Rose bushes are either grown in containers or in fields. Plants that are grown in containers have a fibrous root structure, while plants that are field-grown have a heavy root system. The root structure of a rose bush is an important part of its anatomy.
The roots anchor the rose bush in the soil. They also transport water and nutrients to the rose bush.
Two things determine the size of the root structure: the age of the rose bush and the density of the soil. Mature rose bushes have developed a large root system as opposed to newly-planted bushes.
Rose bushes that are planted in dense soil (fields) have developed a heavy root system by pushing through the dense soil. Rose bushes that have been grown in containers and have not had to push through dense soil have fibrous root systems.
Rose bushes that have been left growing in containers for a long period of time will become root-bound. They will produce a large amount of fibrous roots that wrap around each other within the container.
Root-bound rose bushes can be purchased and planted. Cut the roots before planting, by removing the bush from the container and slicing the fibrous roots selectively around the root ball (using a garden knife).