The M9 and rhizotrons


Last week as I was picking  fruit from some small apple trees I noticed how badly grafted one of them was.   It reminded me  that a few weeks ago I wrote about the history of grafting and prompted me to  take that  story into the 20thc and talk about the M9.

By that I don’t mean the motorway from Edinburgh to Dunblane but a dwarfing rootstock for apples developed, along with many others, at East Malling Research station.

So read on if you’ve ever wondered  what all those strange codes and numbers are on the labels of  fruit trees in garden centres and nurseries? Who, for instance,  is St Julien? Gisela 5?  And what does M27 or MM9 actually mean?   How has M9  reshaped the global landscape and the economics of apple production?

And finally do you know what an apple trees roots look like? If you don’t, you might need a rhizotron…so read on to find out what that is!

In that earlier post I  showed how grafting began to be commercially important in the late 19thc  as a vital way for commercial horticulture  in Europe to overcome pests and diseases.  But it had another use which was equally important.  Commercial nurserymen were experimenting with ways of increasing production [when are they not?] particularly in orchard fruit, and making the care and maintenance of orchards more efficient.  One way of doing that is by getting more trees into the same amount of space, and the easiest way of doing that is by having smaller trees – both in height and spread – so that picking is more less labour intensive and can be done without the use of long ladders.

In that post I also talked about the  introduction of  a dwarf rooting stock which had become known as French Paradise.  There were plenty of others which were gradually introduced too including an English Paradise, sometimes referred to as  ‘Doucin’, which was first described in 1519. This was more vigorous than its French namesake, more akin I’d guess to what we would now call  semi-dwarfing.

As was the case with  many other plants the nomenclature of these rooting stocks became increasingly confused.  For example, in 1870, Thomas Rivers, the Victorian orchard specialist listed no less than 14 different versions of Paradise rootstock in his catalogue.

Being species rather than cultivars rootstocks can be vegetatively propagated but also grown from seed, maintaining their general characteristics, and so  sometimes there were sports or chance differences noted. These are often  eagerly seized upon for further investigation, and the most famous of these sports was  ‘Jaune de Metz’, a chance seedling selected in Metz, France, in 1879 which became a popular choice of dwarf rooting stock on the continent.

The black and white images below of East Malling and its staff and work come from their centenary report: A Century of Research at East Malling 1913-2013

Captain Wellington

By the early 20th century, efforts were being made independently all across Europe to sort out the taxonomic mess  but particularly in England where the work was led by what was to become East Malling Research Station in Kent.   The station had been established in 1913 as the Wye College Fruit Experiment Station under Captain R Wellington to improve fruit production.  It was the latest of a number of such institutions  including the Duke of Bedford’s Experimental Fruit Farm at Woburn founded in 1897, The National Fruit and Cider Institute at Long Ashton, founded in 1903, and  the John Innes Horticultural Institution, founded in 1911.

The original building

The new station was pretty small scale sitting on just 22 acres leased from Kent County Council, but was established  very much with the support of local fruit growers who funded the initial buildings. To minimise costs  Wellington designed and managed the whole project himself. The result was that the offices, laboratory, toolshed, hay store, stable and cart-shed  [remember horses were still the principal agricultural power source]  cost just  £362.  .

Staff in 1923. Hatton is seated in the middle

While many thought it would just be another demonstration fruit farm, Wellington decided to widen its remit to “the study of problems met with in the actual culture of fruit trees and bushes.”   The  Fruit Experimental Station became the East Malling Research Station in 1914 just after Wellington  left for military service and Ronald Hatton took over.  It was a rapid promotion for Hatton since he had only been an agricultural student at Wye until 1912 but it should be remembered that at this stage there were only 3 members of staff!   One of the others was Jesse Amos, who had been a lab assistant at Wye College. He meticulous and practical and turned his hand to anything required.

Jesse Amos photographing an excavated tree – for about what that entails read on!

Hatton’s first task was to complete the classification and cataloguing of apple rootstocks, initiated by Wellington.  He and Amos gathered together as many “Paradise” apples as they could find in nurseries and plant collections across Britain, but also  a few from European sources .   In the end they amassed 71 “different” sorts. What that revealed was, of course, that some sorts were clearly identical and had just been given different names depending on their location.   They  assessed each one and classified them by their ability to control size, their  ease of  propagation, and their  effect on the productivity of the scion grafted on.

By 1917, nine  varieties of ‘Paradise’ root­ stocks had  had their botanical characteristics defined, and were selected as being worthy of recommendation and propagation for commercial sale.  To avoid confusion – although maybe at the time it increased it-  Hatton then renamed them  – or rather he numbered them using Roman numerals: M I-IX.  No more French Paradise, that became M VIII; no more English Paradise or Doucin, that became  M II, while Jaune de Metz became M IX.

In subsequent years  the trials continued and with some additional   criteria, such as disease and pest resistance added to the list of desirable attributes. Gradually other potential stocks were  added to the list, until eventually  the number of East Malling recommendations reached 27.

These were categorised by  size as Very Dwarf, Semi Dwarf, Vigorous, and Very Vigorous.  The Latin numbering was found to be too complicated and soon gave way to the simpler everyday  numbering  of M 1- M27. East Malling then produced large numbers of these rootstocks for the orchard industry, reaching an output half a million stocks a year by 1936.

Bradbourne House




The government finally realised the value of the research in the 1920s when, with a staff of fifteen, East Malling was finally given a reasonably assured future with the award of an annual grant by the Treasury.  However it was the partnership with fruit growers, formalised now as the East Malling Trust, that has been its real strength. So, for example in 1938 when nearby  Bradbourne House and its estate of 200 acres came onto the market they provided half of the £30,000 purchase money.

As a programme it was successful beyond what I’d guess was anyone’s wildest dreams.  The M9 rootstock which produces uniformly low compact trees quickly became the world’s most popular commercial rootstock.  As a pamphlet The Fruit of the Tree published to mark East Malling’s centenary in 2013 points out,  pre-1914 apple orchards consisted of “tall, widely spaced fruit trees” with about 3-4,000 per hectare which yielded about seven tonnes of fruit.  Using trees grafted onto M9 rootstock allows growers to plant twice as many trees and vastly increase the yield from the same amount of land.  East Malling estimated that between 1920 and 1960  it“provided 18 million tonnes of additional apples globally as well as saving 70 million hours of picking time” and because  the fruit could be picked without need for ladders, harvesting labour costs were halved.

The buildings in 1924

Ronald Hatton remained in charge of East Malling for 30 years and oversaw its growth to being an experimental farm of 363 acres, and  a centre for post­ graduate research. He was awarded the Victoria Medal of Honour by the RHS and made  a CBE.  In 1944 he was elected a Fellow of the Royal Society  and then knighted on his retirement in 1949 for his contributions to the success of British fruit production.

aerial view 1937

Hatton realised that  East Malling’s research could make an impact globally so travelled extensively, establishing contacts with growers and research institutes  in all the important fruit growing areas of the Commonwealth.  It didn’t take long to realise that  the original set of Malling rootstocks, while suitable for climatic conditions similar to those in Britain, would not necessarily work for apples growing in areas of extreme cold or heat, nor would they cope with a different range of pests, notably the wooly apple aphid.

It was a bad enough pest in Britain where   infestations cause galls to form. These can split to allow cankers and other diseases to develop on the tree above ground, but it was  a hugely destructive pest particularly in North America and Australia because there it lived underground too attacking the roots system.

East Malling undertook further research in partnership with the John Innes Research Station,  based at Merton on the southern outskirts of London, to breed rootstocks with resistance to the  aphid. This led to the introduction of  the Malling Merton series,  with MM prefixes which were quickly adopted in those areas where the aphid was prevalent underground.

It was not just apples. East Malling has also  carried out similar research into the rootstocks for other tree fruit. In the 1920’s the quince rootstocks Quince A and Quince C were released and became very widely used for pears while rootstocks named Myrobalan, Brompton and St. Julian A were released for plums.  Later releases  included the dwarfing apple rootstock M27, the plum rootstock Pixy  and cherry rootstocks Colt, Cob, and Charger.

one of the laboratories

So what do we know about the way these dwarfing rootstocks work?  The short answer is not much really, even now, although thanks to DNA more information is becoming available.  Pioneering work was carried out at East Malling by Beryl Beakbane, who, in the 1950s, investigated the morphological variation between vigorous and dwarfing rootstocks.

Lab for  the Entomology and Pathology Departments

Later she looked at a whole range of  complicated botanical issues relating to dwarfing rootstocks, including “differences in the bark/wood ratio, the relative proportions of storage to conduction and strengthening tissues in stems and roots and the size of xylem vessels, as well as morphological variation in leaf structure.”  The results  were all closely aligned  with “the most dwarfing rootstocks having more living tissue and less strengthening tissue per unit of root.”

Other scientists at East Malling studied roots systems to determine their role  in water absorption, their association with mycorrhizae, and the impact of soil management, planting density, irrigation, and cropping levels.  But how can you study roots when they’re underground and cant be seen?  The answer was most spectacularly shown at the Chelsea Flower Show in East Malling’s Centenary year, 2013.

They dig them up.  You can see how on this short video clip

Their display of  a dead, but fully grown,  apple tree on M9 rootstock, complete with all its roots exposed and intact, won the first ever show medal  awarded to a scientific exhibitor. Such careful scientific excavation is long and difficult work if done properly  It took 10 people 3 weeks to lift and clean the tree they used.

The excavation of trees to study the root system is nothing new.   It’s been used for over a hundred years to observe the differences between trees grown on different rootstocks and soil types.  It’s also shown that the  root systems of adjacent trees could overlap,  and that half of the mass and weight of the root system was in the top foot or so of the soil, despite many trees having roots that reach deep down to the underlying bedrock.  I suppose from my point of view the most interesting finding was a tree’s root system was  normally more extensive in spread than its canopy.

But excavation only gives a snapshot in time and of course it kills the plant. So in the 1930s a new system was tried.  Trenches were dug and glass panels set into their sides so that the growth of nearby roots could been seen and studied.

A picture of the first ‘root laboratory’ and of roots and associated mycorrhizal fungi appeared in Sir Albert Howard’s 1945 book Farming and Gardening for Health or Disease,  which was an early promoter of  the organic farming movement.

But trenches only go so far and in the 1960s East Malling came up with a new wheeze: a rhizotron.

The rhizotron at East Malling was the first in the world.


Designed by W.S. Rogers and M.C. Vyvyan  it was an observation gallery/laboratory that is built  under rather above ground.  Rather like that well known experiments that children do in primary schools – growing runner beans in a jam jar wrapped in paper  which can be taken off to see the bean’s roots growing,  the walls of the rhizotron walls have glass panels which are normally shuttered but can be uncovered to see the roots system of the plants growing behind them.

These 3 photos of the East Malling rhizotron come from The Root System Of Fruit Plants by V. Kolesnikov, 1971


It was planted with Gala apples on different rootstocks  and although obviously the scientists still can’t see all of the root system they have a close-up view of part of it and can observe changes over long periods of time without damaging the tree or harming its growth.  They also used the system to observe the root growth of plums and blackcurrants.

Since Hatton’s day although East Malling has had a very complex administrative history and relationship with the Ministry of Agriculture & more recently DEFRA, it has managed not only to survive austerity but flourish because of its strong association with the horticultural industry both locally and nationally.  The landholding has continued to grow with the acquisition of neighbouring farmland until it now stands at 600 acres or 243 hectares.


Apple Blossom
George Clausen 1880s

So what does all this work mean? According to  a 2014 consultants report on East Malling & its economic impact the historic impact of their work on apples has been estimated at nearly £9 billion, with an on-going impact of £216 million every decade. For every £1 they spent about £7.50 was returned to the UK economy.  However East Malling hasn’t seen much of that because the original stocks were released without any form of royalties being charged.

An Apple Orchard
William Matthew Hale

In an age where governments know the costs of everything but seemingly the value of nothing,  the work at East Malling shows that research in horticulture isn’t just a nice add-on but a crucial part of improving not just food production but our total economy. Long may it flourish!

If you want to know more a good place to start is East Malling’s website, and their report on A Century of Research at East Malling 1913-2013 And if you want to know about the latest research on Dwarf Fruit Trees try this paper by 2 of East Malling’s staff to the International Dwarf Fruit Association  [I kid you not it does exist] and don’t forget to Google “Rhizotrons”.  Other useful information can be found  at Rootstock Archaeology and in  Capturing the Socio-Economic Impact of East Malling Research,2013And for  the history  try reading” East Malling Research Station” by FR Tubbs, in the Proceedings of the the Royal Society, Dec 7th 1951, and  Edward Bunyard’s article on the “History of the Paradise Stocks” in The  Journal of Horticultural Science. v.1-2 1919-1921.


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