Entitled ‘Trees Rock in Rock’ the event was led by Örjan Stål and Björn Embrén from Sweden.
Stål is an independent consultant and is also employed by the Swedish University of Agricultural Sciences. He has worked on severalinfrastructure projects in Scandinavia and has spent the last 20 years looking at how underground utilities affect trees.
Embren is a tree officer in Stockholm and has worked for the last decade with urban trees. He is the author of ‘Planting Beds in the City of Stockhom: A Handbook’ - see link to download.
The question posed by the two speakers was "Are trees able to live and develop within a rock-based substrate?"
Stål said that had he had spent the last 20 years studying root intrusion in sewer pipes. He admitted he used to think it was only older, weaker pipes which were subject to root intrusion, but his work has shown him even newly laid, modern pipes are also affected. Similarly, he used to think it was fast-growing trees such as willow, poplar and birch which were the main culprits, but again this has been disproved. Many different genera regularly cause pipe damage. A third belief was also blown out of the water - good soil around pipes does not necessarily prevent intrusion into them.
He said that given good conditions, tree roots can and do grow deep. A tree root tip can develop a pressure greater than 12 bars. This was supported by photographic evidence of the depths which roots can reach.
Stål said he had seen roots wrapped round pipes at 23ft deep and more than 50 yards from the tree. Roots growing in sewers make their own soil in which to grow from the small roots they discard and which turn into compost. Summing up his findings, tree roots can travel long distances both downwards and outwards, especially when following pipes. He has even seen instances of roots in pipes emerging in toilet pans.
The conditions provided by the filling-in material around pipes are often favourable for root growth. In Stockholm he has seen the massive root growth which often occurs in railway embankements. The trees in question are often lindens (limes), of which there are many in Sweden. He has also seen much evidence of roots working their way into cable covers for electrical wires, seemingly preferring them to the soil round the covers. In Osnabrück, Lower Saxony, Germany, he has even seen tree roots growing through the concrete roofs of underground World War Two bunkers, such is their strength and tenacity. So are the soils we currently use when planting urban trees the optimal substrate? Stål said that Klaus Schröder, a tree officer in Osnabrück, uses lava and pumice stone when planting his trees. Stal said he has great faith in the Missouri Gravel Bed (MGB) because roots thrive in this, searching for moisture in the many cavities.
Björn Embrén began his session saying Stockholm has more than 30,000 trees in its paved areas. Unfortunately around 10,000 of these are dying. He said in Stockholm it costs £12,000 to plant a new tree in a paved area. The city has a budget of £1.5 million per annum for investment and an annual maintenance budget of £300,000.
He believes that using a rock-based substrate is of benefit both to trees and to the management of stormwater. He likes to use rocks 4-6cm in size in a 30cm layer. He has noted that trees which were 40-45cm on planting in this medium were 70-80cm after six years.
Delegate Jeremy Barrell observed that in the UK engineers are ‘obsessed’ by geo-textiles, creating pots for urban trees with them. Embren told him the situation was much the same in Stockholm. Whatever the plans may say, engineers and those people planting the trees end up doing the job the way they have always done it.
He then went on to tell us us about Hornsgatan in Stockholm. The authorities were told by the European Union they were not looking after the street sufficiently well and should therefore plant trees to improve it. This was always going to be a rather difficult job because Hornsgatan is narrow. Initial estimates suggested the work would cost £6 million, but it will end up costing double that at around £12 million. Hornsgatan had not previously had trees in it and it is a heavily polluted area. Björn has been closely involved with this major project. The work on tree planting began in 2010 and will continue through to 2013.
Tree roots grow up to 1m per year in a layer of rocks, we were told. How can we get more cost-efficient methods of creating good root spaces though? Pumice stone is apparently well worth consideration. It can hold plant-available water and still have a good pore volume. Embrén believes 30 per cent pumice and 70 per cent soil is a good planting mix.