Rhodies danger to foundation?

JennNHJuly 25, 2013

I'm a gardening novice and learning by lots of trial and error. My brother in law told me rhodies were harmful to foundations because of "aggressive roots." I hadn't heard that before and wondered if he was confusing them with lilacs (which abound here in NH). Any thoughts? I planted two lovely bushes along my foundation three years ago and they just started to bloom this year. I would be bummed to have to move them.

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Rhododendrons pose no danger at all to a foundation. Their roots remain fine and threadlike for the life of the plant and grow only in the upper 8-12" of the soil. As for their alleged aggressiveness, many of us who are devoted to rhododendrons would probably wish they were a bit more so - it often takes a good deal of time for their roots to really take hold after they've been planted out.

    Bookmark   July 26, 2013 at 5:48AM
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rhodyman(SE PA, USDA Z6)

What aka says was really brought home to me when I visited the Rhododendron Species Foundation a few years ago. They were transplanting some of their largest rhododendrons. They had the roots clamped down on a shipping pallet for the move. The roots were only about 5 inches thick and didn't even extend all the way to the drip line. Here is a photo of a rhododendron that was 8 to 9 feet tall:

    Bookmark   July 29, 2013 at 9:29AM
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Birdsong72(7/Northshore NJ)

It's the other way around Jenn. Oftentimes planting a rhodie near limestone foundations results in chlorosis, etc from a high alkaline soil.

    Bookmark   July 29, 2013 at 1:04PM
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botann(z8 SEof Seattle)

I thought foundations were made of concrete.
Yes, there is lime in concrete. Very little of it leaches from the foundation. At least not enough to adversely affect the ph to the point rhododendrons suffer. Here in the NW, rhododendrons are a standard foundation shrub.
Left over mortar around a chimney is another matter.

    Bookmark   September 2, 2013 at 11:40PM
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The main ingredient in both mortar and concrete is Portland Cement which is made from limestone, CaCO3. Enough lime can leach from a foundation to effect soil pH when the foundation is relatively new. Once it has weathered for a while, the danger is slight unless excess concrete has been directly dumped into the soil near the foundation - a surprisingly common practice.

    Bookmark   September 3, 2013 at 5:28AM
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rhodyman(SE PA, USDA Z6)

I second what aka says.

Fresh cement and mortar leach a great deal of alkalinity into the soil. Around homes that are a few years old, the pH is always much higher within a foot or two of the foundation, chimney and walkways. This goes away after a decade or two. Application of powdered sulfur is a good antidote, but is slow acting just as the lime in concrete and mortar is.

Regarding the great Pacific Northwest, there are regions near the coast where the soil has iron sulfates and is so acidic that they actually apply lime before planting rhododendrons and azaleas. They can get chlorosis that is caused by low pH. That is completely alien to those of us in the East.

    Bookmark   September 4, 2013 at 4:15PM
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