Planters need to be big—really big. A common mistake designers make with planters is choosing small sizes. You’ll lose plant material in small containers. “Self-watering” planters are a joke. Planters need people to keep the plant in them alive.
Raised planters within streetscapes need to be located beyond the overhang of vehicles. If not, they need to be less than fifteen inches tall to avoid damage from the bumpers.
If trees are planted in narrow, raised planters, they tend to be unstable. I’ve seen elaborate urban landscaped medians trying to do this. The trees eventually had to be removed when they started leaning. It looked great for just a few months, but was certainly not worth the cost.
When designing plants in a planter, try to include things that flow over the sides of the planter. It’s pretty. Add tall, bold focal points. One way to get vertical height in the difficult environment of a planter is to use tall ornamental grasses. They can be very dramatic.
Plan on replacing the plant material in a planter often. It is not a natural situation for plant material. The reflected heat on the sides of planters dries out the root system and the reflected heat from the planter material is harsh. The soil dries out very quickly and can require supplemental waterings as often as three times a day during the hottest days of summer.
Well-maintained planters can be very effective in retail shopping areas, where pedestrians can view their beauty at eye level. They are a hassle, but worth it. They make people want to spend money. They also provide a connection for people with nature and beauty. In urban areas, it may be the only vegetation some people see during the day.