The use of peat and peat free products for propagation purposes

At Glyn Bach Gardens we try to garden as organically as possible and use no herbicides or pesticides on our land.  When propagating our National Collection of Monarda we needed to know the best medium for growing our divisions (stolons from basal growth) in with minimal impact on the environment.  Our research into the three basic types of commercial compost uncovered the following facts:-

  1.  Peat compost.  Peat, from Sphagnum Moss, is singly the most organic medium but, it takes thousands of years to make and provides essential habitat for key species.  Harvesting peat releases methane and carbon dioxide is released into the atmosphere.  However, UK gardeners use only about 5 – 10% of the Irish peat harvested; the remainder is burnt as a fuel in stoves and on open fires in Ireland.
  2. Peat reduced compost.  Usually contain about 50% – 70 % peat and with the remainder made up of non-peat products.   See below for the breakdown of these products.
  3. Peat free compost.  This contains no peat at all.

Peat Free Products can contain any of the following:-

  1. Green waste from council tips.  Predominately grass cuttings in summer and woody material in winter.  However, people can put chemically treated grass cuttings (weed killers) and contaminants such glass, plastics, sharps and stones in the container.  Green waste is chloride and potassium rich which can lock up nitrogen and potassium; essential for plant grown. 
  2. Wood chippings from council tips.  Chipboard, MDF, wood, cardboard and paper can be shredded to make wood fibre but these can contain formaldehyde which is a known carcinogenic.  These are usually the main constituents of peat free compost.
  3. Food waste.
  4. Animal waste.  There are two parts to this one. (a).  Faecal matter from animals and humans (b). Slaughterhouse waste.  Both can contain harmful bacteria.
  5. Coir.  The fibre, found within the mesocarp of coconuts (the thick spongy layer within the fruit wall).  It makes a great filler for peat free composts as it retains moisture.  The snag is that this is imported from India and Sri Lanka by boat.
  6. Perlite and Vermiculite.  These are used to bulk out the compost, but vermiculite has been found to contain asbestos.

Methods to produce a good potting medium vary and are not yet standardised. Heat treatment may not be sufficient to destroy pesticides and pathogens.

The more expensive peat free products contain coir, animal waste and added nutrients; they omit green waste.  Cheaper products are heavily reliant on green wastes – these can be identified on the packaging by the fact that they are unable to germinate seeds.  Unfortunately there is currently no legal requirement to specify the actual constituents on the bags of peat free compost. Gloves should always be worn when handling any of these products and always wash hands well after use.

Trial of Composts

We decided to exclude all peat free composts, with the exception of Sylva from our trials as we did not wish to contaminate our land or soils in any way.  Until better methods of producing non peat composts is found, we will not use them and will look at our own alternative methods.

We carried out a controlled trial to see how well our Monarda divisions, for propagation purposes, developed effective root systems and how they continued to grow in the chosen medium.

We chose the following mediums to grow our divisions in:-

  • Home- made compost (our own non treated grass cuttings and fresh woody material.
  • Sylva Peat free compost
  • Peat based compost.

March 2019 Week 2

Small divisions were taken from assorted Monarda parent plants and placed in 9cm pots with compost as above.  Each compost also included the same amount of Osmocote slow release fertiliser. 

These were then divided in trays according to the type of compost used and labelled accordingly.

All pots were given equal levels of light, water and temperature.

Results

March 2019 Week 3. After one week there was little notable difference in any of the cultivars.

April 2019 Week 1. After 3 weeks, a small number of divisions in the peat based compost and the home made garden compost had died. (Approximately 15% overall).

The Sylva peat free compost showed more promising results at this stage.  Nearly all the divisions took with less than 3% of losses.  One possible reason may be that the compost was more free draining and kept the roots much drier thus preventing the divisions from rotting off.

April 2019 Week 4.   By now the remaining divisions had rooted well and with the additional warmth and longer daylight hours, were beginning to put on good growth. 

The home-made compost showed the best growth spurts, the peat based compost in second place and the peat free compost showed the slowest growth.

May Week 2.  Divisions were now of a good size and ready to be potted on to 1 litre pots.  The home- made compost showed the best results with plants of a good size, with good root systems and strong healthy growth. However there were weed seeds in with the plants.

The peat compost also showed good results with plants of a good size, with good root systems and strong healthy growth, but slightly behind the home made compost.

The plants grown in the peat free compost were healthy, well formed, with no mildew, but were substantially stunted in growth, achieving 50% of the growth of the other plants.  (They looked like bonsai plants)

Conclusions.

The peat free compost is the best medium for striking divisions as it is free draining so there is no damping off or rotting of the division material. But plants need to be potted up after three weeks into a better medium to allow for growth.

The peat compost retained too much water at the initial stage of striking the divisions so there were some losses.  However, those that did take grew well.

The home-made compost initially had some loss at the striking phase, but the plant growth and health more than made up for the losses. Pots did need weeding.

We reached the conclusion that we should make a lot more of our own compost as the healthiest and most organic of all the products tested, but supplement with either peat or Sylva to bulk out the compost.  Sylva is a great compost for striking cuttings and we would be happy to use that in the future.

Until manufacturers come up with a better way of producing peat free composts and list the ingredients used on the bags, we will avoid them out of necessity, to protect our environment and our Monarda collection.

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