Bad news. Forest Hills is about to lose some of its big oaks, the pride and glory of our neighborhood. Look around your block (wherever you live), and you might spot a big oak where all the leaves at one side, or at the top, are already brown and dry. That’s a bad sign.
I realized this a few weeks ago when I walked into my backyard and glanced at the neighbor’s oak. One on side of he tree, the branches were covered with brown dry leaves. When I looked again, I saw that part of the crown was brown and part was green. Then, Marlene Berlin told me that the big oak in the middle of her backyard had big patches of brown leaves, and a tree man had told her it was sick. I began seeing similar trees around our area.
Forest Hills is not alone. Many places are losing oaks. Takoma Park is worried about its street trees. The websites of the Extension Services of Maryland, Pennsylvania and New York write about this as state-wide problems, and arborists at commercial tree companies report getting numerous phone calls.
Excessive rain, heat, and drought bring stress
And in the past year, we’ve had all three. Experts think that the big rains in 2018 and again in spring 2019 caused waterlogged soils, particularly where an oak was growing in a place with limited drainage. Unfortunately our clay soil does not drain well. The present drought (no major rains for six weeks) will add to the stress.
When oaks get stressed they are susceptible to bacteria, fungus, mildew and insect attacks. My neighbor’s tree has now been diagnosed with ambrosia beetles, which bore little holes into the trunks to lay their eggs. This produces a substance that cuts off circulation of fluids in the tree trunk. But it’s pretty clear that some of our trees may have other common diseases, like the leaf scorch shown below.
“We thought they were healthy”
The basic problem is that many city oaks are planted in places where their roots have a hard time getting nutrients and water. They are a woodland tree, and Forest Hills was a woodland area 150 years ago. When our older houses were built, the oak might have had a healthy space around it, but the city, residents and builders have added a number of impermiable surfaces over time, including sidewalks, patios, swimming pools, and cement driveways.
When this happens the roots have a hard time getting to food and water, particularly in the kind of clay soils we have in Forest Hills. I have watched when someone is adding an extension to their home – another impermeable surface – and it often kills an oak.
Experts say that our big old oaks and some species are particularly susceptible, and we have a lot of those species. Younger trees are usually able to manage the stress.
Arborists sometimes recommend that we feed and prune these trees to help them out. (I intend to do this with my oaks.) In a woodland setting, leaves would fall and add to the buildup of nutrient-rich “duff,” a mixture of dead plant and tree material. On our manicured lawns, this doesn’t happen.
What’s the cure?
Unfortunately, there seems to be no good solution. Many of these trees will have to come down. And it’s hard to get at the root of the cause. It’s like trying get a diagnosis from a doctor when your symptoms are common to a variety of illnesses.
What to do
Healthy oaks are still green and will lose leaves in November and December. But if your tree has lots of brown leaves – or leaves that are both green and brown – it may be a problem.
You should call a certified arborist to get an opinion. You will have to use a commercial tree service, but often they will come out and look at your tree for free. Checkbook.org has a good list of tree companies and a good article on tree care.
If a tree has to come down, plant a new tree in its place. Arborists will tell you to avoid oaks. They can recommend other trees that are less susceptible to diseases and better suited for city conditions. And consider letting the leaves collect where they fall.