Q: This weed has probably got into my garden from a pot plant. I can’t identify it and what’s worse, I can’t kill it! The coin is a €1 piece. The weed forms mats and I’ve tried several different proprietary weedkillers to no avail. Please advise. If I do have to dig it out it will be very difficult as it is so small, bits of it are easily left behind. PDJudging by your description and photograph, this looks like Soleirolia soleirolii, or “mind your own business” .
I say “wild” but in fact this low-growing, mat-forming, evergreen perennial is more commonly found in cultivated gardens, growing in lawns, paving cracks and gaps in old stone walls where it readily finds a foothold and can quickly colonise large areas if left unchecked. Happiest in shade, but tolerant of a wide range of growing conditions, it’s generally found in coastal Irish gardens rather than cooler midland parts of the country, but is surprisingly hardy.