HEARTFELT FILIGREE
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Q The concrete planters in the arcade of our 80-unit condo building were piped for irrigation last spring. A fuzzy film is appearing on the outside, seemingly the result of water pushing some elements of the concrete to the outside. There is also a stain on the brick below the planters that was actually black at one time. We removed the dirt from one planter and discovered three small drain holes about a half inch (12.7 mm) in diameter in the bottom, and we assume that all the planters have that same drainage. I am landscape chair of the condo association and have been given this problem to solve. What would you suggest?
A The fuzzy film and stains are efflorescence, caused by watersoluble salts that the irrigation water picked up from the soil or concrete. As the water evaporated, the salt crystallized on the
outside of the planters.
Although it’s possible to remove efflorescence through scrubbing and the use of masonry cleaners, it will keep forming as long as salty water is moving through the masonry. To keep that from happening, it might be enough to simply switch to water-thrifty plants and cut back on irrigation, especially if the efflorescence didn’t start until the irrigation system was installed. Salts in fertilizer can exacerbate efflorescence, so consider reducing that as well.
If that isn’t enough, investigate where the water that drains from the planters winds up. Are the drain holes simply resting on the cap of the brick wall underneath, preventing the water from draining readily and causing it to seep into the brick wall? If you find that there is no good way for water to drain from the planters, you might want to set potted plants in the planters, rather than fill them with soil. Set the pots on trays so that any water that drains can evaporate without having to pass through the concrete. If the pots aren’t as deep as the planters, set the pots on spacers. The added air gap will also help to keep the concrete dry.
If there is a way for water to drain and you want to keep soil and irrigated plants in the planters, consider painting the inside surfaces (after cleaning them thoroughly) with a waterproofing paint or coating. A waterproof liner would probably be even more durable. You might try using adhesive-backed, rubberized material sold as roof underlayment.