Destroying a small number of rhizomes to establish a fresh weight dry weight relation is usually easy and feasible.
There is a difference in getting statistical valid results or an
"unbiased result" of the growth rate. Growth rate depends on the
differences in dry weight and the time passed between the two
assessments of weight. Therefore an estimate of dry weight at the
beginning is necessary. Since all measurements of dry weight are
destructive (dry plants do usually not live) you have to substitute the
initial weight be estimates.
