We are starting to see more and more synthetic grass being installed in front yards and parkways as an effort to reduce lawns and the attendant water consumption. But synthetic grass or turf can be very harmful to trees if it is installed too close to the tree trunk, blocking the flow of water and oxygen to the root system of the tree, explained arborist Nick Mook.
We spoke to Mook, a thirty-year veteran arborist, this week about his increasing concern that homeowners may not fully realize the damage that synthetic grass can do to an already stressed urban tree canopy.
“We often see installers damage the roots when they attempt to level the ground around the tree. Then they install a decomposed granite (DG) base and place the turf on top. Over time, the DG base hardens like concrete. We can’t even get a soil probe in there to check on the moisture level at the roots,” explained Mook. “If I can’t get a moisture probe in, you can be sure water and oxygen can’t get in either.”
Mook said he’s also seen homeowners watering the turf to wash it down or cool it down, because it absorbs so much heat.
“They are still using water and increasing the temperature of the root system dramatically, which can encourage the growth of harmful pathogens that attack the roots system, like botryosphaeria canker, a common fungal disease that attacks already stressed or weakened trees,” explained Mook.
Trees will often suffer silently for years before showing signs of dying, Mook said, so it’s often hard to figure out what caused their demise.
“It can take a mature tree 2-5 years before it dies, so you won’t see the effects of synthetic grass for several years,” he said. “Once something impacts root system, it’s very hard to change it back. Pathogens and opportunistic insects come in and further destroy the tree.”
If you are going to install synthetic grass, Mook urges keeping the turf away outside the trunk and tree roots. A good measure is to start the turf outside the drip line, or the edge of the tree canopy. For a large, mature tree that can be a significant distance. This will insure that you don’t inadvertently harm the root system of the tree, which is the most critical area of the tree.
On a more positive note, Mook said he is seeing hopeful signs that many trees are doing well enough to survive the continuing drought condition.
“So far, they are hanging in there, especially if they are able to get some supplemental water,” explained Mook. “Trees should be watered deeply, Â 8 to 12 inches below the surface, but infrequently.”
When do you know it’s time to water? The best way to know is to use a soil probe and check the moisture level at least 12 inches down, explained Mook.
What’s the best way to water? Â Mook recommended using concentric rings of soaker hoses starting at the drip line of the tree, working inward to the trunk, but never closer than 2-3 feet from the trunk, or you can end up creating a perfect environment for pathogens.
Sounds like trees are fragile, right? Not really, said Mook. Most of the time trees are quite hardy, but human intervention often can harm trees. For example, trees really don’t need to be trimmed except for safety. They are usually trimmed to fit into our landscape plans, so Mook recommends using natural selective pruning to reduce certain lateral branches that have a heavy mechanical to reduce the risk of breakage and potential damage. Mook recommended against thinning a drought stressed tree, however, because it starve the tree of its food source, causing the tree to rely on its energy reserves to put out more foliage, further stressing the tree and exposing it to pathogens and disease.
Over the years, we have reported on a number of large trees falling over. According to Mook, trees typicallys fall when there is a root defect or decay (Phytophthora Root and Crown Rot) caused by aggressive root pruning, invasion of pathogens, or damage from construction caused by trenching or grade alterations.
“Normally, trees are hearty unless they are stressed by too much water and poor soil management,” explained Mook. “Unfortunately climate change has meant that certain trees we used to see are no longer viable here…like the white birch, white alder, and some trees like olives and California redwoods are struggling.”
Mook explains that we can prevent further harm to our urban trees that we are so lucky to have in our neighborhoods by learning more about water and soil management. He highly recommends getting a soil probe to know when you should deep water your trees, and adding a layer of mulch or shredded bark around the base of the tree. Once installed, mulch can save you from pulling weeds and save water too.
“Make sure it’s no more than 2 inches thick and don’t smother the base of the tree,” said Mook. “Keep the mulch at least  8-12 inches away from the trunk. And be sure to get it from a reputable source so it’s clean and free unwanted materials like plastic.”
Mook says it’s up to homeowners to make sure their trees are being properly cared for, because so many gardeners lack the time and expertise. If you’re interested in learning more about tree care in our neighborhood, send us an email at [email protected]. Mook has offered to walk the neighborhood with us and show examples of good pruning and tree care practices.
