Container compost - How often to change?

As & when you change your planting in the containers I would think. When I am re-doing a planter I always use fresh compost because the previous plants would of used up the nutritional value of the compost. Never waist the used compost though, I always put it on my borders. I guess you could always add plant fertiliser to the exisiting compost to use it again, but I find there is often rooty bits left in & just prefer to give new plants a fresh start & the borders benefit as well.

Thanks for the advice Poppy......Its noted....