I read that this is invasive so dug up a 6 month old clump but Im worried I may have left some in the soil.Will Roundup work if I have?
It's not as bad as ground elder and isn't bad to remove unless it's infiltrated other plants. Roundup will work but so will another dig up.
Don't know about Houttuynia but we have been trying to get rid of Acanthus from a small perennial area with geraniums, roses, verbenas and a paeonia for two years and it keeps on popping up. Last year we had new brick path edges infilled with slate slabs surrounded by slate chips, around this bed, removed all the soil, riddled it and replaced it with the same perennials and this time it popped up above the stone edged area. It is as invasive as the bind weed which we have managed to get rid of.