The Critical Factor for Tree Growth

Design for the tree that it could become.  Adding trees to our urban environment is a good thing. They are good for not only our health and well-being, but also […]

Design for the tree that it could become.

 Adding trees to our urban environment is a good thing. They are good for not only our health and well-being, but also that of the planet. The only problem is that trees are by nature, forest plants, they like room to spread out. But we need to find a way for them to flourish in the middle of busy, crowded towns and cities, where space is already in short supply.

It really is a battle for survival. We have to consider soil compaction, irrigation, or lack of, contamination, service lines and pipes… It’s a lot to ask of a newly planted tree, and as far back as 2000, the London Tree Officers Association anticipated a 50% replacement rate as a result of failed establishment. And there’s a lot more buildings, a lot more traffic, and a lot less space now!

In our experience, there are two key factors that help our newly planted trees to thrive.

First – the quality of the growing medium, and secondly, and probably the most important, space.

The image below was taken by Johan Ostberg, a Swedish Aborist. Believe it or not, these trees were all the same size, and planted at the same time

You can see how the trees around the perimeter, have found additional rooting volume in the verges.

They have fared a lot better than the poor trees restricted by small pits in the middle of the car park.

It is a perfect example of how for a tree to reach its full potential, it not only needs good quality soil to grow in… it also needs space.

And remember, tree roots mostly grow laterally outwards, not downwards. For most UK species, 90% of root growth occurs in the top 500mm of soil.

Root growth is far and away the main factor in a tree achieving its full potential, and it is important that tree pits are designed to allow for the correct amount of root growth for the tree to thrive. It makes sense that after spending all that time and money planning, procuring, and then planting the tree, we want to give it the best chance possible of surviving to maturity.

To give some context; a study at the University of Florida by Dr. Edward Gilman tell us that a 40cm (16”) diameter trunk tree (young to middle age), requires 28 cubic metres of soil. A 60cm (24”) diameter (getting mature) tree requires 48 cubic metres of soil. Of course, we rarely see trees of this size being planted. They are usually young trees with a girth of 10-16cm (4-6”), where such large rooting volumes may seem over-generous or unnecessary. However, we need to design tree pits for the tree that it could become, not the tree that is going in.

I often discuss the matter of rooting volume in my CPD presentations, and every time a landscape contractor sends us a plan with tree pits in hard landscaping to price up, rooting volume is the first thing we look for. It is surprising how many times this crucial piece of information just isn’t available. Many times, we are sent a copy of the plans with a simple, “please price up six tree pits”, with no consideration to how big they are, or what tree is being planted within. Much of this is down to the fact that the contractors are not aware how important the rooting volume is, and once their job is done and the tree is planted, it is certainly not their responsibility if it thrives or dies. That role falls on the shoulders of the planners, tree officers and landscape architects.

We design and supply materials for many urban tree pits, and I received one set of plans last week which happened to have a comparison quote attached to it. There were no size details on the plan, but from the quote, it was quite easy to work out the area and the depth. We were surprised to find that there was less than 2 cubic metres of rooting volume. This may be fine for a holly bush perhaps, but certainly not the small/medium trees that had been specified.

It was a prime example of not enough information being given, and a misunderstanding of the basic requirements for tree pit design by each party that had input. Several phone calls later, the species were established, and tree pits of 12-16 cubic metres were specified. For us to simply quote like for like would have served no purpose whatsoever, and in a worst case scenario, those spec’s may have made it right through to the planting stage. The trees would have stood little to no chance of establishment.

There are no set rules or formula for rooting volume, but from all of the arboriculturists, tree officers, and landscape architects that we have spoken to and worked with over the years, here is a rough guide for you to work from.

And to be fair, trees are quite sociable and don’t mind sharing – so if you can give them a larger tree pit to share, not only do you get more rooting area, but you can also save on excavation time, and labour costs.

So, if you have occasion to find yourself planning tree pits, first establish what trees are going in, then work out how much volume they need to make it to maturity. Once you have this information, work out if you can get that volume on site… we know it is not always possible to put in a tree pit 5m x 5m… if you can’t get the correct amount of volume, consider using a different species of tree that requires less.


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