There is only one "rule" in garden design - "right plant, right place". This means that the mature height and width of any plant should not exceed the space available for that plant. In the case of your brainstormed idea, this means that the arbor vitae is completely the right plant in the WRONG place - it'll get too tall and wide for that area, even if it's a Holmstrup Arbor Vitae, which I've seed at 8 feet tall and 3 feet wide. Which plants to put in this bed should really be determined by the amount of maintenance that you're willing to do. Do you have time to deadhead perennials (i.e. remove dead flowers to prevent seeds)? Do you have time to occasionally prune shrubs? How about putting down more wood chips? Personally, I'd match the plants to the time I have to take care of them. If you have a decent amount of time, then go with annuals; a bit of time, then perennials; basically no time at all, then go with shrubs that get no wider than three feet so that you won't need to prune them (this assumes that your bed is four feet wide - shrubs as foundation plantings should max-out at the width of the bed minus one foot - you'll need that foot to prevent damage to the siding and to allow access for maintenance). If you have landscape fabric under that mulch, remove it ASAP. It does not prevent weeds but does indeed kill your soil. Answer from AlwaysPissedOff59 on reddit.com