If a tree grows very tall, it must grow very deep, right?
Not necessarily.
While ultimately, the depth and spread of tree roots are dependent on the soil conditions in which the tree is growing, there are some generalities based on specific species of tree and their specific root growth patterns.
On the whole, tree roots do not grow deeper than 1-2 meters (3-6 feet) below ground level, although in ideal soil conditions, the root system of certain trees with deep roots may extend down to 4 meters (13 feet).
However, that degree of downward growth is considered to be very unusual, and most roots occur in the top 60cm (2 feet) of soil.
But, just like some of us grow taller than others, some trees grow deeper than others.
Taproots
A tree’s downward growing root — or roots, as some do branch off and continue their downward growth — is called the taproot.
All trees have a taproot in their seedling stage; it is this that first emerges from the seed as the radicle. However, in most trees, the taproot soon loses dominance as the tree develops a network of lateral, spreading roots in search of water and nutrients.
Some trees will have a taproot of only a few feet, but in others, it can grow to the 2 meters mentioned above.
Trees That Can Develop Long Taproots
- Oak
- Black Walnut
- Butternut (related to black walnut with a less significant taproot)
- Pine (some species)
- Silver Fir
- Hickory
- White Mulberry
Although all of the trees above can grow long taproots, they will all also adapt to their growing conditions. If soil conditions don’t favor the development of a deep taproot, the tree won’t grow one.
It’s also thought that each tree carries its own genetically pre-determined ability for its roots to cope with sub-optimal soil conditions: heavy clay, for example. This would explain in part why tree growth (and tree root growth) can vary even under the same growing conditions.
Even in those species mentioned above with a propensity to long taproot growth, it’s not unusual for them to be found without any significant downward-growing root system.
But if you are concerned about the downward growth of a tree, it would be best not to choose one of the varieties mentioned above. They are certainly much more difficult to transplant, in some cases impossible.
Lateral Roots
A tree’s most extensive roots are known as lateral roots. These roots grow outward from the trunk of a tree, forming a vast network that will become thicker and woodier over time, especially near the surface of the soil.
Trees like ash, cherry, thorn, and some species of pine have a very shallow lateral root system. Oblique lateral roots, present in birch, lime, and oak, descend diagonally downwards in a similar fashion.
It may not grow very deep, even when you take into account the “sinker roots” that may grow down from laterals, but the lateral root system can grow to at least the same spread as the tree’s crown, with some experts suggesting that a tree’s root system may in fact cover an area three times as big as its canopy.
The UK’s Woodland Trust advises that all trees be given a root protection area, or RPA. The RPA is the minimum zone of protection required for each tree to maintain its health and stability throughout its life-span. This is useful in town planning, zoning, and development, but is equally important in domestic, garden environments.
The RPA for all trees, regardless of their type or age, should be either an area extending 5 meters (16.4 feet) from the trees’s crown or an area with a radius of at least 15 times the tree trunk’s diameter at 1.5 meters (5 feet) from the ground. However, even this might not be enough!
In 2018, updated radar technology discovered large roots, 3/4 of an inch thick, 23 metres from a veteran Pedunculate oak tree. And given that a tree’s fine roots and mycorrhizal network stretch even further beyond its woody roots, even an RPA of 23 meters — 75 feet — would not have been enough to protect that tree fully.
Invasive Roots
Any tree’s roots have the potential to be “invasive”, depending on where the tree is planted. The RPA mentioned above should be a good indicator of the care that must be taken when planning (or planting) trees in your garden.
Bear in mind as well that, while we refer to roots as invasive when they’re where we don’t want them to be, they’re only trying to support themselves and find the nutrients and water they need.
That said, some trees have the potential for their roots to cause more trouble than others.
1. Willow
Willow trees are known for sending their roots far in search of water. As discussed in our article on trees for wet areas and damp soil types, this can have a range of consequences for your home, plumbing, and garden.
2. Silver maple
Their shallow roots grow very fast, so they can easily displace or crack paving, pipes, or even foundations.
3. White mulberry (Morus alba)
These trees have thick, knotty, fast-growing roots which may cause problems if they’re planted too close to buildings or paving.
4. Cherry
Cherry trees have very shallow root systems so are more likely to lift paving or driveways.
5. Sycamore
As with the other trees mentioned here, sycamores have extensive, shallow root systems and can cause damage to paving, pipes, and buildings.
6. American Elm (Ulmus americana)
Another tree with an extensive, shallow root system.
7. Fig Tree
In optimal growing conditions, the roots of fig trees can spread out so far that sometimes gardeners take special measures to keep their growth in check. Two possible solutions are to plant fig trees in large pots in the ground or construct underground retaining structures for their roots: figs require foresight!
8. Beech Tree (Fagus)
Again, while not exactly invasive, a beech tree’s root network may be very shallow and will make planting anything else in the area almost impossible.
General Considerations When Choosing Trees
If you know that your yard or garden is, for example, just a few feet of soil on top of bedrock, then it would probably make sense for you to choose a species of tree that didn’t have a tendency to growing a long taproot.
If, however, you have a wealth of rich, deep soil, and you know that your tree would be able to grow in its chosen spot for many years to come, then you can consider a deeper-rooted and less easily-moved tree like an oak or pine.
But even so: there are pine trees growing happily out of necessity in shallow soil, and oaks that adapt their root networks to match their environments.
When you’re considering planting a tree, your best way forward is to learn as much as you can about the tree and your environment. Do your best to match the type of tree you plant to the soil and climate in which you’ll plant it.
If very shallow roots, an extensive root spread, or deep roots are a problem, think about alternatives that better suit where you are.
Trees are very adaptable; it’s partly this that gets them labelled as invasive: they will go in search of water if there isn’t enough where they are. But adaptation doesn’t necessarily equal thriving. If you want your tree to grow as well as it can, think about where you’ll put it.
Make sure there’s enough space for it to grow down if that’s what it naturally prefers to do. Ensure that it won’t be upsetting anyone’s patio if its natural habit is to have spreading, shallow roots.
A little planning and forethought can help to ensure that you, and your tree, are happy.
