Q. What is the best way to remove a blue stain on flagstone caused by plant fertilizer? It is several days old. The flagstone is an outside border on a raised planter bed.
A. Blue fertilizer stains can be challenging on flagstone, especially outdoors where the stone is typically more porous and textured. The blue color usually comes from dyes and copper sulfate, which can penetrate below the surface of the stone.
Here's what you should do next. Use a poultice to draw the stain out of the stone over time. For detailed, step-by-step instructions on how to mix and apply a poultice correctly, check out our free Stain Management App. The active chemical ingredient for your poultice should be 30 or 40 volume hydrogen peroxide, which is available at beauty supply stores. Avoid the standard brown-bottle peroxide sold in pharmacies, as it is not strong enough to be effective.
You may need to repeat the poultice application more than once, especially since the stain is several days old and flagstone is typically absorbent.
If you do not see improvement after several peroxide poultice applications, try switching to a poultice made with a product designed to remove iron or metal-based stains. While flagstone is generally more tolerant of treatment than polished interior stone, repeated chemical applications can still affect the surface appearance of the stone or any sealer applied to the surface.
If surface damage happens or if you're still having trouble lifting the stain, a professional stone restoration contractor can help. To find a vetted PRO in your area, visit www.surfacecarepros.com and click on Find a PRO.
ABOUT THE AUTHOR Fred Hueston is the Chief Technical Director at SurfaceCarePROS.com and Director at StoneForensics.com. He is also the author of Stone and Tile Restoration: The Manual, a comprehensive online manual for stone and tile restoration contractors. [Learn more about Fred.]
Article ID: 857
Created: January 11, 2026
Last Updated: January 11, 2026
Author: Fred Hueston
Online URL: https://surfacecarepros.com/kb/article/how-to-remove-blue-fertilizer-stains-from-exterior-flagstone-857.html