The Answer to Tree Health Struggles May Need a Closer Look

Our tree commission made a mistake often repeated in the diagnosis of declining plant health. We speculated from a distance without investigating more closely. We had replaced trees along a street frontage in Meadville, PA in 2015, and we knew the site would challenge their survival. The tree spaces consisted of 3-foot by 3-foot sidewalk planting pits, and the area was heavily salted for pedestrian safety during winter. We had planted Japanese tree lilac (Syringa reticulata), and two of the four had died in the past 8 years.

In an effort to help the remaining two trees survive, I decided to adopt the trees in mid-June of 2023. I started watering the trees a gallon each day as I worked the trees into my garden watering schedule. However, close inspection revealed what I least expected. Under the top mulch layer was an impermeable plastic sheet. It was likely placed by a well-meaning landscape volunteer to inhibit weeds were growing in the original mulch layer. However, it also kept moisture from reaching the planted tree.

Investigating below the mulch layer revealed a layer of impervious plastic sheeting placed as a bed liner over an original second mulch layer below. Photo: Scott Sjolander, Penn State Extension.

In looking more closely, at the bottom mulch layer, I found the original landscape fabric underlayment. The fabric was a permeable layer intended for the purpose, and it was placed well. Over the years, however, site challenges had left their cumulative stories visible in the tree trunk. The cambium on the south trunk face had been killed, perhaps from heat reflected off the sidewalk, or from root injury.

A layer of landscape fabric was found under the original layer of mulch. Visible on the tree trunk is a wound from the dieback of the inner bark (cambium) of the south-facing side of the tree. Photo: Scott Sjolander, Penn State Extension.

The combined effects of water shortage stress, heavy safety-related winter salt applications, and reflected sidewalk heat had taken a toll on the landscape. Two trees had died, and the remaining two showed dead twigs in the top of their canopies. I learned once again that it pays to look closely at a landscape when considering a tree health question. Answers may be revealed when peeling the layers of prior care back from the surface. Thankfully, these two Japanese Lilac trees still survive to serve the street landscape. We apply these lessons learned as we plan a salt and drought-resistant landscape in our northwestern Pennsylvania community.

In late summer, dieback of branch tips and wilting and dieback of foliage reflect the impacts of drought, salt, and other stress on the Japanese tree lilac street trees. Photo: Scott Sjolander, Penn State Extension.