Hi!
TKHooker gave you some good pointers so I'll concentrate on the "dirty" part...
Definitely do not recommend the use of topsoil in planters! What you need is POTTING soil. This is a lighter mix which will not compact nearly as much as topsoil would. Suggest 1st removing old
plant material to trash or compost pile. Remove aproximately half the existing soil (another gift for the compost pile). Once you have your
plants purchased, you'll know how big their containers are so...using potting soil (ammending it with a time-release fertilizer would be a good idea) refill the planters blending the old and new soil together. Firm the soil, don't pack it too tightly. Don't fill them all the way! Instead, add 1 inch to the heighth of the container of your largest potted
plant. Place the
plants in the planter adding soil below them if they're in shorter post and filling in around them with more potting soil to within 1 inch of the top of the container, then water them. They should be checked twice a week (more often if the weather is dry & hot) to maintain moisture.
A rule of thumb locally for container planting is One
plant for height, three for color and two to trail down the sides of a 12 inch wide pot. Seems to work pretty well. Oh! All this is assuming you are using annuals because I really doubt many perennials would make it through the winter in pots or planters in your area unless the planters are absolutely huge.
By the way, I find it fascinating that you're doing this in Edmonton and would really enjoy hearing what
plants you decide to use. I'm curious to see if there's much of a difference in what you and I (Joliet, Illinois) have to choose from at the nurseries.
Good luck with your project!