"Azaleas prefer an acidic soil. Soils near concrete sidewalks, building foundations, driveways or roads are often too alkaline for azaleas. If azaleas are planted in an alkaline soil, the result is iron chlorosis, or yellowing, of the area between the leaf veins."

Osceola County Extension Service
Kissimmee, FL

Jennifer Welshans
osceola.ifas.ufl.edu

"Alkaline building materials, such as limestone gravel and concrete, and high pH irrigation water may also contribute to a soil's alkalinity."

Eldon Everhart, Department of Horticulture
Iowa State University

"Concrete slab foundations, brick, mortar, plaster and other building materials are strongly alkaline. These materials leach into surrounding soil, drastically changing the pH over time."

Pat Ann
University of Florida
IFAS Extension

"These plants will not tolerate alkaline (lime) conditions. Keep this in mind when planting around a new home where lime, masonry material, or cement often become mixed with the surrounding soil. This eventually could cause discoloration and death of the plants."

Ray A. McNeilan
Oregon State University

Also see the chart in the Section titled "What Makes Wet Cement So Dangerous?" in http://neutralite.com/

Do understand that you can plant azaleas/rhodies near these places but you might have to amend the soil if the shrub shows signs of iron chlorosis. The problem will not be immediate and might take years to harm the shrub such that it eventually dies. Most people take preemptive action before allowing that to happen. The damage is bound to be higher if you plant in contact with the foundation; the damage should also be reduced the further away you plant, even if we are just talking inches.

Like you I have seen many rhododendrons growing near concrete in this area, where people plant them everywhere. I think most specimens showing poor foliage color are having other problems, like infertility, excessive exposure and drought. But it could certainly be the case also that in some sites there is an alkalinity problem. However, interveinal chlorosis is not what is often seen.

During the 1970's, when I belonged to the local rhododendron society the editor - a nurseryman - was starting to advocate winter applications of dolomitic limestone to rhododendron plantings. The thinking was many gardened sites here were so leached and acidic that such applications could be expected to produce a visible improvement even from rhododendrons.

In parts of China rhododendrons grow wild on dolomitic limestone formations.

Of course, a given site should be investigated using sampling and testing of the soil before any particular kind of corrective regimen is undertaken.

The point I'm trying to make is that concrete doesn't leach enough calcium to harm Rhododendrons in our acid soil. Look how much calcium you have to add to a lawn to change it's ph. If concrete leached that much lime it would disintegrate.
One of the above references mentions cement. Cement is not concrete. Cement is an ingrediant of concrete.
Concrete slab foundations and concrete foundations have a roof over them and are relatively dry and therefore can't leach much. Cured concrete can't leach much anyway, no matter how much, or little, water it's subjected to.

As bboy says, if a Rhody is having problems next to a foundation other causes are more likely to be at work, soil too dry, too compacted, poor, etc. The one place next to a foundation that can be bad for Rhododendrons is adjacent to a brick chimney because of the mortar left by the brick mason. I have never seen one clean up their mess yet and I've done a lot of new construction landscaping over the last 35 years.

I have a question...we planted 3 different rhodedendrons 2 1/2 years ago in our backyard, along a fence, under a tree(not sure what kind of tree this is). Perhaps 30 feet, or so away, are a line of black walnut trees. Right next to these rhodies is a concrete slab. The slab is just below the soil line. The people that owned the house before us had a doghouse on this slab.

The first spring, two of the three bloomed & they all looked healthy all of that summer. Last spring, none of them bloomed, though they did produce quite a few buds. The buds consistantly disappeared (we have a lot of squirrels...the black walnut trees). At some point last summer, one of them began to wilt a bit and, eventually, a portion of it broke off. This spring, two of them look as if they are on deaths door. One has bloomed...one teeny flower. They are all producing new leaves, but they all have brown or grey in their leaves and a lot of the leaves are wilted.

Is this a result of being just close enough to the black walnut trees? Is it the concrete slab? We are at a loss as to what to do.

I would move them away from the Walnut tree as far as you can. Growth inhibiting chemicals can be draining off the concrete from fallen Walnut leaves. A lot of plants can't be grown under or near Walnuts.

Thank you, that is what I thought. Sadly, I'm not sure we have another place for them. The only other place that's suitable is out front & we have deer. Only the back yard is fenced off.

First, yes black walnut kills rhododendrons. The roots of Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea L.) produce a substance known as juglone (5-hydroxy-alphanapthaquinone). Persian (English or Carpathian) walnut trees are sometimes grafted onto black walnut rootstocks. Many plants such as tomato, potato, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be injured or killed within one to two months of growth within the root zone of these trees. The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree enlarges. Young trees two to eight feet high can have a root diameter twice the height of the top of the tree, with susceptible plants dead within the root zone and dying at the margins. The juglone toxin occurs in the leaves, bark and wood of walnut, but these contain lower concentrations than in the roots. Juglone is poorly soluble in water and does not move very far in the soil.

Second, new concrete leaches lime for a while. If you are familiar with concrete work, you know that you must saturate new concrete with water to complete the reaction that hardens concrete. This results in much runoff of lime water into the surrounding soil. Lime continues to leach out of concrete in a decreasing amount. County agents have many photos of rhododendrons and azaleas that became chlorotic from being too close to relatively new concrete. Old concrete is no problem lime wise, but it is a problem in that it reflects light and intensifies problems related to sunlight.

I had the soil tested where my evergreen bushes are growing and dying 5 feet from a hearty rhodie. Lime was recomended for the soil. Can I safely put lime in at the dead bush site without harming the rhodie?

I am in Seattle Washington. I sent a soil sample into a Washington University office to see if the soil was in some way the culprit for the dying bushes. I got a complex report saying there isn't enough lime. I don't know the name of the bushes but they are the classic ones planted to provide a barrier from the sidewalk. Several turned brown on their sides as if someone had thrown acid on them (that didn't happen) but the bushes around them are healthy and green. I got the lime, but didn't use it once I read what rhodies don't like. Thank you for your interest. I've been curiousas to what "near" means when I read don't put lime near rhodies.

We can't possibly tell you what's wrong with your plants or how to fix them if we don't know what they are :-) I lived in Seattle most of my life and can think of a couple dozen shrubs that would fit the description of a sidewalk barrier or hedge plant. Are you able to post any photos?

I will tell you this - Seattle soils are simply not acidic enough to require lime unless you are growing plants that prefer alkaline soil conditions. And there are just not all that many to worry about and especially any that would be used commonly in the Seattle area. The vast majority of plants grown for landscape purposes prefer soils that are moderately acidic to neutral or with a pH of 6.0 to 7.0 and PNW soils tend to be right there at the lower end.

We just need more information to be able to help......other than to tell you it is highly doubtful you need to add lime. That's typically a recommendation made by someone who doesn't understand local soils and plants.

Thanks for the information. I'm not really asking what is wrong with my shrub but really a query on the lime situation, as in if I were to put it down, how close could I put it to the rhodie. Several agricultural seers at Washington University have looked at my brown shrubs and pronounced them - dead. They then directed me to send in soil samples for a chemical analysis.

However I think your comments will seal the fate of the lime as in no lime. I was just going on the reports from the state as to what my soil was lacking. Soil samples were sent to them along with names of plants that were planted there so it wasn't just a blind response.

Given your information I will forgo the lime, keep tending to the living bushes left and put some garden art in the blank spot.

Thanks again.

Soil testing is only meaningful for a particular plant or crop. Each plant or crop has different soil requirements. In parts of the Pacific Northwest there are places, especially on Vancouver Island where lime is necessary since the pH is too low. In most other areas, calcium sulfate, gypsum, is used for a calcium deficiency. It provides calcium without changing the pH. Rhododendrons do need calcium, but need fairly low pH.

If you think it is low pH, then there should be chlorosis just like with high pH. If no chlorosis, it is unlikely it is nutrient related. In the east we had lots of plants with dieback last winter. Rhododendrons and boxwoods in particular. They had individual branches die during the winter. This is not borers in winter even though it looks the same. In severe cases, all branches died back. It is because when the ground is frozen for months, and there are brief warm periods, the plant suffers drought damage. We saw a lot of dieback this past winter.

I'm not sure what you are reading, but there is nowhere on Vancouver Island where the natural pH is too low. In fact, a soil survey conducted jointly by the BC Ministry of Environment and Ministry of Agriculture and Fisheries characterizes island soils as having "soil parent materials in the survey are generally slightly or medium acid" and they go on to classify these as having a pH of 5.6 to 6.5. Liming is only suggested if soils have a pH below 6.0 and then only for specific applications/crops.

Since the poster is located in Seattle, an area I am intimately familiar with, both in terms of plant variety and soils, the liming or no liming is a non-issue. pH of Seattle garden soils runs from 6.3 to 6.7, hardly an overly acidic situation. Lime is simply is not needed, not even to provide Ca, which is normally present in very adequate levels. And there is no issue of dieback from winter cold - we do not have cold winters here and even the occasional Arctic Blast generally results only minimal foliar damage of fully hardy woody plants.

Really appreciate all these informative posts.

FYI for questions on the free soil testing I referred to. The King Conservation District provides free analysis for up to 5 soild samples. They then send a detailed print out for you to interpret.