In applications dealing with plant growth modeling, increasing attention is being devoted to the topological structure of plants. Different models, based on tree-graphs, have been introduced to represent plants. These models assume that the scale of description is fixed. However, this hypothesis is too restrictive for new modeling applications that aim to tackle analysis or simulation of plant growth at different time and space scales. In order to make such multiscale descriptions available to computer applications, we have defined a general methodology for measuring and representing multiscale plant topological structures. This paper discusses the design of a model of plant topological structures and sketches out its general formal properties. The model supports multiscale, attributed and time-varying descriptions of plants. It is intended to be used for plant analysis methodologies and plant growth simulations.