The Sonoma Garden: Signs of troubled trees

By Karen Boness — I recently got a call from a woman who was very distressed about her Japanese maple trees. As she described it, the trees were dying.

Based on her description I suspected that her trees were succumbing to verticillium wilt. caused by the soil borne Verticillium fungus. It ultimately chokes the tree’s vascular system thereby preventing water and nutrients from moving throughout the tree. One of the most obvious symptoms of verticillium wilt is that entire branches of leaves will wilt, brown and die at the same time.

I came prepared to collect soil, root and foliage samples so I could send them to the lab.

When I was introduced to the trees I was surprised to see that from a distance the trees looked completely normal. They had nice form. They were green. There were no obviously dead branches. This did not look like verticillium wilt.

There were other obvious culprits, though. And these culprits had nothing to do bugs, bacteria or fungus.

Upon closer inspection it was obvious that numerous leaves had dropped from throughout the tree. There was quite a pile of leaves under the trees in an otherwise extremely well-manicured garden. My clients informed me this happened at the end of the last heat wave. Ah! Water and heat-waved stressed trees will shed their leaves in order to preserve the core of the tree. This is especially prevalent in shallow rooted trees such as maples. Maybe the trees simply didn’t get enough water.

I also noted that there was a new, concrete, empty pond just beyond the drip line of the trees. The pond was placed in a small lawn just down slope from the Japanese maples. They were installing a koi pond but there had been months of delays. My clients were anxiously waiting for the contractor to return and complete the job. A tree’s root zone can extend up to three times the tree canopy. So root loss was also another likely culprit.

The irrigation valves for the lawn had been turned off for the koi pond construction. Unfortunately, the maples depended on the lawn sprayers for their water and hadn’t been irrigated in months due to the construction delays. To my client’s credit she had tried to hand water the maples during the heat wave. But it just wasn’t enough.

The site factor that concerned me the most was the mounded soil piled up and compacted around the trunks of the trees. This soil looked like it had been added at some point well after the trees had been planted. It buried the trees’ root crowns and several inches of the lower trunks.

Grade changes around mature trees can be very harmful. If you scrape the soil away you take away important surface roots. If you mound up extra soil you can suffocate those surface roots. Worse yet, circling roots may form in this added soil and ultimately strangle the tree. When I inspected the soil around the trunks I noted it was full of small fibrous roots. My clients reported that this mounded soil had been there when my client’s bought the house. I suspect it had been there for years.

I normally instruct my clients to remove mounded soil that is burying tree root crowns. But I decided to delay that remedy. The risk was too great. The Japanese maples were already stressed and they didn’t need to lose even more roots. I instructed them to provide a deep watering to the trees three times a week. I instructed them to connect drip line emitters or soaker hose for the trees to an existing nearby drip irrigation zone so the trees can get regular water. We will evaluate slowly removing the mounded soil — over the course of several years — in the future.

The don’ts of tree care

• Do not put your trees on the same irrigation zone/line/valve as your lawn. They should be on a separate independent line irrigated with drip emitters or soaker hoses.
• Do not forget to turn your irrigation system back on after the rainy season or after construction.
• Do not bury the root crown of your trees with excess soil or construction debris.
• Do not forget to give all your plants extra water during heat waves.
• Do not remove roots under the canopy of the tree if you can help it. These are the most critical roots. I was happy to see my clients’ new pond was beyond the canopy. Still, I’m sure that some of the trees roots had been severed and weakened the trees.