Let There Be Light: Plant Health Care and the Tree-Turf Conflict

Access to sunlight has more effect on turf performance than root competition. All photos courtesy of the author.

We call it plant health care, but our focus as arborists is tree health care. While our clients are interested in attractive ornamental and shade trees – after all, that is why they call us – they often are more interested in maintaining a uniformly green lawn beneath their trees. They also want turf heath care.

This is where the problem with plant health care begins. Our clients want attractive trees and turf, and the two really do not do well together. Trees belong in forests, where they are associated with other trees and a ground layer of forbs, not grasses. Grasses belong on prairies, surrounded by other grasses and forbs, where a tree might be a distant dot on the horizon.

The difference between these two ecosystems is more profound than just their obvious appearances. Prairies produce a bacterial-rich, organic soil, while forest soils are fungal dominated. The plant community has as much influence on the soils as the soils do on the plant community.

So, planting a tree in a prairie or a patch of grass in the middle of a forest places the newcomer at a competitive disadvantage. It has to adapt to foreign plant neighbors as well as soil microbes. Grasses in forests tend to die out, but once a tree gets a toehold in a prairie, eventually, in the absence of fire, the prairie will become a forest. In the end, trees are going to win.

A common management tactic for solving tree-turf conflicts is similar to how parents manage sibling conflicts – separate them. Many plant-health-care programs suggest giving each its own space. Place mulch (or herbaceous plants) beneath trees and keep trees out of the lawn. This is the best solution, but it’s not one that our clients necessarily want. They envision their front yard having an emerald-green lawn beneath the canopy of a large, spreading tree.

If we have to manage both in the same environment, we first have to recognize that we cannot achieve optimum performance from the turf, and the tree does not benefit from the situation either and may even be harmed, depending on the herbicides used. The tree canopy limits resources to the turfgrass by limiting access to light and its roots through competition for water and elements. But which part of the tree, its canopy or its roots, has the most impact on turfgrass performance?

Surprisingly, it’s the canopy. So let’s review how tree shade impacts turfgrasses. First, not all trees impact turfgrasses to the same degree. There is a difference in the canopy architecture among tree species, a fact apparent to any keen observer of the winter silhouettes of trees. While the shape of tree canopies is influenced by a number of environmental stresses, there are inherent open-growth forms for each species. American elms have an upright form with high, arching branches that support numerous slender, hanging branchlets. Sugar maples, on the other hand, have a more oval form with compact branching. These differences in canopy architecture influence two very important properties of the light penetrating to the ground below, the intensity and the quality.

The intensity of light impacting a leaf surface has a major influence on its photosynthetic capability, the process by which light energy is used to synthesize the carbon compounds essential for plant growth, maintenance, reproduction and defense. Shade has a profound influence on forest development, so it has long been studied by foresters, who classify tree species as to their tolerance of shade.

The most shade-tolerant trees, American beech and sugar maples, as examples, may live for 300 years or more in a forest. In the absence of disturbances, these species will be dominant, as their seedlings and saplings can grow within the intense shade of the mature trees. The leaves of shade-tolerant tree species are very efficient in low-light intensities. The interior leaves are able to photosynthesize in as little as 15% full sunlight. This adaptation to low light permits a very dense canopy to develop. In fact, less than 10% of full sunlight is able to penetrate the dense canopy of these shade-tolerant trees.

Quaking aspen, on the other hand, is a shade-intolerant species. It is one of the first trees to grow in the sun-rich environment following a fire or other severe disturbance. The leaves of shade-intolerant species – birch, aspen and cottonwood, among others – need full sunlight to maximize photosynthesis.

If the lower and interior leaves of shade-intolerant species are heavily shaded by the branches and leaves above them, they may not be capable of photosynthesizing enough food to support their own needs, and subsequently turn yellow and are shed. Eventually, if enough leaves cannot survive in the lower and interior light intensities, their supporting twigs and branches also die and are shed, thus creating a more open canopy. If you look up into an aspen canopy, you’ll notice there is much less self-shading than you’ll find with a sugar maple. About 30% or more of the full sunlight will penetrate these canopies.

These differences in shading from shade-tolerant to shade-intolerant species have a major impact upon the turfgrasses growing beneath them. Many cool-season turfgrasses, the fescues and ryegrasses, may reach light saturation – the light intensity above which greater photosynthesis does not occur – at approximately 50% full sunlight. The canopies of aspen and other shade-intolerant species generally allow enough light to penetrate to support many turfgrass species, while sugar maples and other shade-tolerant species may cast too deep a shade.

Light intensity is not the only influence tree canopies have on turfgrasses. Tree canopies do not just block sunlight, they mine it as well. Tree canopies also alter the spectral composition or quality of light that passes through them. Tree leaves absorb light from the violet-blue and orange-red portions of the spectrum, leaving the remaining light disproportionately higher in the green and yellow wavelengths.

This shade can have deficiencies in the wavelengths of photosynthetically active radiation (PAR) compared to the far-red and ultraviolet ends of the spectrum. This is one reason for the difference in growth between turfgrasses in the shade of a building versus the shade of a tree. The tree has essentially “mined” the light of certain wavelengths of light, whereas building shade, even of the same intensity, still contains more blue and red light proportional to the green and far-red.

As observed with light intensity, there are differences in light quality of the shade among tree species. The shade beneath conifers has more blue light than the shade beneath broadleaf trees, since the opaque needles of conifers serve as neutral filters, reducing all wavelengths equally. Shade-tolerant broadleaf tree species absorb a greater proportion of the red light than shade-intolerant trees.

It is difficult to separate the influences of light intensity and quality on plant growth and development. Generally, they are collectively referred to as shading. Turfgrasses respond to shading with more upright growth and form longer, but narrower, leaf blades. Root, stolon and rhizome growth is reduced proportionally more than leaf growth.

This is why the grass beneath very shade-tolerant species such as Norway maple appears so tufted and sparse; the grass is stretching in response to the changes in light quality (less red light proportional to the far-red) and is robbing resources allocated to the roots to achieve this. This is a good strategy in a prairie where you steal from the roots to get your leaves above your neighbor. Unfortunately, the turfgrass does not realize its neighbor is 50 feet tall.

A Quantum meter used to measure light beneath tree canopies.

There are also many indirect influences of tree canopies on the turfgrasses. The tree canopies can restrict air movement, so there may be more disease pressure on the turf. Some shade-tolerant turfgrass varieties are not necessarily more shade tolerant, they are more disease resistant.
The solution to having trees but still maintaining the turfgrasses? First, try to keep trees as single specimens rather than in groups. While trees are “herd animals” and do better surrounded by other trees, planting as solitary specimens means less collective shading and more sunflecks, the light passing through the canopy from the wind movement of branches and leaves.

Maintaining adequate light beneath trees usually requires pruning. However, the old concept of “spiral pruning,” where limbs are thinned out through the canopy in a winding pattern up the tree, is not generally effective. Thinning to increase light penetration is best limited to shade-
intolerant tree species. Many shade-intolerant species self-prune enough to allow turfgrasses to grow beneath them and might just need a branch or two removed to achieve sufficient light penetration.

Shade-tolerant tree species develop and maintain a thicker array of branching to support a denser arrangement of leaves. It is difficult to thin out enough branches to achieve adequate light to support turfgrasses. A demonstration I conducted on several mature (40-foot), open-grown Norway maples (that were scheduled for removal for building construction) found that we needed to remove about 30% to 40% of the canopy to achieve a midday 30% PAR compared to full sun. Might as well remove the tree at that point.

During a sunny day, the meter may register between 1,400 and 1,600 umol per square meter per second.

As a general rule, you need 30% to 40% sunlight for cool-season turfgrasses (low end for fescue, upper end for bluegrass) and more than 60% for warm-season turfgrasses for at least five hours or more per day for good performance. But how do you know how much shade the tree is creating? Simple, buy a PAR meter. These photosynthetically active radiation meters measure not just the intensity of light, but also the quality of the light.

The meters are easy to use; most just have an on-off switch. Flip the switch on and hold it up in an open lawn on a sunny day. If it’s the middle of the day, the meter will probably read about 1,400 to 1,800 umol/m/s (micromoles of photons per meter of surface per second). Now walk over and stand in the shade of a large tree. If you try to guess the percentage of full sunlight, most people guess about one-half or more (700 to 900 umol/m/s), but it might register only 100 to 200 on the meter! Why? Your eyes are poor light meters. You have a difficult time determining intensity (other than bright or dark), and you cannot determine quality (hmmm – seems there is a little more red light compared to far-red light today).

A PAR meter is an excellent means (and relatively inexpensive, about $200 to $300) for explaining to a client the challenge of trying to grow high-quality turf beneath a large, mature tree – there may not be enough light. If the reading is 600 umol/m/s beneath a honeylocust (mid-
tolerant) over a fescue lawn, a little pruning may achieve the objective. If it’s reading 80 umol/m/s beneath a Norway maple over a buffalo-grass lawn, the only way you can achieve sufficient light for the turf is to basal prune the tree!

While thinning is not usually going to provide sufficient light, crown raising is an option for isolated specimen trees. Pruning out the lower limbs, up to a height of 12 feet, will increase the daily exposure to direct sunlight for the turfgrass, specifically in the morning and late afternoon.

Raising also improves air circulation, and this, along with more morning direct-light exposure, reduces retention of dew. As mentioned earlier, tree canopies allowing moisture to persist longer on turf, which increases disease issues, may be more of a problem than the reduction in light.
Unfortunately, these lower limbs are also usually the largest, and their removal will leave large wounds that are subject to decay. Obviously, crown raising is best addressed during training, when the tree is young, rather than delayed until the tree is mature, but no one calls us then.

Moving to a shaded grove, the flux may drop to less than 200!

The best compromise (other than removing the tree or turf) may be a combination of tactics. A little thinning if that is all that is necessary to achieve sufficient light. Maybe removal of at least the tips of the lower limbs if they are too large for complete removal, so more of a crown reduction than raising, to expose the turfgrass to more direct sunlight.
Also, don’t forget that the shade will alter the turf management as well. The shaded lawn should have a higher mowing height and reduced nitrogen fertilizing (maybe reducing the amount by half), as well as less irrigation. There are also some situations where the use of a growth regulator such as Primo (a gibberellic-acid inhibitor) can be used on the turfgrasses to compensate for the imbalance in light quality (too little red light) that triggers excess shoot growth at the detriment of root growth.

So, the solution to the tree-turf conflict is truly plant health care – managing both the tree and the turf. Plant health care also includes considering the expectations and desires of the client, so don’t forget to pull out the PAR meter to explain to them what is possible and practical!

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