A new phase for Greenwood’s PhD carbon research
An update on the study measuring how much carbon nursery-grown plants store, two years in
Plants are one of the few practical tools available for taking carbon out of the atmosphere and holding onto it. The figures used to account for that carbon in parks, domestic gardens, and green infrastructure within developments are still surprisingly rough. Forestry has decades of careful measurement behind it, while cultivated green spaces do not, and the equations built for woodland rarely transfer cleanly to a street tree, a shrub bed or a border of perennials.
Greenwood is funding a PhD study to help close that gap, in partnership with the Royal Horticultural Society (RHS) and the University of Sheffield. The research measures how much carbon nursery-grown trees, shrubs and herbaceous plants hold above and below ground, how that changes as the plants mature, and how the picture could shift under the warmer conditions projected into the future. The work is led by PhD researcher Louis Kosowicz, supervised by Professor Jill Edmondson and Dr Holly Croft at the University of Sheffield, alongside Dr Mark Gush, Head of Environmental Horticulture at the RHS.
Greenwood wrote about the project last autumn, when the field trials at RHS Wisley were newly established and the climate-controlled chambers at Sheffield were running their first cohort of young trees. The study is now approaching the end of its second year, with a programme that runs through to 2028. A good deal has moved since then, including the closing stage of one experiment and the opening of another.
Research of this kind does not deliver headlines every quarter. It delivers samples, spreadsheets and slow accumulation, and then numbers that hold up.
Why this data matters
Planting schemes are increasingly asked to demonstrate an environmental return. Carbon, biodiversity and water now appear in specifications, planning conditions and post-completion reporting. The awkward part is that the evidence base behind the carbon figures is thin, and much of it rests on assumption rather than measurement. Root biomass, for instance, is often estimated as a fixed proportion of what is visible above ground, rather than dug up and weighed.
Greenwood grows and supplies the plants that go into those schemes, which puts a nursery in a useful position to help produce better figures. Every plant leaving the nursery carries a quantity of carbon with it, and at the moment nobody can state that quantity with any precision. The same is true of what happens next, once the plant is in the ground and growing.
The research is designed from the outset to produce something usable rather than something that stays in a journal. That means measured values and practical guidance that planting decisions can genuinely rest on.
The first chapter at Sheffield reaches its final stage
The climate chamber experiment compared young trees grown under present-day conditions with trees grown under the temperatures projected for 2100, roughly four degrees warmer than today. Native and non-native species were paired within the same genus, so that the comparison isolates where a plant comes from rather than confusing it with everything else that differs between species.
That experiment has now finished its practical phase. Over the summer, Louis worked through the backlog of samples from last year’s cohort, reducing the harvested trees to a form that can be analysed for their carbon and nitrogen content. Sending those samples away for analysis was the last piece of laboratory work for the chapter, and the results feed directly into the calculations of how much carbon each plant had accumulated.
A new experiment on water and growth
The next phase has already begun in the glasshouse at Sheffield, and it moves the focus from temperature to water.
Irrigation is one of the largest variables a nursery controls, and one of the most consequential. Water is a precious resource, increasingly restricted in dry summers, and directly tied to how quickly a plant grows and how much carbon it lays down. What is not well understood is where the tipping point sits. At what point does giving a shrub less water start to compromise its growth and its carbon storage, rather than simply making it tougher?
The experiment sets out to find that point. Shrubs were delivered to Sheffield earlier in the summer, potted on and left to settle into the glasshouse conditions. A pilot run is now under way to establish how the plants respond and over what timescale, which will shape the design of the full experiment that follows. Getting that groundwork right matters, because a drought study that starts measuring too late misses the very thing it is trying to catch.
With increasing water insecurity, it is important to identify species that are well suited to reduced watering, helping to create sustainable, beautiful gardens, nurseries and green spaces, while also contributing to carbon storage.
Louis Kosowicz
For a grower, this is the strand with the most immediate practical use. If the research can identify how far irrigation can be reduced without costing growth or carbon, that has a direct bearing on how water is managed across the business, and on the resilience of the crop in a hot summer.
The trials at Wisley
The field trials at RHS Wisley continue to be monitored through the growing season. The trees planted for the study, along with the shrubs that went in more recently, have come through an intense summer in good health, which is a useful result in itself given how the last few years have gone.
Field trials work on a different clock to glasshouse experiments. The plants at Wisley are being tracked over several years, with groups measured at intervals so that their carbon content can be established directly at different ages rather than inferred from a single snapshot. It is the only way to see how storage builds over time, and it is why the study needs the full length of its programme to produce something meaningful.
Measuring the carbon in nursery stock
Running alongside the experimental work is the strand with the most direct commercial bearing on Greenwood, which establishes how much carbon is already held in a plant at the point it leaves the nursery.
Nursery stock is dug, sampled and measured above and below ground, across different sizes and ages, to build a picture of what a plant is carrying when it arrives on site. That figure is the starting point for everything that follows. Without it, any calculation of what a planting scheme sequesters begins from zero and quietly ignores the carbon that came in on the lorry.
This strand depends on bare-root material, so it is paused until the end of the growing season and will pick up again as the bare-root season opens. It also serves as a control for the other experiments, and it will help refine the relationships between what can be seen above ground and what is happening below it.
What the data will be used for
The intention is a body of evidence that can be applied rather than admired. Measured carbon values for the kinds of plants that actually go into cultivated landscapes. A model that projects how storage develops over decades rather than seasons. An understanding of how species differ, including how they are likely to perform as conditions warm.
From that, practical outputs follow. Guidance on species selection where carbon is part of the brief. A clearer basis for the sustainability claims made about planting schemes. Better information for anyone trying to reconcile a planting design with a net zero commitment. The RHS has its own target of net zero by 2030, and Greenwood has its own sustainability commitments, and both need real numbers behind them rather than plausible-sounding ones.
There is also value in what the research design itself contributes. Pairing native and non-native species within a genus turns a long-running argument into a measurable question. Digging up roots rather than estimating them replaces an assumption with a figure. These are unglamorous methodological choices that will make the resulting data considerably more trustworthy.
Two years in, the study has a completed experiment moving into analysis, a new one running in the glasshouse, established field trials being tracked at Wisley, and a nursery inventory building in the background. It is progressing as research progresses, which is to say steadily and without much drama. Findings will be shared as they are confirmed, and the figures they produce are the kind the industry has been missing.
For more information about the carbon research project and Greenwood’s wider sustainability programme, speak to the G Team today.
Glasshouse photography by Laura Clark.