Maryland Grows

Sphagnum peat moss is valuable in horticulture because its fibrous structure helps it retain a lot of water and air while draining excess water. This has made peat a primary ingredient of soilless growing media (potting mix) around the world. These stable, light-weight, and porous products have been filling the benches, flats, and containers of greenhouse and nursery operators and flower and vegetable growers for decades. You’d be hard-pressed to find a gardener who has not benefited from soilless potting mixes for starting and growing plants, inside and outside.

What’s the problem with peat?

Peat is an organic substance formed from mosses, reeds, and sedges that accumulates and decomposes very slowly in waterlogged soils (bogs). Peatlands hold 30% of the earth’s soil carbon and occur mostly in cold, temperate regions. “Peat moss” used in horticulture typically refers to mosses in the Sphagnum genus.

The problem with peat is three-fold: stripping off peat from peatlands disturbs complex ecosystems; excavation releases enormous amounts of CO2, a major greenhouse gas driving climate change; and demand for peat-based soilless media is growing.

For decades, there have been calls to conserve the U.K.’s dwindling peatlands. Timelines are in place for soon phasing out peat as a growing media for gardeners and commercial growers. Most sphagnum peat is from Canada and there are no indications that Canada, with its vast peat reserves, will follow suit. But public demand for peat-free alternatives will drive the industry to develop new products.

Reducing the use of peat in horticulture will mitigate climate change and increase reliance on local materials as peat substitutes.

Think before you switch: what makes a good soilless growing media?

Any growing medium for plants has three main functions: 1) supply roots with nutrients, air, and water, 2) allow for maximum root growth, and 3) physically support the plant. This becomes especially important for plants growing in cells, flats, and containers because they are dependent on a relatively small volume of growing medium. Container plants only have so much space to find what they need and can’t grow around problems, the way that plant roots in your garden can grow around the rocks.

That small amount of growing media must supply the right amounts and proportions of air, water, and nutrients. The volume of pore space (“empty space” filled with air and water) in a high-quality potting mix is 50%-80% of the total. So to get your flower, herb, and vegetable transplants off to a good start you are basically paying for pore space!

Soilless growing media should also be free of pests, pathogens, weed seeds, and other contaminants. The manufacturers also adjust the pH and add fertilizer. The end products must also be very stable and consistent to be acceptable to growers and gardeners.

Make some peat-free moves this year

 Here are some ways you can begin to reduce your use of peat and make your garden more climate-resilient:

• Stop using peat moss in your landscape and garden beds. Instead, use compost to improve soil. Gardeners sometimes use peat moss to reduce soil pH. This can be done more precisely with sulfur or iron sulfate, both readily available.

• Buy commercially available peat-free growing mixes. A lot of work has been done by the industry and university researchers on developing peat-free mixes that substitute materials such as coconut coir, compost, rice hulls, and small wood particles for peat. Here are two regionally-produced examples:

• Ask your local businesses that sell potting mixes to start carrying those that are peat-free.
• Make your own compost or buy locally-made compost. This can help reduce greenhouse gas emissions from transportation.
• Mix together a soilless growing medium with a high quality, screened compost (made from yard waste and/or food scraps). Experiment with mixes that are 25% compost and evaluate the results. If plant growth is good, increase the amount of compost to 50%
• Develop your own blend by experimenting with different components and ratios. The goal is a mix that drains well but also holds enough air and water for quick, strong root growth. Compost and coconut coir are good primary peat alternatives. Gardeners are also experimenting with bark fines and coarse sand.
  • Compost- researchers have obtained good results growing bedding plants, including vegetable transplants in soilless mixes that range from 20%-50% compost by volume. This is always tricky for commercial growers because compost batches can vary even when the same ingredients and processes are used. The concerns with compost– high nutrients, salts, and pH– may not be as big a problem for gardeners. Since we can make our own compost or buy locally-produced compost, it makes sense to increase our use of compost in growing transplants.
  • Coconut coir is natural by-product of coconut harvesting. It consists of the coarse fibers extracted from the husks of coconut shells, a renewable resource. Coconut coir is screened, washed and graded for premium horticultural use. It is similar to sphagnum peat in terms of bulk density, porosity, water retention, and water drainage. Sources countries are Sri Lanka, the Philippines, Mexico, India, and Costa Rica, increasing transportation costs. There can be some variability between products, so experiment with different options.
  • Research shows good results growing a variety of plant species in mixes where coir is 35-80% of the total volume.
  • Try coir/compost mixes that are 3:1, 2:1, and 1:1.
  • Monitor plant appearance and growth and fertilize with a soluble complete fertilizer if needed.
  • Don’t use garden soil- even a small amount. It’s too dense for germinating and growing seeds in flats and containers and could introduce pathogens and weeds.

Challenge yourself to reduce your use of peat this year and please share your experiences if you make and use reduced-peat or peat-free growing mixes (send to: jont@umd.edu).

Resources:

Joe Lampl podcast on peat featuring Merritt Turetsky

Royal Horticultural Society peat-free web page

UK Horticultural Trades Association- Growing Media (detailed information on the movement toward more sustainable products)

By Jon Traunfeld, Extension Specialist, University of Maryland Extension, Home & Garden Information Center. Read more posts by Jon.