preparing containers for spring.

How much dirt should I replace each year in my containers? Each one is about 4ftx3ft x 1.5ft deep. And I use them for annual flowers as well as each one holds a base for my grapevines which are 20 years old.

Doug says that all he does is loosen up the soil and adds compost to top up the container. This give a ton of good organic matter and some nutrition to the soil. I always feed containers during the season in any case but topping up is compost. (I don't replace soil - but top it up as it sinks every year)