Heavy pruning – also called “wardrobe” – is never good for ficus and other evergreen trees, but pruning in high heat is even worse.
Evergreen trees like the ficus, which was heavily pruned last weekend outside Universal Studios, where notable screenwriters and actors were picketing, need a big canopy to stay healthy, especially in the summer.
Just like humans, trees are stressed by high temperatures, and just like humans, they need their own shade to protect the more delicate parts of their bark, which is used to growing beneath their canopy, said Don Hodel, tree expert and retired horticultural landscape consultant for the UC Cooperative Extension’s Los Angeles County office.
“Professional certified arborists definitely frown on cloakroom or hatracks where you prune the tree so much that it looks like a hatrack,” Hodel said. “They may do it at the urging of the owner, but a reputable arborist will tell them this is bad for your tree, and some will simply refuse to do it at all.”
Protesters sounded the alarm on TikTok and Twitter over the loss of shade in 90+ degree heat, sparking a heated discussion over what is jokingly called that “tree law” – as in the rules for pruning city-administered trees in the public right of way. NBCUniversal told the Times that tree pruning is done by licensed arborists every year at this time of year. The Los Angeles City Controller’s Office is investigating the issue.
Heavy pruning is harmful because trees use their leaves as solar receptors for photosynthesis, Hodel said. Trees use the water at their roots, sunlight on their leaves, and carbon dioxide to create the carbohydrates — sugars — they need to survive and the oxygen we need to breathe.
When that canopy is removed, the trees depend on stored sugars to survive, Hodel said, adding to the stress of living in extreme heat.
“That’s why trees really need their canopy during hot summer temperatures,” said James Downer, a plant pathologist and horticulturist who teaches tree care at Cal Poly Pomona and has just retired from the Ventura County office of the UC Cooperative Extension.
“Pruning these trees in this massive heatwave is unacceptable,” Downer said. “Everything depends on photosynthesis, so you’re crippling trees in the process. This is the worst time to prune a tree unless you really want to stunt its growth. Then it’s a good time to do it because it will definitely have an impact.”
Like humans, Hodel said, trees have parts of their anatomy that are used to being in the shade. When that canopy is removed, the shade-giving bark is more susceptible to sun damage, leading to wounds that make the tree more susceptible to disease and invasive pests.
In the case of ficus, Hodel and Downer say this is a very serious problem as the trees are very susceptible to the fungal disease Botryosphaeria spp., also known as ficus canker disease or ficus branch dieback, a slow death that causes branches to turn yellow leaves and eventually wither branch by branch, causing the tree to die within a few years.
This pathogen is ubiquitous in Southern California, Downer said, and researchers suspect it’s spread through pruning.
“It’s not proven, but there’s a lot of circumstantial evidence; You can see the pathogens spreading right down the street after pruning,” Downer said. “It’s killing trees all over Beverly Hills, Santa Monica. Pathogens want wounds and pruning causes wounds, and pathogens also like the stress that comes with high heat. Therefore, pruning at this time of year is the worst thing you can do if you want to avoid ficus canker disease.”
Ornamental trees like ficus don’t need pruning at all to stay healthy, Downer said. “People only prune trees for their own needs – because they don’t like the shape, for example, or the garbage truck hits one of the branches.
Downer warned against relying on tree advice from “licensed” arborists because the term “licensed” does not certify anything related to tree knowledge. “If someone says they’re ‘licensed,’ that should be a red flag. It could mean anything, like a city business license, because there are no statewide credentials.”
Certified arborists must pass an arboriculture and care exam to maintain their qualification International Society for Arboriculture, and they would need to continue their education to maintain their qualifications, he said. The organization provides a List of certified arborists by zip code.
If you do need to prune, evergreens like Ficus should only be pruned in late winter or early spring (February through April), Hodel said, just before they start producing new leaves. And you only want to remove a small percentage of the overall canopy, he said, to ensure the tree has enough leaves to stay healthy.
A ficus pruned properly in early spring shouldn’t need another pruning later in the year, he said.
Deciduous trees that shed all their leaves in the fall, including fruit trees, should be pruned before they sprout again, which is January through March in Southern California, Hodel said. Some fruit trees can be pruned slightly in the summer so they don’t get so tall that you can’t reach the fruit, but again you want to keep as much canopy as possible.
When temperatures are soaring, it’s best for trees to give them a deep, thorough soak every few weeks, Hodel said. He uses a drip system around his trees that runs for about an hour every ten days or so.
If you don’t have drip irrigation, just use a very slow-drip hose and run it somewhere between the tree trunk and the outer leaves of the tree, called the drip line.
The aim is to get water deep into the ground, at least 30 cm deep. You can use a shovel to see how deep the water gets into the ground, or you can invest in an inexpensive shovel soil moisture meter With a 12 inch probe to measure the watering depth.
Downer said he has little faith in moisture readers; “It’s better to understand your own soil,” he said. Clay soil holds water longer than sandy soil, he said, but generally, “If you can stick a probe in the soil, the soil is probably wet.” If you can’t put it in the soil, it’s probably very dry.” A safe bet is to water the trees deeply at least once a month during the hot months, he said.
Hodel also recommends placing a few inches of mulch under your trees to conserve moisture and keep the soil cool.