By Eithneelgin
United Kingdom 
We have an invasion of sorrel. At first I weeded it out but the roots are so thin that they tend to break and the whole thing starts again. I tried spraying on weedkiller last year and thought that had improved things but am finding that it is just as vigorous and has found its way into the middle of large plants like lupins. What would you recommend? We have ericaceous soil. Eithne Nutt, Elgin
18 Mar, 2012
I assume you mean wood sorrel, or one of the Oxalis varieties, rather than sorrel itself.
I always dig this out, carefully extracting the roots. Where its invaded other plants, I dig those out, extract the sorrel and then replant. Other people use weedkillers, but like you, I don't find those particularly effective, although sometimes a dose of glyphosate (where possible) followed by digging out a week later is more effective.
18 Mar, 2012