If you’ve ever been through the experience of dealing with creating a sustainable landscape in the immediate vicinity of a fully grown tree, you’ll relate to this article immediately.
But if you’ve never had the pleasure you might wonder what the fuss is about.
The truth is, whether over the course of renovating an existing landscape or in the case of a new landscape over time, eventually, most home landscapers will face the challenge of landscaping underneath a mature tree.
And they’ll soon discover just what a challenge this is.
By their very definition, all trees will eventually grow to be quite tall.
Unless they are rigidly columnar, they will also develop some degree of spread ranging from a dozen feet to a hundred feet or more under the right circumstances.
In any case, mature trees are a grand spectacle of nature and are a force to be reckoned with from the perspective of a homeowner looking to establish some semblance of landscape or gardens in their immediate vicinity.
You can develop an appreciation for the challenge when you look at the inner workings of the native forest habitat.
Here these majestic giants are clearly dominant, dictating everything else that is permitted to grow below and around them.
They are the kings of the forest, and they attempt to retain this regal standing in your home landscape despite what you may wish to have growing in their company.
Think about it – how many glorious gardens have you seen growing in the vicinity of one or more mature trees?
Have you ever noticed that nothing seems to grow near the trunks; even the grass is thin and yellow as it struggles to survive in the presence of an established tree?
What is behind this apparent behavior of plants around mature trees, and what if anything can the homeowner do in terms of a landscaping solution?
A Story Of Competition
To understand what’s going on, we need to look at the nature of competition in the plant world.
In the forest, trees are the dominant species, and anything else that is to grow in their shadows must be able to compete with them.
And that’s no small task; trees didn’t come to rule the forest by being weak and accommodating to other plants.
Let’s take a closer look at the various types of competition that trees impose upon their surroundings, which will help us gain a handle on how to landscape in their presence.
Let’s start with the obvious issues. Mature trees cast a dense shade onto the ground below them.
Their leafy canopies extend high and wide into the sky where they can capture sunlight freely, but little light is able to penetrate this canopy to the ground below.
In the case of a grouping of trees in close proximity, the shade may be absolute, while with one or two trees, there may be varying degrees of shade and light, with more sunlight on the south side of the tree and increasing away from the trunk.
Dense trees cast a deeper shade, while trees with more open canopies let some light through.
The canopy also contributes to a second effect which is a little less obvious; the creation of a “rain shadow”.
You may not realize this, but it is many times drier directly beneath a dense mature tree than it is in the open just a few feet away.
We intuitively know this when we seek the shelter of a large tree during a rain shower, yet we don’t always connect this to the drought conditions that plants growing underneath are forced to endure.
The rain shadow effect is the result of the leaves directing the rain away from the tree trunk towards what’s known as the “drip line”, an imaginary shadow that the tree would cast if the sun was directly overhead.
You see, most trees absorb the majority of their water from the soil at a distance away from the tree equal to this drip line; very little water is actually collected near the trunk.
This is because the finest roots with the most absorbent root hairs are abundant at their farthest from the tree.
So nature has designed the canopies of trees to guide water to this outer periphery, and thus little water gets directly below a tree.
This segues us nicely into what I consider to be the greatest challenge with growing anything underneath a tree; the root system.
Let’s face it, it requires a massive and extensive network of roots to both anchors such a large tree firmly to the ground against storms and winds, and to supply the amounts of water it will expend through its leaves during the process of transpiration.
Trees have the most extensive root systems of all plants, and it’s a tall order for any perennial to compete head-to-head (or root-to-root) with tree roots.
But different trees have different extents of root systems.
Certain trees such as oaks, hickories, and walnuts have what are known as “tap roots”, long carrot-like roots that extend directly beneath the tree to tap into deep reserves of moisture.
These trees tend to be a little less competitive near the surface, although the tap root makes them difficult to transplant.
On the other hand, the roots of some trees extend near the surface for great lengths away from the tree.
Certain tree species such as maples, poplars, and willows are notorious for their competitive surface root systems.
As much as 20’ or more from the trunk, you can find extensive tangles of fine tree roots which will literally choke out any smaller shrubs or perennials.
And as you get closer to the trunk, the roots get thicker and nearer to the surface, making planting and cultivation virtually impossible.
In Search Of Solutions
So we’ve got shade, artificial drought, and masses of roots to contend with underneath a mature tree.
Now you can appreciate why it’s so very difficult to grow anything nearby. Even the toughest grass isn’t up to the challenge.
But there are effective landscaping solutions and tricks at your disposal, so let’s have a look.
For one thing, not all trees exhibit these tendencies equally; a few will actually permit a happy coexistence with other shrubs and perennials.
The graceful honeylocust has fine leaves and a very open canopy which permits a fair amount of light and rain to penetrate through to the ground beneath, making it a favorite with home gardeners.
In general, trees with deep tap roots will permit gardening a little closer to the trunk, although not right up against it.
And narrow upright trees allow for more light and rain below, although the root systems may be deceptively wider than the canopy.
Instead of having delicate gardens in the immediate proximity of your mature trees, why not use durable groundcovers?
Certain groundcover plants are aggressive enough to compete with tree roots and can tolerate the drought and shade beneath; look to perennial favorites like Japanese spurge, barrenwort, cranesbill, dead nettle, or even the dreaded bishop’s goutweed (one of the very few places where this noxious weed has a landscape purpose) for reliable performance.
Shrubs require a greater degree of moisture and might be more of a challenge to grow around mature trees, but tough hombres like bush honeysuckle or Virginia creeper may prove up to the challenge.
Some creative gardeners will plant annuals like impatiens or petunias around the base of mature trees, first creating a little raised garden of fresh soil in which they can grow without competition from the roots.
The effect is beautiful, but a word of caution to the non-diehard gardeners in the crowd; you should expect to water these gardens at least twice a week all season long, they’ll require replanting every spring, and you’ll have to remove and then replace the raised soil every year.
Forget perennials here; they have deeper roots than annuals and would be up against the thick roots of the tree in no time.
Other “gardeners” try and force raised planters up and around the trunk of the tree, using it as an anchor as you might the foundation of a house.
This is a very bad idea! For one thing, mature trees are highly sensitive to grade changes, which is what you are affecting around their roots.
For another, you can’t just load up wet soil against the bark; it will begin to rot and you may ultimately lose the entire tree.
For the toughest of mature trees, you might want to consider installing a hardscaping groundcover, for example, a decorative bark mulch, gravel, or rock mulch.
The landscape effect is both stunning and enduring, and you’ll never have to worry about watering or maintenance.
I am fast becoming a fan of this hot new trend in suburban low-maintenance landscaping; trust me, in a few years it will be all the rage.
As long as the mulch isn’t too thick, it should be of no consequence to the tree.
Final Words Of Caution
No matter what solution you choose to employ, remember that it has to work for the mature tree as well.
Established trees despise having their roots disturbed in any way, so don’t go planning to dig up a new garden bed underneath a mature tree without serious consequences.
In the case of poplars, plums, and chokecherries, you’ll probably trigger a vigorous suckering response that’ll have you cursing the day for years to come. Other trees may just outright die!
And be careful about changing the grade around your trees. They have developed and matured relative to a certain grade and drainage pattern, and changing this may stress them to the point where they turn up their roots in dismay.
Changing grade can change drainage patterns, creating pools of standing water where there were none before, or burying established roots under soil thus depriving them of necessary oxygen.
As a rule, don’t dig any deeper than 6” around a tree, and don’t increase the grade by any more than about 2”, except in the first couple of feet from the trunk (but not up against).
Forget trying to grow grass right up to the base of your mature trees. If they have any canopies whatsoever, the wicked combination of shade and drought is all but a death knell for our sacred suburban turf species.
Even the root systems of otherwise columnar trees will eventually outcompete the grass for moisture and leave it a dry and crispy brown by mid-summer. So don’t even bother; look for alternative solutions.
And finally, some trees exhibit a tendency known as allelopathy, whereby their roots secrete a substance that is toxic to other plant species, killing them within the radius of the drip line.
Walnuts are the best example of this, secreting a substance known as “juglone” which is toxic to rhododendrons, tomatoes, and a number of other plants.
For mature members of the walnut family (Juglans spp.), it’s best to simply go with hardscaping solutions.
So there you have it; a few suggestions of what to do and what not to do when landscaping in the presence of your stately landscape trees.
By the way, there’s nothing wrong with developing complete shrub and perennial gardens around young and newly transplanted trees; it will probably be 5-10 years before the trees begin to exert their dominance over the other plants.
But do be prepared to eventually lose out the balance of the gardens to the trees – it is inevitable, so don’t say you weren’t warned!