Removing staining on concrete - Sycamore Seeds

Looking for advice for removing brown staining caused by sycamore seeds (helicopters) on concrete. Tried a pressure washer which had little effect, soak of bleach and scrubbing helped, but it's hard work. There must be an easier solution? or perhaps I need a better pressure washer?

 

Probably the stain has been absorbed by the concrete if its not sealed, its porous.

Can only really think of a bleach solution and allow it to be absorbed, before rinsing away, but watch out for any plants nearby.

There are possibly patio cleaners that might work.

 

Would that Wet and Forget stuff work? I used it on a concrete paver where my air conditioner sits and it not only got rid of the green stuff, it bleached the stone itself. Looked like I'd just laid it!

 

Likely to be a tannin stain:
Tannin Remover - 5L | Total Janitorial Supplies
Also sodium hydroxide, as drain cleaner

 

Maybe something based on oxalic acid? Or mashed up rhubarb leaves (not sure if it's allowed). I've found oxalic is a good bleaching agent for stains on wood, though I use farmer's bleach on bricks and render - it's about 3 times the strength of household bleach.

 

Oxalic acid rings some bells, always remember French polishers having some of that around.

 

Last place I worked we fitted some oak windows and the debris from the trees around the house stained the oak cills.
I think oxalic acid sorted it out.