Workshop summary: practical management of peat-free growing media

Key points, presentations and videos from the RHS/HTA peat-free workshop on 14 September 2023

About the workshop

The first of a series of peat-free workshops by the RHS in conjunction with the HTA took place on 14 September at Kingstone Village Hall, Herefordshire and Allensmore Nurseries Ltd, Herefordshire.

The workshop focus was practical management of peat-free growing media, with speakers including Gracie Barrett of the Farplants Group, Neil Bragg of Bulrush, Andrew Wilson of ICL, and Mark Gush and Raghavendra Prasad of the RHS Transition to Peat-Free fellowship

Topics included the progress made in peat-free trials and cultivation, including the latest developments within the RHS Transition to Peat-Free fellowship’s peat-free trials; quality control of peat-free growing media; and how growers approach technical or scientific trials and instigate crop monitoring to manage plant quality and prevent nutritional deficiencies.

Introductory presentation by Mark Gush (2.1MB pdf)
Summaries of key information and presentation slides from each of the speakers can be found below, along with a series of short videos in which the speakers share top tips for growing peat-free.

Andrew Wilson, ICL

Wayne Brough, HTA

Dr Gracie Barrett – Farplants

Nursery quality control of peat-free substrates

Download Gracie’s presentation (5.3MB pdf)
Gracie works for the Farplants Group, and has been looking into peat-free growing both as a researcher and a grower for more than a decade. In 2021, the Farplants Group began looking in more detail at various peat-free media to build knowledge and confidence. They wanted to find out which substrates would work for which products, as no one substrate would cover everything.

We’re used to growing in peat, which is a very stable substrate with few microbes (old and decayed). The physical and chemical structure of peat is predictable between batches and years, and can produce thousands of plants to a similar standard.

Peat-free mixes are different, as they contain much younger organic materials, and every mix has a different spectrum of properties. They are less consistent, which adds uncertainty. This means trials are especially important.

Uneven water distribution can create a challenging environment for roots (see photo in presentation). Slumping led to poor rooting after initial positive results for some plugs.

Some peat-free mixes were less resilient to changes in weather, but higher amounts of composted bark were more stable outdoors.

Trials need to last for at least a year so that you can check successes – there are points in a growing season when certain problems like chlorosis are more likely to occur, especially post potting (3-4 weeks) in early spring outdoor crops, as there are fewer available nutrients from controlled release fertilisers in lower temperatures.

Make sure you understand your materials – indoor and outdoor cultivation will have different nuances, and growing media storage is important. Keep peat-free mixes dry and cool, and use them as soon as possible.

Key messages

- Every peat-free mix has a different spectrum of properties, so trials are especially important

- Match the substrate to your crop and production methods – e.g. mixes high in composted bark were more stable outdoors. No one size fits all

- Make sure you understand your materials – indoors and outdoors will be different and growing
media storage is important – keep peat-free dry and cool and use as soon as possible

- If you only get one piece of kit, get an EC probe to give you a better picture of your growing media

Neil Bragg – Bulrush

Understanding methods of analysis in relation to peat-free mixes

Download Neil’s presentation (0.8MB pdf)
There is a huge library of methods and results for growing in peat, but we are only just starting to build this up for peat-free, and different mixes all need testing. We have less experience with peat-free in terms of a knowledge bank.

Specifications from the manufacturer will help you understand the breakdown of nutrients in the mix. Be aware that nitrogen may be in the form of urea. When urea is the N source in growing media, the soil pH can affect the rate of availability of N to plants in the first few days after potting – so it’s especially important to have information from the manufacturer on where the N comes from in your mix.

Again, the storage is important – microbes will proliferate in the sun. Check mixes and if they are very wet, reduce moisture before storage by using mixes as soon as possible after delivery. Store mixes in shade and cover with a waterproof covering if outside.

Key messages

- Store in shade, keep dry and use as soon as possible if mix is wet

- Pay attention to manufacturer’s specifications regarding nutrient composition of the mix, particularly what the nitrogen source is

Neil Bragg, Bulrush

Gracie Barrett, Farplants

Dr Raghavendra Prasad – RHS

Monitoring crop performance

Download Raghavendra’s presentation (1.4MB pdf)
The RHS Transition to Peat-Free Fellowship is looking into key challenges across the industry by working with a range of industrial partners.

It is important to standardise peat-free trials against your regular growing media mixes as a control. Replicates help to ensure that human error or environmental factors are not giving a false result.

Trials are randomised and replicated. We do not give peat-free mixes any special treatment – we need to see how they perform commercially, and ensure any changes in inputs that could affect costs can be planned for.

Key messages

- Trials are being carried out across many plant groups in a commercial setting as part of the RHS Transition to Peat-Free fellowship

- Set out what you want to achieve before you start your trials

- Ensure there is a control using your regular mix, and trials are repeated and randomised within your setting

- Test the growing media at the beginning and the end of the trial, and also during trials if possible

- Assess various parameters including physical and economic at the end – are the plants saleable and healthy?

Andrew Wilson – ICL

Management of crop nutrition

Download Andrew’s presentation (4.3MB pdf)
If you can get the nutrition and wetting agent right, then roots develop well in peat-free. Peat can hold nutrition well due to binding properties; being aware of this can help anticipate problems.

Use information from a weather station alongside data from pots as you test.

Coir can add a lot of potassium – make sure you test media before use for every batch, not just from a new supplier.

Irrigation little and often will reduce leaching – wetting agents help here. Even when the top is very dry, lower down the media may still be moist and have good rooting, so check before you water. A dry surface can be beneficial as it reduces moss and algae growth too.

The first watering should be thorough, as the controlled-release fertiliser will not have started working. This will help prevent leaching of nutrients in the first few weeks. Overwatering can also lead to iron deficiency.

It’s important to ensure messages are being passed on to garden centre assistants, as they need to know how to feed and water your plants. Slow-release feeds are best to stop leaching.

Calcium deficiencies are especially common in peat-free, especially with soft water, so know what to look out for.

Pay attention to what your pot is telling you – e.g. algae indicates wetness.

Key messages

- You might need to adapt your irrigation if your mix is holding more water – this can save water in the long term

- Your water type is important, and will affect irrigation and nutrition (e.g. soft water can lead to calcium deficiency, so check pH of substrate regularly)

- Some mixes will have different effects on nutrients, e.g. wood-based media will alter nitrogen. In general, peat-free leaches nutrients more easily

- Try and review before you grow so you know the nutrient profile of your media – then you can plan for extra fertiliser needs

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