University of Maryland Extension

Hard Decisions When Managing Ash Trees

Stanton Gill, Extension Specialist, University of Maryland Extension
Emerald ash borer adults showing color variation
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Protect it or cut it down?

This is the question that managers have to answer over the next couple of years if there are green or white ash trees planted in the landscape they manage. Green and white ash trees were one of the top selling nursery trees for over 40 years and a lot of them were installed in new communities around office parks throughout the metro area. Many of these trees are now big caliber trees of 15 – 25” DBH. You have heard the saying “The bigger they are the harder they fall”. This situation will be very true in many landscapes.

In May 2013 a meeting was organized to help city managers and arborists understand what we can expect as emerald ash borer continues to establish itself in Maryland. Urban forest managers from mid-west cities have already had to deal with the fall of the ash in the mid-west. They comment that you go through about 3 or 4 years of discovery of the pest in the area, followed by a tidal wave of dying trees which is called the Exponential Death of Trees. You really need to start planning in these early stages because when the Exponential Death of Trees starts you will have a large number of dead trees that have to be taken down.

If you can keep your ash trees alive through the tidal wave then the population goes down and you are hopefully left with standing trees. The questions are how much are you are willing to spend protecting the ash and for how long? There is no natural resistance to the emerald ash borer among ash trees so the choice is with which chemical do you treat the tree to protect its life. You also have to decide when the tree is taken down. The decision process is not simple at all. A big tree not only shades areas reducing temperatures in summer, it absorbs large amounts of runoff water. Taking down a large tree is going to dramatically change the environment in the area where it was growing. It is also expensive to remove a tree and stump grind out the roots. A replacement tree takes years to grow and fill in the area. Inject and save the ash tree or cut it down? I lean toward removing the tree and starting over with another species of tree but it can be argued it is best to inject and save a tree a little while longer until the tidal wave passes by. It may be too expensive to take down all of the ash trees in an area preemptively.

Some city foresters are choosing to go out and rate the quality of the ash tree and evaluate its potential hazard if it dies. They rate the trees on a scale of 1 – 5 with 5 being the trees that should have priority on being removed. One city in Maryland that has 22,000 ash trees in its tree inventory is choosing to remove 500 ash trees each year, picking the ones with a ‘5 ‘rating. They are injecting 25 - 30% of the better ash trees to give them more breathing room before these trees have to be removed. Eventually the plan is to replace the ash trees with other species of trees to have a mixed species stand of trees to protect against outbreaks of other insects coming in the future. This is probably the best long range way of dealing with EAB.

The adult beetle emerges in May at 450 degree days and at the time that black locust trees (Robinia pseudoacacia) are in full bloom. If you are taking action with insecticide treatments then May to June is the best treatment time for most of the treatments.

Imidaclorpid Dinotefuran Emamectin
Application method Soil drench
or Injection
Basal trunk spray Trunk injection Trunk Injection
Time of application Fall or May - June May - June May - June May - June
Residual control 1 year 5 - 7 months 2 years 2 years

*The product must be stored at 40 – 70 °F and will last one year in storage. When you take it out to the field it is recommended that you pack the material in a cooler with ice packs to keep it cool. The formulation is very thick, and the product is injected using large tubes. It takes 30 - 60 minutes to get it into the trees.

What to Do

At a program in May 2013 in Maryland, Chad Tickle, City Forester in Fort Wayne, Indiana started his talk with the comment “As the population of EAB goes up, the options go down.” His approach is to remove a certain number of ash trees each year and treat a set number of trees to buy time until you have to take the trees down. There just is not enough money to take all the ash trees down in most cities on one year’s budget. He mentioned that while in the early stages like is the case in most of Maryland that you can get lulled into thinking you don’t need to do anything. This is a bad decision because in 2-3 year you are suddenly faced with many standing, but dead, ash trees.

During the meeting, Nevin Dawson, forestry expert, University of Maryland Extension, commented that the City of Baltimore has over 300,000 ash trees in the city so it is one Maryland city that better start developing a plan for dealing with EAB and soon.

When Chad started in his job in 2008, there were 59,000 street trees in the Fort Wayne city limits. In 2013 they have 46,000 trees in the city limits. The reduction in tree numbers is mainly due to the removal of ash trees. They still have ash standing, but not many.  Green and white ash was 25% of the Fort Wayne city tree plantings back in 2008. They started their management program with a tree inventory to know what they had and where the trees were located. They then evaluated tree health to determine which trees needed to be removed immediately and which trees could be treated with imidacloprid to give them one year of control and at least 2 more years until the tree would be killed by EAB and have to be removed. It is a buying time strategy. Originally in Fort Wayne they treated 11,000 trees with soil injection of imidacloprid to buy time. Now they treat 1300 trees per year. Chad also noted that emerald ash borer seem to like green ash the most with white ash being less preferred but it still will be attacked. Chad told the group that the 14,000 trees in Fort Wayne has cost $7.2 million to take down and treat from 2008 – 2013. Even with this aggressive plan it is not enough for many people and Chad still receives over 250 complaints about dead or dying trees in the city from citizens who want immediate action to remove the dead trees in their neighborhood. He said you better be organized and prepared. This is good advice for us here in Maryland.

We are not trying to scare anyone with this article but you need to make sure your clientele understand the choices and the consequences of “no action.” Hope this short summary is helpful.

A list of chemicals with time of application and how long they will give control of EAB is available at

Article posted July 2013
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