Dear Miss Floribunda,
About a year ago, I noticed fungus-like growth on my azalea bushes (see attached picture). It’s slowly been getting worse, and some of the branches have died. What’s going on here, and what should I do about it? I’ve had these shrubs for a long time and don’t want to lose them. They border the steps going up from the street to my door, and what I love about them is that even when they’re not in bloom they are handsome and provide curb appeal.
Frustrated and Anxious on 42nd Avenue
Dear Frustrated and Anxious,
What you have on your azaleas is lichen. In itself lichen doesn’t harm plants, but it is a symptom that the plant isn’t doing well. As you know, Aunt Sioux and I made a house call to examine your ailing azaleas. Noticing that your steps were made of concrete, we called in our soil expert, Dr. Agronomosky. He confirmed our suspicion that the probable cause of your azaleas’ poor health is that lime leaching from the concrete has changed the naturally acidic pH of your soil to alkaline. Azaleas require quite acidic soil to thrive. Although almost all gardeners are aware of pH, just as we are all aware of electricity in our homes, few of us really understand much more than what we see when we flick a switch, or when we add lime or sulfur to change soil pH.
PH, or “potential hydrogen,” concerns the level at which a plant can absorb hydrogen, an element vital to plants. Without hydrogen, the ions of which are absorbed from the soil through water, a plant cannot take in any other nutrients. The concept was introduced in 1909 by the Danish chemist Søren Peder Lauritz Sørensen, who was working at the Carlsberg Laboratory studying ion concentration on proteins. He invented the pH scale based on electrons. The level of pH is measured from 0 to 14, with 0 indicating highest acidity and 14 highest alkalinity. Soil with a rating of 7 pH is neutral and is acceptable to most plants. It’s important to be aware that the amount of hydrogen needed by specific plants differ, and soil that is too acidic might not attract enough hydrogen for one plant, while soil too alkaline would attract too much hydrogen for another. No plant likes either extreme. Azaleas and most other evergreens generally like soil with a pH as low as 4.5 but no higher than 6. In general, soils with high alkalinity have a large component of limestone in them — as does the concrete forming your steps.
You can add sulfur, peat moss or iron sulfate to your soil to acidify it, but Dr. Agronomosky advises getting your soil tested professionally before you make any such decision. You can find information on labs that test soil at the University of Maryland Home and Garden website: extension.umd.edu/hgic, or you can call their Agricultural Nutrient Management Program at 301.405.1319. Dr. Agronomosky doesn’t recommend the purchase of a home soil testing kit because considerable expertise is needed to interpret results. It usually costs from $5.50 to $27 to have testing done by experienced technicians.
There is another solution to your problem. Late next February, or very early March — before your azaleas wake from their winter dormancy — you can dig them up and move them to a part of your garden away from those steps. (Almost any area where you haven’t added lime or bone meal would be fine. Unamended garden soil in our area is generally acidic enough for azaleas.) In their place you could plant something that actually likes an alkaline, or “sweet” soil. I suspect from your appreciation of the way azaleas stay attractive when out of bloom that only a similar evergreen shrub would do. The only evergreen shrub I can think of that would tolerate alkaline soil is boxwood, but this could be researched further for another column.
In the meantime, please come to the Hyattsville Horticultural Society’s autumn plant exchange on Saturday, Oct. 21, after a brief meeting at 10 a.m. It will take place at the home of Joe Buriel and Dave Roeder on 3909 Longfellow Street. There will be many gardeners there who can advise you on possible choices.
