Soil Made Simple, Part 3: Peat-based vs. Compost-based Potting Mixes

Before we conclude our discussion of potting soil, we have one more item to discuss. We have reviewed potting soil basics and the world of mixing your own, and there are tremendous online resources for the serious grower and hobbyist alike. Now, we shall sink our proverbial hands into the how’s and why’s of the mixes Wellfield chooses to employ.

Wellfield, like many public gardens, has a public display side and a grower/production side to the enterprise. I use my best, most expensive potting mix for plants growing in containers on display for all the world to see. Plants growing in our behind the scenes “production” space are planted in a far less expensive and quality soil media made of compost and sand.

I have spent my entire career advocating for, and moving away from, the use of peat in my potting mixes. While working at Longwood Gardens, I developed a soil-based mix for display containers on a small scale. However, I did not love the quality of the displays and time required to mix my own onsite. I switched to a commercially produced mix (sand/pine bark) when I came to Wellfield. This mix drained well, but did not have great water and nutrient retention. Finally, I remembered Mark Highland, a classmate of mine, who developed a compost-based mix for Longwood Gardens as a part of his Master’s research. He also eschewed peat, and he launched Organic Mechanics as a result of his ongoing rigorous research.

Organic Mechanics Potting Soil used at Wellfield

So what is the concern with the use of peat? First of all, it is important to remember any sustainable action one takes is a trade-off decision. That is just the way life works. There are no perfect solutions and “one size fits all” products in the world of sustainable horticulture. There are two concerns I have with the use of peat:

  1. Ecosystem Disturbance- Peat, technically a renewable resource, comes from peat bogs which require long stretches of time to produce the peat harvested for soil amending. To the best of our knowledge, peat grows approximately one-sixteenth of an inch a year. Harvesting removes centuries of growth at once (9 inches per year). Peat bogs are incredibly biodiverse as well. These ecosystems grow some of the rarest and unique plants on earth due to the unique environment they provide. Most of the sphagnum peat, the most common type used in potting mixes, largely comes from Canada, which contains a quarter of the world’s peatland. There are efforts to rehabilitate bogs mined for peat; however beautiful, the created wetland it is not an undisturbed peat bog. It is incongruent with Wellfield’s mission (a portion of which is to promote the inseparable relationship between plants, humans and water) to utilize a wetland sourced material.  
  2. Carbon Sinks- Peatlands contain a third of the world’s carbon, a concern for those with an eye to climate change.

The Royal Horticultural Society is looking to phase out the use of peat from horticultural use for the same reasons listed above. However, more research is needed to find the best material alternatives, since the green industry is so reliant upon peat-based soilless medias. The concerns of growers needs to be adequately addressed. Sound economics and market forces must be considered in any trade-off sustainable policy, or the policy is doom to failure.

The ingredients (compost, rice hulls, coconut coir, aged pine bark and worm castings) in the compost-based recipe we use for display containers are the by-products of other industries, which creates a closed loop system. The results, thus far, are a definite improvement over the previous commercial product I used. The downside, as previously discussed regarding any soilless media, is the “rapid” breakdown of the potting mix. I must change out and refresh the mix every few years as the media loses volume.

I am not sure if the entire ban on peat moss from potting soil is practical or doable, but there is absolutely no sound reason to use peat as a general soil amendment in the flower bed. Yes, peat is very stable in the soil, lasting for years. Yes, it can help lower the pH a little. Yes, it holds twenty-five times its weight in water. However, for the reasons listed above plus more, it is no longer recommended when there are plenty of alternative organic matter sources.

Josh Steffen, Horticulture and Facilities Manager