USDA researcher defends horticultural peat as a sustainable growing substrate

Containers were filled predominantly with field soils in the early 1900s. The use of peat for horticultural substrates began in the late 1950s.

Today, peat has revolutionized container plant production by creating a root-zone environment with less salinity, greater drainage and fewer root-borne diseases than those filled with mineral soils. It is unofficially considered the gold standard in growing media by most horticulturists and the media to which all other components are compared.

With the ever-growing focus on western Europe and their plans to phase peat out, its use in the United States has become the focus of negative press. How much of this bad press is deserved? Extracting peat for horticultural purposes has an environmental cost. But let’s look at the realities of this process and weigh the pros and cons. And instead of using hyperbole and vague declarations, let’s use science and facts.

Peatlands occur in Asia (38.4%), North America (31.6%), Europe (12.5%), South America (11.5%), Africa (4.4%) and Australasia (1.6%). However, the U.S. gets more than 85% of its sphagnum peat from Canadian peatlands, and Canada exports more than 90% of its peat to the U.S., so for the remainder of this article, we will focus on North American sphagnum peat supply and demand.

North American peatlands

Ombrotrophic peatlands (or bogs, the type of peatland used for horticultural peat harvest) are a type of wetland ecosystem with a broad diversity of plant life but dominated by species of Sphagnum moss. As mosses grow slowly year by year on top of previous years’ growth, older layers below the living surface partially decompose. These partially decomposed peat layers are characterized by low pH due to the buildup of humic acids and are anoxic due to being saturated with water and consequently low levels of oxygen.

These characteristics, plus the relatively cold soil temperatures throughout Canada, slow the decomposition rate. This leads to a buildup of partially decomposed sphagnum peat in a near-permanent condition, and thus a long-term storage bank for atmospheric carbon. Preparation of the peatlands for horticultural peat harvest allows the partially decomposed peat to decompose more rapidly and emit CO₂ into the atmosphere, becoming a source of CO₂ instead of a sink.

There are 294 million acres of peatlands in Canada, according to the United Nations Global Peatlands Assessment. That’s almost three times the land area of California. Of that area, approximately 43,500 acres are currently used for peat extraction (with another 34,500 acres having been harvested in the past). That’s about 0.03% of the total peatlands in Canada used for horticultural peat.

Greenhouse gases from peat

The crux of almost every argument against horticultural peat is that it results in the emission of greenhouse gases. None of the gardening blogs, nor the respected nationally syndicated newspapers or magazines, have bothered to offer any details or numbers on this crisis. But I will.

Carbon storage and loss from landscapes (or any system) is often described in terms of net ecosystem exchange (NEE). Intact peatlands in an undisturbed or natural state have a negative NEE of about -286 kg of CO₂ per hectare (1 hectare is equal to 2.4 acres), meaning they accumulate and store more carbon than they emit. For obvious reasons, this is very good in terms of reducing the impacts of global warming.

The process of draining the peat and exposing the subsurface layers to the atmosphere for harvesting allows peat to decompose and release CO₂ into the atmosphere. The Intergovernmental Panel on Climate Change estimated (using the crudest models and data available) that an exposed peat bog prepared specifically for horticultural peat extraction releases about 2.8 tons of CO₂ per hectare per year.

Research by Dr. Hongxing He at McGill University used a coupled heat and mass transfer model for soil-plant-atmospheric systems to predict carbon loss more accurately at 1.5 tons of CO₂ per hectare per year.

Dr. He validated these models with extensive on-site measurement of CO₂ flux, soil moisture, soil temperature and water levels over a period of three years and found the revised estimates far more accurate than the IPCC estimate.

To put these numbers into perspective, a single flight on a private jet from Dallas to Chicago releases over 4 tons of CO₂ into the atmosphere.

Restoring peatlands after harvest

Another argument against the use of horticultural peat is that peatlands can never be restored, at least not in our lifetime. That is a gross oversimplification that glosses over several important details. Let’s look at the numbers.

Once a peatland is opened for extracting horticultural peat, it will usually be harvested for 20 to 40 years, although some deeper bogs could be harvested longer. If harvesting conditions are optimum, up to 3 inches of peat is typically removed each year.

A typical peat bog is usually harvested to a remaining depth of 20 inches before it is restored back to a functioning peat bog. The coveted layers of partially decomposed peat only accumulate at a rate of approximately 0.04 inches per year.

If a depth of 75 inches were harvested or removed, it would then take about 2,500 years for that peat layer to reform. This is the crux of the argument suggesting that peat bogs can never recover.

There are two practical rebuttals to this assertion. First, considering the infinitesimally small areas being harvested for horticultural peat compared to the massive land area in Canada covered by peatlands, the total mass of stored carbon in partially decomposing peat layers is accumulating every year despite the harvests.

Let’s assume liberally that 4 inches of peat is harvested annually from 43,500 acres currently being extracted, while just 0.04 inches accumulate on the undisturbed 294 million acres. That would equate to approximately 23 million cubic yards of peat harvested, with about 1,500 million cubic yards of new peat accumulated. The peat bogs are growing despite the harvest of horticultural peat.

The second rebuttal is far more important. While it may take millennia for the bogs to reaccumulate the same depth of peat, the bogs return to a carbon sink relatively quickly. After peat is no longer harvested from a given bog, it is restored through a process called the “moss layer transfer technique.”

This technique was developed by Dr. Line Rochefort and her team at Laval University in Quebec. It involves taking living fragments of Sphagnum moss from a donor bog and applying them to the surface of the bog being restored or transitioned back to a native-like ecosystem.

Rochefort and her colleagues from the University of Waterloo and McGill University found that 14 years after restoration of a post-extraction peatland, the bog returned to an annual carbon sink, with an NEE of -78 grams of carbon per meter per year. There are many factors that can affect the rehabilitation of a post-extraction peatland. Nonetheless, this and other research has shown that peatlands become functioning carbon sinks between 10 to 20 years post-restoration — well within our lifetime.

Alternatives to peat

Bark, wood fiber, coconut coir, rock wool, perlite, sawdust and compost are among the most cited products that could serve as alternatives to peat. Many of these products are excellent in very specific applications or can replace a relatively small fraction of the normal peat volume in horticultural substrates. However, none of them (except possibly coir) are suitable to replace peat over the broad range of applications in which it is currently used.

Coir is fibrous, abundant, compressible for shipping, easy to mix with other substrate components and amendments, has a relatively high water-holding capacity and has been shown to be an effective substrate across a broad range of crops and applications.

Furthermore, fertilizers, plant growth regulators and pest management products that have been developed over the past six decades (largely for use in peat-based substrates) work similarly and effectively in coir-based substrates. There are few modifications that growers need to consider when switching to coir. Other substrates require extensive modification in irrigation, fertilization and pest management. But is coir more sustainable than peat?

Figure 1 shows the global warming potential (GWP) of peat, coir and rock wool (or perlite) as calculated by three different groups of scientists, each working with different models and assumptions. Perlite/rock wool was included (depending on the study) to compare a mineral-based substrate, as they tend have extreme impacts compared to organic substrates like peat and coir. As previously discussed, the only fair comparison for this discussion is between peat and coir, as they are the two most interchangeable substrates.

In two studies, peat has a slightly higher GWP than coir, but in one, coir has the higher GWP. The authors of this study attribute the higher GWP from coir to the high rates of fertilizer (urea and PO42-) used for producing the coconut crop. Is it fair to attribute the GWP from fertilizer to the coir substrate, since that fertilizer was likely applied to improve fruit yield? Your opinion may vary. Understanding these assumptions is one of the challenges with interpreting life cycle analyses, despite their quantitative nature.

Figure 2 shows the environmental impact from three substrate components across three different studies. Environmental impact includes many factors, such as ozone formation, terrestrial acidification, freshwater and marine eutrophication, land use, water consumption, etc.

Because these impacts involve many factors, the data is often normalized and expressed as a percentage of the highest value. For example, the Vinci et al. study calculated that rock wool had the highest environmental impact, while the impact from peat and coir had 2% and 87%, respectively, of the impact caused by rock wool.

In every case, the environmental impact from coir is higher than peat, and in two of the three studies, the impact from coir is higher than rock wool. This was attributed to the retting process to extract coir pith from the husks, a process that uses a lot of water, as well as the shipping of coir from South Asia.

The blanket statement that peat is more damaging to the environment from greenhouse gases (CO₂) and other impacts is not supported by scientific literature.

Should we harvest peat?

There are three primary reasons that I believe peat is necessary for the horticulture industry and society.

Horticultural peat is used to feed the world. For example, enough peat is harvested each year from a single acre of peatland to grow between 14 and 40 million vegetable seedlings (depending on plug size). Other substrates could be used, but they often require more time, more energy, more fertilizer or more water to grow the same crop as peat.

Peat also reforests the world. The volume of peat harvested from a single acre each year can grow more than 3.3 million forest seedlings, which is enough to replant more than 6,400 acres of forestland.

Finally, peat beautifies the world while providing ecosystems services in our landscapes and improving overall human health. The volume of peat harvested from a single acre each year can grow over 56 million seedlings of ornamental plants.

As horticultural professionals, you might have felt pressure to stop using peat due to the “consequences of its continued use on peatland habitats” or its “whopping carbon footprint.” I believe these claims are exaggerated, misguided and in many ways factually incorrect. Horticultural peat can be, and is, harvested in a sustainable and responsible manner. Canadian peat harvesters have been doing so for more than three decades.

James Altland is research leader at USDA’s Agricultural Research Service’s Application Technology Research Unit. Contact him at james.altland@usda.gov. The views expressed in this article are those of the author. This article first appeared in the April 2024 issue of Greenhouse Management.

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