Landscape companies come in Several flavors.
The typical landscape company is the Design Build or "do it all" Company; These companies are "Full Service", that is they will Design, Install, and maintain the landscape. There a few down sides to this; The First is that few can afford to have a trained landscape designer on staff, this means the owner or one of the crew will draw up a design on his lunch break or after he is exhausted from pushing a shovel all day and whip out your design. Another Issues is that the title of the person while drawing a landscape design is sales and not designer, the main purpose of a landscape design for a design build company is the sell landscapes. This mentality is the root of most of the poor designs you can find in every new neighborhood, way too many plants (Profit) planted too close together or in the wrong spot. This is were I started my career and new their had to be better options.
the Next option is the Construction Only Companies; These companies usually work with independent designers and will just install landscapes. This can be a great benefit to people who have a clear idea of what they want, can draw their own design or have hired a Landscape Designer.
Design only is my recommended approach (not just because that is what i do). The philosophy of design only is much different then the other options. Since designs are our only income we must spend more time on creating the best solutions and using the most creative ideas. With out these we would have no referrals or reputation. When you hire a Landscape Designer they should have knowledge of the local climate and plants as well as other factors such as elevation, and local wildlife (Deer, Rabbits, Etc.). And on the economic side, owning you plan allows you to shop the contractors for the best bid or invest some sweat equity and install some or all of the design yourself.