I have a theory that trees can’t read. My theory is based in part because I’ve never seen a tree purchase a book and in part because trees don’t always do what they are supposed to do – according to the many books about trees at least. I can think of several examples where trees haven’t died when they should have, or where limbs should have failed according to available literature, but haven’t. But when comes to roots it seems trees are simply playing with us, they just don’t always do what they are supposed to do. I suspect the fault here doesn’t actually lay with the tree at all, but us. Because we can’t see roots (they are mostly underground after all) it seems that we have made some ‘engineering’ assumptions and then added in a bit of artistic licence to justify our guesses.

Too many quaint notions still exist about the location of tree roots; most of these seem to have no bases in reality. Tree roots do not grow as a mirror image of the above ground part of the tree, they don’t have a single massive stabilising ‘tap’ root thrusting deep into the earth under the main trunk. Tree roots don’t just stop conveniently at the outer edge of the canopy (at the drip line), there is no equal but opposite root distribution (i.e. if the majority of the canopy is to the left, then the majority of the roots will also be to the left) or is the counter loading growth theory true (that if the majority of the canopy is to the left, then the tree will put out roots to the right to counter the load). Having dug around hundreds of not thousands of trees, I can tell you that in almost every situation the convenient ideas about root placement and location are nothing but artistic guesses. While it is possible that some trees may have roots distributed in a similar fashion to the patterns above the overwhelming majority of trees wont; in fact, the overwhelming majority is so overwhelming that its simply safer to say roots don’t grow like that at all.

So if roots don’t conform to some convenient pseudoscience growth pattern, engineering assumption or artistic licence, where are they?

Without digging them up there is no easy way to tell, but we can use what we know to make a calculated guess.

For starters, roots don’t suck tree food from the soil. Tree food, as such, is a product of photosynthesis which takes place above the ground. When you fertilise a plant, you are not feeding it. Roots don’t extract building materials from the soil either, the bulk of the tree is mainly carbon which comes from the air.

Roots provide stability and extract nutrients and water from the soil. Nutrients are used for metabolic pathways and processes and water for almost everything. Stability is key, not so much to keep the tree in place or stop it running away, but to keep the roots still. Roots harvest nutrients from the soil mainly through root hairs. Root hairs are single cell protrusions that grow just behind the root tip. These single cell protrusions aren’t very strong as you could imagen, so if the tree is shaking and the roots are moving about in the soil the root hairs will be damaged and nutrient harvesting will stop. No metabolic pathways and processes means no growth and eventually no life either.

The other thing that we need to keep in mind, is that roots are alive; they are actively living and breathing entities – maybe not breathing in the traditional sense, but gas exchange takes place. This is really important because the further under the ground you go the less gas there is. Gas exchange is most active in the upper layers of soil so that is where the majority of the roots are too. How close to the surface will depend on the soil texture – how pours the soil is. Typically, the majority of roots sit in the first 300mm but in more pours soils this may extend down more than a meter.

Roots harvest nutrients and if all things were equal you could assume that the roots would radiate form the base of the trunk in concentric circles consuming nutrients as they go; but all things are seldom equal (especially in the urban environment). If a tree root finds a readily accessible and bountiful nutrient supply it won’t necessarily invest in additional root growth to find more nutrients elsewhere or to conform to some artistic publication. It won’t think to itself, ‘this is nice, but I must grow some roots over there’, no it will most likely respond by setting up camp and harvesting all it can with the lease effort required; which means, don’t pin too much on the artistic notion of concentric circles.

So with all of this in mind and accepting that trees can’t read where will the roots be?

The distance roots spread away from the trunk and into the soil is as a function of how much and what type of soil there is, how compacted that soil is, the amount of moisture, what the nutrient availability is and where those nutrients can be found. In short, it mainly depends on the soil and there is not a rule-of-thumb that holds true especially if it is based on some artistic publication; but unless removing the soil is an option a rule-of-thumb is required.

My thought here, is that we stop using the pseudoscience engineering assumptions artistic licence approach and start using Tree Protection Zones (TPZ).

Tree Protection Zones, being the radius of 12 times the stem diameter measured at breast height [TPZ radius = DBH X12].

While it is true that TPZs are also generalised models (rules-of-thumb), at least they were created by someone that knew about trees and not some artistic or engineering interpretation based on something unseen. So until such time that trees can read and tell us otherwise, let’s use TPZs to suggest where the roots are.