Garden Detective: Ajuga die-back may be linked to ash tree

12/28/2013 12:00 AM

12/27/2013 5:23 PM

Where did my ajuga go? Last spring I planted a flat of Ajuga reptans in an area that previously had a Modesto ash. The tree was not doing well and split down the trunk in a storm several years ago. I cut it up and had the stump ground out. I then removed as much of the “mulch” that was created by the grinder as I could.

Two years ago, I planted a California sycamore in its place. The sycamore is doing very, very well. It’s really happy. It gets lots of water and has doubled its size since I planted it. The ajuga that I planted last spring did really well over the spring and summer but since the early part of September, I started noticing it declining. It went from nice little 1-foot clumps to nothing. It has virtually died off. What’s going on?

D. Bowden, Sacramento

Your letter prompted debate among the master gardeners. Most agreed: Ajuga reptans is subject to root rot when planted in an area with poor drainage or too much water.

You might try adding 2 or 3 inches of organic matter to the soil, which will help with drainage, and try again.

Your problem might be traced to your Modesto ash. These trees are subject to verticillium wilt, a fungus disease that inhibits a plant’s vascular system. The disease organisms are always present in the soil and become active in cool wet soils. Over time, the disease killed your ash and left you with contaminated soil.

In order to plant shallow-rooted plants (such as ajuga) in that kind of soil, the area needs to be solarized by covering it with clear plastic for six weeks from mid-June to late July.

However, you can not do this as the process will kill many of the young roots on your sycamore. (Sycamores, by the way, are not affected by verticillium wilt.)

Sacramento County master gardeners cannot offer proof positive that the wilt killed your ajuga, but it is within the realm of possibility.

If you decide to not replant the ajuga and re-seed the area to grass, make sure you keep the grass at least 2 feet away from the trunk of your tree as grass growing up to the trunk of young trees inhibits their growth.

Additional details are needed for a definitive diagnosis. However, other potential reasons for your die-back may include fertilizer burn and/or inappropriate sun exposure in addition to poor drainage or excess irrigation. Ajuga prefers to be irrigated sparingly with deep watering every two weeks. By comparison, traditional lawns are irrigated three times a week.

Ajuga is usually pretty easy to grow. Related to mint, this attractive ground cover – also known as carpet bugle – is remarkably tough. Forming a dense mat that blocks out weeds, ajuga tolerates a wide range of soil conditions, helps control erosion and makes itself at home in sun or shade. The scalloped foliage hugs the ground, staying under 3 inches tall.

Ajuga’s dark-bronze leaves tend to be smaller in full sun but can make an unusual border along paths. In late spring, 6-inch spikes of vibrant blue flowers attract beneficial insects, especially butterflies. Another plus: Deer don’t like ajuga and tend to leave it alone. One drawback: Its underground runners can be invasive.

Although ash trees are susceptible to verticillium wilt, we were unable to find information on ajuga’s resistance to verticillium.

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