A couple of weeks ago, Connie, my neighbour, asked me to take a look at her beloved Japanese lilac. She was worried about its gradual decline. I could see why Connie was concerned. Half the tree had lost all its leaves. Something was obviously wrong, but what could it be?
At first glance I thought it could be verticillium wilt. Not only are Japanese lilacs susceptible to veticillium wilt but the tree also had the classic one-sided dead branch symptom. But appearances can be deceptive. On examining the cut branch, I didn’t see the telltale black streaking in the xylem (just under the bark). It had to be something else.
Then I remembered Consulting Arborist, Jason Miller’s #groundchat on tree roots. Misunderstanding the needs of tree roots can lead to many trees’ demise. I widened my search. I looked around for more clues. Connie’s next-door neighbour renovated her backyard 2 years ago. The garden transformed from a grassed natural area to a concrete patio. This could be it.
Connie confirmed that the Japanese lilac started to weaken the year after her neighbour installed her concrete patio. AHA! We have contact! Something must have happened to the tree roots.
To confirm my suspicion, I emailed the photos over to Jason Miller, consulting arborist at Tree Resolution for a second opinion
“I’m confident the patio was the initial causal agent (likely followed by water stress, weakening immune response and subsequent infection by several microorganisms). Also noteworthy, it appears the subject tree had been planted a bit too deep, which could have made it more susceptible to root damage.”
Root systems are vital to the health and longevity of trees because roots take up water and nutrients. The most important roots are found in upper the 18 to 24 in. (45 to 60 cm) of soil, and can spread two to three times further than the branches. Clearly, Connie’s Japanese lilac roots had spread into the neighbour’s yard. Digging and installing a solid concrete patio must have cut through the roots. To add insult upon injury, the poured concrete patio, no longer allows water or air to filter into the soil, killing any roots that may have survived and eliminating the possibility of root regeneration.
But couldn’t the other roots in Connie’s backyard compensate?
Morton Arboretum sheds some light on this question.
“Balance between the tree’s crown (top) and root system is important for maintaining healthy trees. When roots are lost for any reason, the imbalance creates stress. A tree usually has 4 to 7 major roots. Cutting just one of them within a few feet of the trunk can remove up to 25 percent of the root system.”
That’s huge! Installing the patio may have removed up to 50 percent of the root system. No wonder the tree start to show stress. Connie loves the tree and wants to save it. Consulting Arborist Jason Miller and Morton Arboretum confirm that it’s possible.
Morton Arboretum advises to give the tree extra water during summer dry periods to minimize decline.
Consulting Arborist, Jason Miller advises against thinning the crown.
“Research has shown that crown thinning in attempt to balance root loss results in a substantial reduction in carbohydrate storage, antimicrobial chemical production, and annual ring growth in favour of hormonally induced vegetative growth production (furnishing the illusion of tree health).
I assure you that both older and modern arborist textbooks strictly advise against crown pruning to compensate for root loss unless wind throw is a concern. Where pruning is concerned, removal of dead or diseased branches is advised.”
To further enhance the chances of recovery, Gary Johnson at the University of Minnesota Extension says, “Seriously consider removing the turf from the trees’ root systems and replacing it with mulch and low-growing shrubs and herbaceous perennials.”
Rx for Connie’s Japanese lilac
- Prune the tree’s dead or diseased branches
- Remove the turf and replace it with mulch and low-growing shrubs and herbaceous perennials
- Give the tree extra water during the summer dry periods.
It may take a couple of years before Connie’s Japanese lilac recovers, but it beats cutting it down or letting it slowly die over the next 5 years. Connie will be rewarded not only with a healthy, floriferous Japanese lilac tree but also with the knowledge that her trees increase her property values by 9 to 27 percent.
References and more information
Ash, Cynthia L. University of Minnesota Extension. Verticillium wilt of trees and shrubs
Johnson, Gary. University of Minnesota Extension. Protecting trees from construction damage: a homeowner’s guide
Minnesota Department of Natural Resources. Construction damage causes and remedies – Tree Planting and Care
Ohio State University Extension. Plant Facts: Syringa reticulata – Japanese Tree Lilac (Oleaceae)
The Morton Arboretum. Tree root problems.