I’m concerned with my purple leaf plum tree. Originally, I was only concerned with the dark crusty limbs, and then I noticed some stuff like dark coffee grounds at the trunk. How bad is this? And how can I help my tree?

— Lori

You have a little monster called peach tree borer. Peach tree borer is the most common serious insect pest in trees such as yours.

In spite of the name, this insect is a problem on members of the genus Prunus that set a “stone type” fruit. This includes peaches, plums, cherries, apricots and almonds.

It also can affect their ornamental relatives such as cistena plum, pink flowering almond, purple leaf plums, European bird cherries, Schubert chokecherry, flowering cherries, double flowering plum, Nanking cherries and more.

This sneaky little guy tunnels around under the bark like other shade tree borers, but it does it low down on the tree near ground level where many people don’t notice the problem until serious damage or even death of the tree occurs. Those “dark coffee grounds” around the base of the tree are evidence of this.

As with most all borers, treatment involves wetting the bark of the tree where this borer is active. However, it’s also important to carefully time the applications to achieve good control.

The way to treat the problem is to drench around the tree (use a watering can or a bucket) with Permethrin. On ornamental trees such as yours, we’d recommend using a 38% concentrate.

Be aware that it’s not labeled for use on edibles. You can only use it on ornamental plants with the problem. Apply the 38% concentrate on June 1, July 1, Aug. 1 and Sept. 1.

On edible fruit trees, you want to use a 2.5% concentrate of Permethrin according to label directions. With the weaker concentrate, we recommend applying the drench around trees approximately on June 1, June 20, July 10, Aug. 1 and Aug. 20.

Wet the bottom 6–8 inches of the trunk and soak the ground immediately around the trunk. It really doesn’t take all that much of the insecticide solution to treat each tree.

Is there an organic alternative to Permethrin? Is this something we have to do next year? My husband is really resisting the use of Permethrin, and I’d rather use something easier on the world, if there’s an option.

Peach tree borer is so prevalent locally that I recommend treating for it every year.

Sooner or later, the tree will get it, and it’s best to prevent infestation in the first place. Permethrin isn’t systemic so it doesn’t get into the tree or the fruit; it’s strictly a contact insecticide.

The only organic solution I’m aware of uses parasitic nematodes. Our experience has been pretty hit-or-miss with these guys. You’ll have to order them online.

They are delicate, so handling them properly is important as is the environment you create around the tree when you apply them. If they dry out, they die.

The company you get them from should be able to give you some guidance on how to handle them.

Also, since there are multiple generations of the borer throughout the summer, I’d say you’ll want to apply them four times each summer: June 1, July 1, Aug. 1 and Sept. 1.


Dennis Hill is the nursery manager at Bookcliff Gardens, bookcliffgardens.com. Send questions to info@bookcliffgardens.com.

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