably be a concrete pad large enough to turn the windrows frequently. It should be at a higher elevation than the nursery growing areas to prevent runoff from entering the bark handling area. A drainage system is also necessary to prevent swampy conditions in the bark handling area.

Effects on growth

Comparisons of fresh and aged pine (Pinus spp.) bark on plant growth have been made with several crops.

A 2008 study funded by Oregon Association of Nurseries and conducted by several Oregon State University researchers and James Altland, a research horticulturist with USDA Agricultural Research Service, researched growth differences between fresh and aged Douglas fir bark. A secondary objective was to document the immobilization and decomposition rates in fresh and aged Douglas fir bark to better understand those differences. The Oregon study, published in the Nov.-Dec. 2008 issue of HortTechnology, found that geraniums grown in fresh bark were uniformly smaller than those in aged bark.

In an oft-cited Australian study comparing fresh, aged and composted Pinus radiate bark, shifting from fresh to aged created a modest shoot size increase in petunia and gardenia grown in a mix with 40 percent sand and 60 percent wood waste. But the weight of the shoots more than doubled from fresh to composted bark.

In a 2004 study from North Carolina State, Tiffany Harrelson, Stuart L. Warren and Bilderback compared fresh and aged pine bark and three rates of controlled release nitrogen fertilizer; bearberry cotoneaster (Cotoneaster dammeri ‘Skogholm’) grown in aged pine bark were larger than cotoneaster grown in fresh pine bark.

For more:www.berger.ca; www.nurserycropscience.info; horttech.ashspublications.org/content/18/4/619.full