You should assess plant parameters starting from germination to maturity + effect on soil + effect on plant quality depending upon the nature of the material and and the use of plant itself. Hence you can use germination percentage, relative growth rate, total biomass, leaf area, photosynthesis, agronomic characters, yield, soil content for the material used and constituents of the material in the plant itself.
This indicators are important: physio-chemical plant indicators, root system, Location of leaves, Palatability, climate conditions, soil conditions, stage of growth, growth form, Carbohydrates storage, Location of Carbohydrates storage in plant, Location of plant Sensitive organs, kind of animal grazer, season, Repetition frequency of grazing, Grazing time period, plant composition in habitat, plant Access. Vigor in habitat, frequency, kind of Species, Adaptability.