Trees and grass - Do they naturally co-habit well?

Guy Watsonin Conservation & Ecology

Spring is nearly with us and, whilst some of you may not really have stopped cutting grass, for others you will be starting to think about the season ahead. Grass is great; it's green, lush and looks good but it can be an issue for the trees that grow in it. Guy Watson of Certhia Consulting Limited explains why.

I don't need to tell you that grass grows - sometimes very fast. It is hungry, taking a lot of nutrient to continue that fast growth. It needs a lot of moisture and therefore it is very competitive. To keep it looking neat and well cared for, it will need constant maintenance. We cut it, it grows, to keep it neat we cut it a lot, it grows - and every time it grows, it takes resource from the soil. And so, we get to the conflict. Trees and grass do not necessarily co-habit well together.

Grass is an interesting plant and engenders many emotions. For some, grass has to be managed to the nth degree. Cut at least once a week, contrast lines, even height, colour, type, not a weed in site. For others, letting the grass grow on to a more natural height, letting it develop a meadow effect is the way forward. Depending on the function of your grass, there may be no choice as to how it is managed. Where lawns are required and trees are close neighbours, there may be problems.

Physical harm often occurs where mowers are run close to trees. Many trees show bark wounds around the base and this is an obvious result of that impact damage. Understandably, any bark wound is detrimental to the tree. Tree bark is made of several layers of cells. The outer layer is the impervious protective layer of corky material that prevents water loss and protects against insect and disease predation. Just behind the cork layer are the vital phloem and cambium layers. The phloem moves the sugars from leaf photosynthesis down and around the tree, mostly for storage in the roots. Cambium is the meristematic layer that produces new cells, becoming xylem, wood to the inside, and more phloem to the outside. This layer is a single cell thick; if the bark is removed, this layer is also removed, meaning no new cells can be produced. That part of the tree will then simply become dysfunctional, the exposed wood may become colonised with decay organisms, the transport of water and nutrients will be disrupted and will therefore affect the vitality of the tree.

Not all mower impacts result in clear bark removal. Bumping a tree repeatedly with the deck or other parts of a mower will bruise thin-barked or young trees to the extent that over time the same issues will occur. Mowers should be kept at a safe distance from the tree base to prevent impact damage.
Filament line trimmers, strimmers, brushcutters, call them what you like, cause significant harm to the bark of newly planted young trees and trees with thin bark (examples include Beech, Birch and Maples). Whilst adverts still imply that you can cut against trees with no harm, I'm afraid the simple truth is that repeated cutting in close proximity will cause bark cutting and death. In many cases, the only part of the tree remaining unaffected is the part immediately behind the stake - where the trimmer line cannot reach. Please, please, make sure that young trees have suitable mulch and/or mulch mats to prevent the need to cut grass.

When considering new trees think about the other land use and management you anticipate. If you want a formal lawn, then maybe trees do not have a long-term place within it. If your scheme requires that formality, then the design and management should include a hand cultivated geometric clear soil area around the tree.

Some trees produce large 'surface' roots close to the stem and these may extend several metres from the tree, snaking across the ground. These are very vulnerable to damage, both from the cutting action of mowers and also from the friction of wheels etc. running across them. Depending upon tree species, the damage to these roots can result in other issues similar to the physical damage to the bark (see earlier). Poplar, Elm and False Acacia for example are renowned for their suckering reproductive strategy. However, the tree generally produces suppressing hormones from the terminal buds that are filtered down the system to prevent suckering. As the tree declines in health and vitality, the hormone production slows. If damage occurs between the two points, then suckering can be triggered. Once suckering commences, it can be very difficult to stop. If it occurs in formal lawns, it can be very disruptive. The best cure is prevention: keep the trees in good vitality and prevent damage to the surface roots. This may mean restricting mowing, lifting decks above root height, a minimal covering of soil to allow mowers to run unobstructed. All these have the additional benefit that it will prevent damage to the equipment used.

You could avoid this conflict by preventing grass growing around the base of the tree. This means that no mower or strimmer need get anywhere near the tree or its buttresses. Hand cultivation in a formal lawn can work well, but is time consuming and expensive. In a wider landscape context, it may be incongruous. Mulching is an obvious choice and has many benefits to the tree. In an ideal world, the mulch ring would extend to the 'drip-line' (the vertical edge of the branch extent). This may be a very large area but will improve the soil structure under the tree, increase nutrient, prevent water loss, increase earthworm activity and encourage beneficial micro-organisms. If an extensive mulch area cannot be achieved, any area of mulch will be beneficial. Mulch has other benefits of application; many people do not want to walk over mulch areas and would prefer to walk on grass. Therefore, mulch will prevent increased target values and reduce any footfall related compaction (more later). However, there have been instances recently where mulch rings have become self-defeating. Kew Gardens have applied extensive mulch rings around important trees in the gardens, but the combination of sandy soils and high visitor numbers have compacted some of these into hard, impermeable surfaces under which is a sterile, dry, inhospitable environment. These have had to be physically broken up and removed. New mulch is now incorporated into the upper soil horizons to ensure contact with the natural soil micro-organisms is maintained, thereby assisting degradation and nutrient release. When the mulch rings are topped up, these too are incorporated. Gone are the days of simply spreading layer after layer of mulch and standing back.

Herbicide could be considered, but please use with care. A non-selective herbicide kills all it touches, including trees! Any tree with basal growth, for example Lime, will be particularly at risk. Trees are vulnerable to inappropriate application. Any spray that drifts onto the leaves will be uptaken by the tree and, whilst it may not kill it outright, will build up over time and reduce vitality. Trees with recent damage, exposing active tissue, may also be at risk. Typical symptoms of herbicide damage are misshapen, small yellow leaves. The common herbicide, glyphosate, is inactivated by being bound to clay and organic matter particles in the soil. Trees in very sandy soils, or sand bedded paving areas, may be particularly vulnerable as the active ingredient may then be available for uptake from soil waters. This has been shown to be the case in a number of civic planting schemes. I will leave all the other arguments for and against glyphosate for someone better qualified!

The use of selective herbicides may also be implicated in the decline of tree vitality. Grass is a monocot. Selective herbicides are designed to eradicate the broadleaf, dicot weeds (buttercups and dandelions for example). Trees are simply dicots and may be affected by regular applications of a selective 'weed and feed' type solution.

Have you considered replacing grass with other appropriate plants and so remove the need to get a mower anywhere near the base of the tree? Shrubs tend to have a lower water requirement and create a more natural environment. Choosing the right shrub may have increased benefits for pollinating insects, food and nectar sources, nesting, roosting and hibernating habitats etc. Bear in mind any plant that may hinder future inspections around the tree base due to their particularly thorny nature, sheer density or proximity should be avoided.

The underlying cause of failure in a lot of young trees is the conflict between weed and grass control and the damage that may occur to the tree. As trees mature, the bark thickens on most species and impact damage may become less of an issue. Similarly, as the canopy develops, the shade generated by the tree may slow grass growth. But, any damage is detrimental, may increase the likelihood of problems and reduce retention spans. Do you really need to manage the grass so close to the trees? Certainly, in the wider landscape, parkland, less formal school grounds, larger gardens, away from the very formal areas, there is no need to do so.

Another issue that may not be immediately obvious is the compaction that running mowers on a regular basis causes. Mowers can be heavy - lots of mechanical parts, fuel, hydraulic fluids, a body sitting on top. A mid-range mower can easily weigh in the region of one tonne, larger models more. Generally, they are on relatively small wheels and the ground pressure they exert can result in harm to the soil structure underneath. There is very limited information in relation to actual ground pressure on a machine by machine basis available. A very rough calculation with nominal tyre sizes and weights indicates that a mid-range mower could exert a psi of 6-8 pounds per square inch. This will vary with tyre pressures, fluid levels and the machine configuration. As the grass growing season seems to be getting longer and longer, and mowing runs on into the autumn/winter months when the soil gets wetter, the increased risk of compaction intensifies. Particular care must be taken on clay-based soils where compaction becomes more problematic. Having said that, pedestrian mowers may have similar compaction issues. An average human male, if there is such a thing, may exert a ground pressure of 8 pounds per square inch.

How much pressure is 'too much' is impossible to answer. If you run machines over the same area on a regular basis, if those areas are wet and have a reasonable clay content, and if your trees are showing signs of decreased vitality, this could indicate compaction issues. It may be time to investigate further and review your management schemes.

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Conservation & ecology