Ever notice that the grass around the trees in your lawn isn’t as thick as the rest of the yard? Or, if you have a lot of trees, maybe there is no grass growing under them at all.
Trees can pose a problem for grass. First, the roots of these two plants compete with each other for water, nutrients, oxygen and soil. Second, there are some trees that will not allow grass to grow – in other words, the two just don’t mix. Let’s explore this interaction of trees and grass in more detail.
For turf to stay healthy, it will need a minimum of four hours of sunlight during the course of the day to maintain itself. Those four hours don’t have to be consecutive hours – it can be one in the morning and three in the afternoon for example.
Generally, grass growing in low light conditions will be etiolated, or tall and thin. This is due to the cells within the plant elongating and becoming thinner as a reaction to the lack of sunlight. It is a natural reaction by the grass to catch more sunlight on its surface area to produce food via photosynthesis. Therefore, you should mow this area high, so the grass can capture enough sunlight to sustain itself.
Lack of sunlight is the number one concern when attempting to grow grass under trees. The grass should be a shade-tolerant variety, and the trees should be pruned to allow enough light to reach the grass. Otherwise, the grass will never be fully healthy like the rest of the lawn.
Two methods of pruning to allow more light to the grass underneath are thinning and “lifting the skirts,” or removing the lower branches. Thin trees by taking out some of the branches all the way back to the trunk or a larger branch. You can remove up to 1/3 of the branches in a tree this way without altering the appearance of the tree. Thinning must be repeated every two or three years.
Removing some lower branches may change the looks of the tree substantially if its branches sweep the ground, but it makes most shade trees more attractive. Lifting the skirts is permanent; the lower branches won’t re-grow.
Generally, both of these pruning methods are used in combination to let more light through a tree canopy. If the trees are large, don’t attempt the pruning yourself-it’s dangerous. Get a certified arborist to do the job.
Grass growing under and around trees is in direct competition with the tree for sunlight, nutrients, water and oxygen.
By cutting back the limbs on the tree, you can help the grass receive an adequate amount of sun. You should also make sure this area receives more water than the rest of the lawn. Water the drip line, and not right up against the trunk of the tree. The drip line of a tree extends out as far as the tree’s branches. Applying a little extra fertilizer in this area will also help.
Trees that don’t receive adequate amounts of water tend to have shallow roots, and the roots in the upper zone expand and end up on the surface. Some trees do this naturally in order to survive. Compacted spoil will also cause the roots to expand to the surface. Either way, it is impossible to grow grass on top of the tree’s roots.
Grass Won’t Grow Here
Finally, some trees are not conducive to ever growing grass under the tree.
If the tree has an extremely thick canopy, then sunlight will not penetrate through it. Examples would be pines and many other conifers.
Other trees produce natural toxins that have an allelopathic effect on the grass – that is, the toxins negatively affect its growth. Black walnut, honey locust, sycamore, eucalyptus, hackberry, and silver maple have shown some allelopathic reactions with grass.
Finally, the organic breakdown from some tree material will prevent grass from growing. This is true of pine trees, where the needles and bark that fall from the tree are acidic, and over the years their breakdown causes the soil to become acidic, which is not conducive to healthy grass. Plus, pine trees typically do not allow enough sunlight to penetrate to the ground below, and they are fierce competitors for food and water.
Trees Don’t Like Grass, Either
Grass, because of its dense root system, out-competes the trees for food and water in the top foot or so of soil. Since this is where trees have most of their roots, trees growing in lawns usually grow more slowly than trees growing in groundcovers or shrub beds. You can overcome this problem by providing extra food and water under trees growing in lawns. It also help to place fertilizer for the tree below the grass roots. One way to do this is with fertilizer spikes.
And Trees Don’t Like Mowers
Lawnmowers and string trimmers often bruise tree bark, crushing it and causing dead spots or even peeling it off. This problem is so common that arborists refer to it as “lawnmower blight”. Since the circulation of the tree passes just under the bark, damage done to the bark slows down water passage to the top and sugar passage to the roots, starving the tree. Damage to 1/3 of the circumference is significant and causes a marked loss of vigor.
The best defense against lawn mower blight is to keep a ring of mulch or plantings around the trunk, so there is no reason for lawn mowers to go there. If the mulch or ground cover extends to the drip line of the tree, it also saves both the grass and the tree from competition. Many shade-tolerant plants are adapted to growing under trees and will thrive there without harming the tree.
Tips for trees in the lawn:
- Make sure the grass around the tree receives at least four hours of sunlight.
- Plant a shade-tolerant grass variety around the tree.
- Consider a mulch ring around the tree or a ground cover instead of grass.
- When watering a tree, water inside the drip-line and water deeply to encourage deep root growth.
- Fertilizing a little extra, not extreme will also benefit the grass.
- Check with an arborist or your local county extension agent to find out if a tree in your lawn may have an allelopathic reaction with the grass.