As a sun-loving species, grass might grow in a spotty pattern under the deep shade of a tree's canopy, leading to an unhealthy look that doesn't fit in with the rest of your landscaping. It can also be difficult to mow without damaging the tree. If having grass around your trees is too much trouble, there are several ways to kill it.
Why Kill Grass?
Trying to mow grass under a tree can be a challenge. You have to dodge low branches and avoid visible roots because the mower blade can carve into them and damage the tree, and the mower as well. The same is true of a string trimmer -- touch the trunk or roots with the string, and you chop away bark and wood. During drought times, the tree competes with the grass for moisture and nutrients. Even in years when rain is plentiful, grass and trees don't have the same nutrient needs, and caring for one might keep the other from getting the food it requires.
Using Elbow Grease
To keep chemicals away from areas where your kids play, choose an organic option to kill the grass, such as digging it up. This doesn't need to be done in one day. Take your time, and get your family involved so you can systematically eliminate the grass surrounding the tree. Water the grass thoroughly so the soil will be easier to dig into. Start near the trunk and dig carefully, paying close attention to the location of roots. If your shovel or spade doesn't go in easily, don't force it and risk digging a chunk out of a shallow root. Push down on the shovel or spade until it's 3 to 4 inches below the surface, and then push down on the handle to lift the head, breaking through grass roots to kill the grass. Toss the lifted grass into a landscape bag or your compost pile. Work outward as far as you like. You might want to eliminate all grass under the tree's canopy, or you might only need a few feet of open space around the trunk.
Covering the Grass
For less intense labor that's still an organic option, cover the grass to kill it. Newspaper works well because it degrades relatively quickly, usually within a few months. Separate black-and-white newspaper pages into stacks of six or eight sheets, and then have your kids help you soak the stacks with water. As you lay the soaked newspaper stacks over the grass you want to kill, overlap the edges by an inch or two to ensure full coverage. When all the grass is covered, dump wood or bark chip mulch over the newspaper to hold it in place as it dries. Together, the paper and mulch will block sunlight the grass needs, effectively killing it. There's no need to remove the mulch later because it's biodegradable. If you're short on newspaper, soak a single layer of cardboard instead.
To kill the grass quickly -- often within a couple of days -- chemical herbicides such as glyphosate serve as your best option. Keep your kids away when spraying herbicides, and wear protective gear such as long sleeves and pants, gloves, eye protection and a dust mask. Choose a day that's not windy. Glyphosate won't hurt most trees if it touches the bark, but any that drifts onto the leaves can kill them. If enough gets blown onto the tree, it can kill the tree. Glyphosate doesn't leach into the soil, according to herbicide manufacturer Monsanto, so it's safe to spray beneath a tree. The spray is unlikely to touch or harm buried roots, but it's best to avoid exposed ones. Keep the spray away from suckers -- small trees growing up from the base of the mature tree's trunk. These don't have thick bark yet, and they can absorb the herbicide and transport it throughout the mature tree's vascular system.