Posted: 04/05/2013 at 21:41
These containers.....18" deep but how wide? Are they troughs? Are they pots? You said "flowerbeds" ....?? Are you talkIn g about raising the height of your garden soil?
It depends on what you want to grow in them. If shrubs, they need compost for the entire depth. If annuals.....plants just for the summer....then you can put anything half way down and then just use compost for top half.
Let us know what you want to grow and then we can suggest right "soil"