​FAQ for Urban Landscape & Tree Planting | GreenBlue

Questions and answers

Root Management

How deep should I install the root barrier?

This will depend on what you are trying to protect or achieve. As a general rule, don’t direct roots deeper than you need to. For example, don’t go 1m deep to protect a pedestrian curb structure where a 300mm deep barrier would be sufficient.

How close to the tree can I install root barriers?

Always give the tree as much space as possible. For smaller species, we can use small root directors to manage roots downwards. If we are installing a barrier deeper than a RootDirector all around the tree then we need to take into consideration the tree’s need for anchorage and access to soil nutrients.

If I’m protecting a service or utility, how deep in relation to that should I go with a root barrier?

We would normally suggest that a barrier extends to a depth of 2-300mm below the invert level of the service or utility.

How close to the utility can I place the barrier?

The recommended distance is again about 300mm away from the utility. This will ensure that any pressure against the root barrier is not transmitted directly to the service utility. The 300mm is effectively a buffer zone.

What about NHBC and house builders, do they allow use of root barriers?

Generally we find that if you can show that you have planned the tree planting design to incorporate protection for their utility, they have no difficulty. What they do not like is indiscriminate planting without regard to their investment. We do have on file some copies of letters from utilities suppliers approving the use of RootStop 2000 for protection of their installations.

What about NHBC and house builders, do they allow use of root barriers?

Generally we find that if you can show that you have planned the tree planting design to incorporate protection for their utility, they have no difficulty. What they do not like is indiscriminate planting without regard to their investment. We do have on file some copies of letters from utilities suppliers approving the use of RootStop 2000 for protection of their installations.

How should I finish the top of the root barrier installation?

Any root barrier used must finish at least 10mm above any planting medium (i.e. topsoil) on the tree side otherwise roots could grow over the top. This top edge can be incorporated in a pedestrian curb detail or disguised by groundcover plants or suitable edging material. It does need protecting from traffic or mowers which could damage it.

Will the barrier rot or break down over time?

No, our barriers are resistant to biodegradation and photo degradation (light).

Do you use recycled plastic in your root barriers?

Yes, Root Directors and RootStop are made from recycled plastic and are in turn recyclable at the end on their life.

Can you use root barrier horizontally over service runs?

Yes, in some cases this is the only way trees can be planted near pipes. We recommend that the barrier forms a shallow arch over the service to ensure that it does not collect standing water but drains off both sides.

Can we then plant trees directly over the top of the service?

This would be for the utility company to decide if they would allow, but technically, providing there is sufficient depth of soil (Minimum 800mm) over the root barrier there is no reason why not.

What will happen to a RootDirector when the tree within it out grows its size?

Eventually the barrier could split but by then its purpose will be fulfilled as the root plate pattern will be established at a safe level.

When would you use the flexible linear ribbed root barrier instead of Root Directors?

ReRoot 300/600/1000 are extremely versatile products and can be used in different ways. They allow the flexibility of working around underground obstacles and protrusions when trying to create tree pits in congested urban situations. They are also useful for grouping trees together in clusters rather than individual pits – giving the advantage of root space sharing for trees.

Soil Cells

What do StrataCells/RootSpace do?

RootSpace provides a load bearing structure beneath hard landscape areas, which we can load with quality topsoil for tree root systems without the fear of settlement leading to surface subsidence.

How do they work?

By protecting the soil from over compaction, RootSpace/StrataCells are high strength, interlocking modules keeping weight off the soil, rather like a soil skeleton.

Have they got enough space for roots to develop within?

Yes – the void space ratio in RootCells is 92%, in StrataCells 94.63% and in RootSpace 96%. The space between the support columns is large enough to allow roots to develop and thicken for long term stability and transport of moisture, nutrients and movement of plant sugars around the tree.

What volume of RootCells/StrataCells do you recommend?

The greater the volume you provide for the tree, the more the tree will succeed. Please consult our standard tree pit details for a good starting point. Obviously the answer to this will be species dependent. Three cubic meters of rootable volume should be regarded as a minimum start although a mature tree root system will frequently occupy more than ten times this volume.

How do roots react to growing in these structures?

As the roots grow into the structure, they meet the support columns and either follow them around or divide. This produces a multiple rooting pattern which is very beneficial, particularly for trees in confined spaces. Graft unions may occur as roots rejoin around columns and proceed through the structure.

How do we know this occurs?

RootCells/StrataCells were developed following research into the use of rock soil mixes which showed that even with a soil void ratio of only 18%, roots could proceed through the structure. What RootCells/StrataCells do, is take the advantages of this system and further improve it by increasing the soil content dramatically. This removes the long term disadvantage of rock soil mixes which was the lack of void space for secondary thickening of the root system.

How do you assemble the structure?

Simple interlocking modules are linked together and assembled in the pit. The modules interlock both horizontally and vertically. The soil is then poured into the structure and lightly compacted either by treading or using a small plate compactor. The plate compactor simply rides across the RootCells/StrataCells, vibrating the structure and allowing the soil to settle, eliminating large unwanted voids.

How long have RootCells/StrataCells been in use?

The first installations were successfully completed in 2001. The current RootCell has built on the experience gained and is further improved.

You are burying topsoil – isn’t this a bit unnatural?

Yes, but to a degree, planting a tree in a city is unnatural for the tree which is still essentially a living forest plant. What we are doing here is creating a forest floor environment for the trees’ benefit, but at a slightly lower level within the ground.

But how will the soil stay alive?

RootCells/StrataCells should always be installed with adequate drainage and equally importantly with a root ventilation system such as the GreenBlue Infrastructure Solutions Arborvent with two inlets. This will allow some air movement over the RootCells/StrataCells to allow gaseous exchange to take place. This will allow the soil to breathe and live in the longer term.

Will the Roots push the Cells apart?

In practice this doesn’t happen due to the fact that a RootCell/StrataCell structure is normally installed at a depth of 300mm below finished levels and the weight of granular sub base material above prevents surface heave occurring.

Can I use RootCells/StrataCells right against the root ball?

Best practice would be to allow the tree the maximum volume of unsupported topsoil against the root ball possible in the circumstances. The RootCells/StrataCells only need to start where their load bearing capability is required. This will allow for the tree’s zone of rapid taper within the tree pit before entering the RootCell/StrataCell soil structure.

Do I need to load the cells with Amsterdam tree soil or similar load bearing soil mixes?

No – the RootCells/StrataCells need no additional strength from the soil so it is much better to load with premium sandy loam topsoil to BS3882.

How do I protect the top of the RootCells/StrataCells from subsequent road base layers above?

GreenBlue Infrastructure Solutions supply a twin wall geonet textile fabric to protect the top of the structure.

What about protecting the sides from sideways ground movement?

The sides of the RootCell/StrataCell structure are faced with multiple columns giving good lateral support. Void spaces between the RootCell/StrataCell structure and surrounding sub base should be filled with either further RootCell/StrataCells and soil or the edge of the structure should be lined with geotextile and the reverse side be filled with a suitable compactable base material.

My tree pits are smaller than I would like to see – is it still worthwhile installing some RootCells/StrataCells?

Yes. Many tree pits are smaller than we would like but the idea of the RootCells/StrataCells is to maximize the value of the volume that you have got to the tree. Thus by providing optimum rooting conditions within your small tree pit you are giving the tree an excellent start in life and the vigor to grow out further in the long term.

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