RHS Science reveals peat-free doesn’t need additional water

Extensive data analysis from RHS trials of peat-free and peat-reduced growing media across multiple nurseries, plant groups and growing seasons has shown that peat-free blends do not necessarily need more water than mixes containing peat, and may in fact have better water retention properties.

This is contrary to popular beliefs around the water-holding capacity of peat-free media, with the 2023 RHS Industry Survey citing irrigation management as a primary concern faced by growers during their transition to peat-free.

Addressing these concerns, through developing new growing media technologies and optimised irrigation protocols, has been a key focus for RHS and industry collaborative research.

Now, 18 months of commercial trials, carried out under standard nursery production conditions at four leading UK nurseries and including shrub, herbaceous, alpine and ericaceous plant groups grown in media from six major manufacturers, have brought news on the water-retaining properties of peat-free mixes.

The results from the trials, which were led by RHS Transition to Peat-Free research fellow Dr Raghavendra Prasad, offer new insights for growers on optimising their peat-free irrigation.

What do growers need to know?

Peat-free doesn’t necessarily need more water under standard nursery production

In all six sets of trials, the growers used their ordinary irrigation regimes. None of the trial plants were treated any differently to a normal commercial batch and no adjustments were made for the peat-free media, since these were preliminary trials, gathering baseline data to provide a yardstick for ongoing research into irrigation management.

Analysis of volumetric moisture content (volume of water contained within a medium relative to the medium’s total volume, as a percentage) over time revealed that five out of six peat-free media tested had similar or higher average moisture content than the peat-reduced control media throughout the course of the trial, despite all mixes receiving the same irrigation.
 
“This indicates that when treated the same as commercial nursery controls, the majority of peat-free mixes tested did not need more water than media containing peat,” says Dr Prasad. “Additionally, all mixes produced equally saleable plants regardless of volumetric moisture content during production.” 

Mean water retention, as measured by mean volumetric moisture content, of different peat-free and peat-reduced control growing media during trials of Lupinus from December 2023 to March 2024. Source of data: Allensmore Nurseries and RHS Transition to Peat-Free fellowship trials

Some peat-free media have better water retention than peat-based mixes

“Growing media manufacturers are very aware of grower concerns around irrigation and water retention, so some manufacturers have specifically designed peat-free blends with enhanced water retention to avoid the need for extra inputs by growers,” says Dr Prasad.
 
A more detailed long-term analysis of moisture retention, indicated by volumetric moisture content (VMC) percentage over time, of a range of mixes under the same irrigation regimes has revealed that some peat-free media in fact have better water retention than the peat-reduced nursery controls – revealed by a VMC that remains consistently higher under the same conditions. 

“Though the same growing media will yield slightly different results on different nurseries,” explains Dr Prasad, “what we consider a moist mix is consistent across the board. The patterns in volumetric moisture content remain the same.”  

Water retention, as measured by mean volumetric moisture content over time, of four peat-free and one peat-reduced nursery control growing media. Source of data: RHS Transition to Peat-Free fellowship trials by Dr Raghavendra Prasad

Growers can reduce their water use by improving irrigation efficiency

The commercial trials found that volumetric moisture content during the production cycle did not significantly influence the saleability of the end product. All products were of similar, saleable quality – as judged by nursery staff – regardless of whether moisture content remained at 60% or 40%.
 
“This means growers may be overwatering, and keeping growing media moister than they need to, owing to the deceptive ‘dry on top’ look of some peat-free media,” says Dr Prasad. “There is a potential opportunity for growers to reduce their water use by better understanding the water retention properties of their individual mix. Irrigation efficiency in these new-generation peat-free mixes also needs to be improved to optimise crop production.” 

All trialled mixes produced saleable plants, despite the same water input and each growing media maintaining a different average moisture level (VMC) throughout their production. Source of images: RHS Transition to Peat-Free fellowship trials at Allensmore Nurseries in spring 2024

Better irrigation management will improve nutrition management

When plants are overwatered,

To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key plant nutrients usually derived from soil are nitrogen, phosphorus and potassium, while carbon, oxygen and hydrogen are absorbed from the air. Other vital soil nutrients include magnesium, calcium and sulphur. Gardeners can add nutrients by applying fertilisers (either artificial or naturally derived) to boost plant growth and improve flowering and fruiting.

Water retention following a single irrigation event, as measured by volumetric moisture content over time, of four peat-free and one peat-reduced nursery control growing media. All mixes were irrigated to saturation, followed by no further irrigation for the next two days. Source of data: RHS Transition to Peat-Free fellowship trials by Dr Raghavendra Prasad

Awareness of water retention differences between mixes can increase success rates during establishment 

Moving a plant from one pot into a slightly larger one, once its roots have filled the current pot. Most container plants need potting on every few years. If you leave a plant in the same pot for too long, its roots will become overcrowded (pot bound), hindering growth.

potting on

It’s imperative for growers to understand the individual characteristics and behaviour of their particular growing media

What next?

The RHS Transition to Peat-free Fellowship will be conducting further research to better understand the varying water-retention properties of peat-free mixes.
 
The growing media moisture levels seen in the trials (40-60%) can be considered quite moist. Future research will focus on determining the container capacity for peat-free mixes (how much water a growing media can hold while still containing air pockets), and the plants’ permanent wilting points. The aim will be to establish an ideal range of moisture content (VMC) percentages for various peat-free mixes, to help growers maximise the performance of peat-free growing media and improve efficiency of irrigation and nutritional management.  
 
Another programme of trials is also underway to investigate how nutrition regimes can be optimised for peat-free media.

Research background

The RHS Transition to Peat-Free fellowship, working in partnership with eight major UK commercial growers and seven growing media manufacturers, aims to provide UK growers with the science-based, nursery-tested information and results needed to navigate differences and assist growers in successfully moving away from peat.

These results were shared at the RHS annual Peat-Free Horticulture Conference, which brings together horticultural experts, scientists, industry leaders, and policymakers to share insights, research, and practical solutions for transitioning to peat-free.

Trial details