How to remove potted plant stains from unsealed exterior travertine?

Q. I have unsealed travertine outdoor tiles that are badly stained from a large pot. This pottery pot had a large palm in it. It became clogged and, with so much rain over time, started oozing dirty water from the pot. The breakdown of the pottery has badly stained the tiles. I have since smashed up the pot and removed it. However, the area has remained badly stained with a brown mark and ring where the pot was. I seek assistance on how to remove it. I have tried a high-pressure washer to no avail. Looking for your help.

A. Thanks for reaching out. What you’re describing is a very common situation with exterior travertine, especially around large planters. You did the right thing by removing the pot, and now the challenge will be to deal with the stain that’s been left behind.

Travertine is naturally a porous, calcium-based stone filled with pits and voids that act like little storage spaces for moisture. Outdoors, travertine is especially prone to pulling in anything that sits on the surface, such as rainwater, dirty runoff, plant matter, and the like.

Just so you know, don't be too hard on yourself for not sealing your travertine. Many people assume sealing outdoor stone will prevent stains, but exterior conditions make sealers tricky and often more harmful than helpful. Outside, stone constantly takes in moisture from rain, humidity, and the ground below. Sealers can actually trap this moisture. If pressure builds inside the stone, it can result in flaking or surface damage. This is especially true in climates with freeze-thaw cycles. 

Anyhow, your unsealed stone behaved exactly as expected when it absorbed the dirty pot water. The brown discoloration and ring you describe are almost certainly deep organic stains from decomposed palm matter, soil tannins, or dissolved minerals, and maybe even some pigments from the pottery itself. The organic pigments must have settled deep inside stone, since pressure washing only cleaned the surface and could not reach what was already under the surface.

Here's how to treat this type of stain. Start by cleaning the affected area with hydrogen peroxide. Use a 30/40 volume hydrogen peroxide, sold at beauty supply stores. The regular drug store peroxide is too weak. Test in an inconspicuous spot first, since it is possible that the hydrogen peroxide could slightly lighten certain types of travertine. Follow the manufacturer's safety precautions, and do not mix peroxide with any other cleaners. Rinse thoroughly and allow the area to dry fully. Repeat if needed. 

Deep-set exterior stains can be stubborn, especially when the stone has absorbed liquids over a long period of time. If cleaning with peroxide does not eliminate the stain, try applying a poultice. Use the same type of peroxide in your poultice mixture. For detailed, step-by-step instructions on how to mix and apply a poultice correctly, check out our free Stain Management App.

If the stain is severe or if the results look uneven, a stone restoration professional may be able to achieve better results. To find a vetted PRO in your area, visit www.surfacecarepros.com and click on Find a PRO.