“Zombie trees” sound straight out of science fiction, but don’t worry: Your trees aren’t going to bite you. They’re just thirsty.
Although seven years of drought in California finally relented this March, high heat and lack of water have caused a severe decline in the health of some trees, with many now essentially suspended between life and death, Sacramento-area arborist Matt Morgan said.
“The whole zombie tree issue came about after years of drought stress,” Morgan, assistant district manager with The Davey Tree Expert Co., said. “They structurally declined and the health decreased to a point where the trees are just there right now.”
By “just there,” Morgan means that though the tree may look like it’s alive, it could be suffering from long-term dehydration. Its root structure may not be strong enough to support it, it may be starting to crack, or its foliage may be discolored and dying back.
These traits can be hard to spot if you aren’t looking for them, and Morgan said you should be. Zombie trees are at risk for falling over, which could damage buildings and injure people.
Root structures that grew under dry conditions are weaker than they normally would be, Morgan said, so if rains come in and make it harder for the already weak roots to hold onto the soil, the tree may tumble.
“When you have an unhealthy tree, these roots decline and decay,” Morgan said. “They may be good enough to keep the tree alive for now, but not good enough to hold the structure of the tree.”
California had a wetter-than-usual winter this year, and Sacramento reported February was its sixth-wettest in history with nearly nine inches of rain over 11 days. More than 16 inches of rain fell at Sacramento Executive Airport between October and February, which is 3.3 inches greater than the average year.
Trees most susceptible to becoming zombies are usually older, Morgan said. Growing up, they were likely used to daily watering. But during the drought, they got less than the amount to which they were habituated, causing their health to decline.
Morgan said species like birch and tulip are highly susceptible to changes to their normal environment. But some larger trees may have enough reserve energy to stage a comeback out of the zombie state given the right kind of maintenance practices, he said.
Property owners can evaluate options for their trees by calling a registered arborist to perform an inspection. Cutting down old trees can be sad, but trying to rescue drought-damaged trees can be expensive, and some may be too damaged to rescue. Unhealthy trees may also be at a higher risk for pests, Morgan said, because they have been under too much stress.
One of the biggest concerns caused by the zombies is that they may discourage people from planting trees, Stephanie Robinson of the Sacramento Tree Foundation said.
The danger of trees “going zombie” could make people think they require too much upkeep, and some Californians have also been worried that watering trees might result in fines because of drought-related water restrictions.
“Drought messaging was ‘cut back your water use,’” Robinson said. “It should have been ‘cut back your water use, but save your trees.’”
During the drought, Californians were told to use only 55 gallons of water per day. But that number is “only a guideline for water agencies” and “only applies to indoor water use,” according to the Tree Foundation. Fines would be incurred only for watering during restricted hours or overwatering.
Confusion about drought policies led to a loss of the benefits trees provide, Robinson said. And replanting efforts to make up for that loss will have to take climate change into account, she said. These considerations led the Tree Foundation to “overhaul” its Shady 80 list of recommended trees for planting in the Sacramento area.
“We were looking very critically to see if (trees on the list) are climate ready trees,” Robinson said.
The Tree Foundation is phasing out high water using trees like the red maple and the littleleaf linden to prepare for a hotter, drier climate. They were replaced by more “climate ready” trees like olive, deodar cedar, desert willow and strawberry, Robinson said.
Morgan said the city of Folsom is engaged in a shift similar to the Tree Foundation’s recommendations. He said Folsom is looking at adjusting its tree planting practices to mimic the Phoenix, Ariz., area to prepare for the drying effects of climate change.
If Sacramento residents see a public tree that looks like it could be a zombie, they can report it by calling 311, Robinson said. She also said it’s important for private property owners to call arborists to do inspections yearly, particularly in light of the recent drought.