Disclaimer: This post was originally posted by Full Member, LaurainFL. This user is located in Zone 8b/9a, NW Florida. This was originally posted on July 15, 2013.
No, that's not necessary unless you had some problem with the mix. Good quality potting mix should be reusable for several years. Eventually it will break down and you'll notice that the mix seems to stay too wet - then you need to replace it.
To replant the box, you:
Remove the old plants and their large roots (small roots will decay on their own pretty quickly), and remove the leftover fertilizer strip. This is a good time to check and make sure that there are no roots blocking the drainage hole, too. Sometimes you need to add more dolomite - I don't think you need to add it more than once per year unless you are growing plants that need a lot of calcium or higher pH. So you can probably skip the dolomite this replant, but DO add more dolomite next spring.
After your dolomite is mixed in (if needed), put in a new fertilizer strip, and add enough new potting mix to cover the strip and mound the mix up. Put on the cover and you're ready to plant!
You may find that the cover needs to be replaced; the elastic tends to break down in one season in strong sunlight. Normally you get two covers with a new box, so hopefully you have a spare cover. After that, you can either order more covers or make your own with garbage bags or rolls of 3 mil plastic.
Some people choose to empty the box to do a more thorough job picking out roots, and some people empty out the box and mix in some new mix (say 1/4 or 1/3 of the box) every time they replant. That way they never need to do a full mix replacement. But that's not required.
Enjoy!
