Compost and weeds.
I finally managed to get out into the garden for a shortish time today to tidy up the pots. What struck me was that all the newly planted pots i.e. planted last Spring are thick with weeds whilst the older ones have very few. I've never noticed such a dramatic difference in the past so suspect compost which hadn't got hot enough to kill the weed seeds. This is commercial compost, not home grown.