UKTC Archive

Problems with tree sand

Subject: Problems with tree sand
From: Ben Rose
Date: Dec 01 2017 07:31:51
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Dear all

I've been working on a project during the past year that has required me to 
find out why some semi-mature trees planted about 4 years ago are now dying. 
After a lot of study I'm now fairly sure that I know the reason behind it - 
and it’s an industry wide problem. The problem is that the tree pits have 
been filled with sand, I'm talking about Amsterdam tree sand and the like. 


It doesn't work. 

Engineered tree pits are very often an isolated island surrounded by 
compacted soils, and now the trend is to surround tree pits with root 
barriers, and this means that the poor tree has no option but to survive on 
the soil that it is given, and SAND ISN'T SOIL. Think of the plant pot on 
your windowsill - would you fill that pot with sand or soil? 

The water holding capacity of the soil is the key issue. Sand doesn't hold 
water as well as any other soil (see attached diagram), and so planting a 
tree in sand is sentencing it for a life of drought stress.

It is a common specification to fill tree pits with structural sand. Tree 
roots can survive in structural sand…..but you need loads of it. Not all soil 
nutrients are in a form available to plants, some are compounds of rock 
minerals or organic compounds that must be simplified before they can be 
exploited by plants. The simple forms are water-soluble or easily available 
because they are on the surface of clays or organic matter. For this reason, 
on a volume basis, sand-based structural soil is disadvantaged given its 
lower inherent soil content and corresponding nutrient pool. Studies have 
shown that sand-based structural soil is approximately 50% as efficient as 
loam soils (Smiley & Urban 2014), and so twice as much sand-based structural 
soil is required in the tree pit to support a healthy tree (as well as a 
constant supply of water in hot weather). 

Part of the problem is that landscape architects are designing the tree pits 
and putting everything from the GreenBlue catalogue in them, but after the 
‘dead men’ guying system, the root deflector, the watering pipes there is 
little space left over for soil, and so we should be making the most of it. 
Tree sand is currently used for when a paved surface is required above, but 
in such circumstances we should be using a suspended surface above the soil 
such as tree grilles or a crate system. 

Trees grow best in open tree pits with mulch on top and soil mixtures with 
close to natural soil profiles, and so there should be sub-soil in the tree 
pits as well as topsoil (300mm topsoil over 600mm subsoil). The best guidance 
out there that I am aware of has been produced by Ed Baker at Cardiff Council

Tree sand can be used to extend the size of rooting areas under paved 
surfaces, and so it is sometimes a good idea to put trees sand under paved 
surfaces AROUND the tree pit. 
As arboriculturists we need to communicate this knowledge to others 
(particularly landscape architects).

As you may have gathered I needed to get that off my chest, I think I'll have 
a cup of tea now.


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