## 4 Conclusions  
The impact of three studied factors, PGRs, soil type, and year of vegetation, were evaluated for two vegetation seasons during the growth of the second-generation energy crop \(M\). \(\times\)_giganteus_ in marginal post-military soils supported by the treatment of rhizomes with PGRs. The M-GLM statistical approach showed that these factors influenced plant bioparameters (stems, shoot and root lengths) and dry biomass yield. As a result, increased plant height and a larger amount of \(M\). \(\times\)_giganteus_ biomass were ultimately achieved. However, stimulation of other aspects of individual plant growth can increase competition between individuals, which may not increase the proportional growth of biomass and its energy output. The results of two years of production of \(M\). \(\times\)_giganteus_ stimulated by various PGR treatments emphasized that treatment by Charkor in higher concentrations positively influenced biological parameters and dry biomass value, and the effect was much more vigorous for the second vegetation year. Therefore, this PGR can be recommended for practical application on \(M\). \(\times\)_giganteus_ plantations to enhance plant development and biomass production. However, other studied PGR treatments were not as effective in terms of biomass production despite certain combinations of treatments showing satisfactory results. To the best of our knowledge, the results reported here represent the first systematic study comparing the influence of the common PGRs Charkor, Regoplant, and Stimpo on the production of \(M\). \(\times\)_giganteus_ over two years in lab conditions. The results obtained permitted the conclusion that when a crop is grown at a field scale on post-military marginal soil, only Charkor may be utilized; in particular, the effect of its positive influence has to be evident with years of growing. Two other researched PGRs (Stimpo and Regoplant), despite earlier reported positive influence on the development of common crops (corn and barley) [7; 8; 9; 10; 55], appeared not to be so effective in the case of the development of \(M\). \(\times\)_giganteus_ and cannot be recommended for broad utilization in field conditions when economics is among the main decision-making factors. Future research should focus on verifying the field-scale recommendations for \(M\). \(\times\)_giganteus_ plantations established on post-military land and on comparing the results of lab and field-scale experiments. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12020881/s1](https://www.mdpi.com/article/10.3390/app12020881/s1)]([https://www.mdpi.com/article/10.3390/app12020881/s1](https://www.mdpi.com/article/10.3390/app12020881/s1)). **Table S1. ** Descriptive statistics of morphometric traits of _Miscanthus_; **Figure S1. ** The effect of PGRs on number of steams (**a**) depending of soil's type (**b**). Squares and circles show least square means. Vertical bars denote 0.95 confidence intervals; **Figure S2. ** The effect of PGRs on shoot number (**a**) depending of soil's type (**b**). Squares and circles show least square means. Vertical bars denote 0.95 confidence intervals; **Figure S3. ** The effect of PGRs on root length (**a**) depending of soil's type (**b**). Squares and circles show least square means. Vertical bars denote 0.95 confidence intervals; **Figure S4. ** The effect of PGRs on stem's length (**a**) depending of soil's type (**b**). Squares and circles show least square means. Vertical bars denote 0.95 confidence intervals. Conceptualization, V.P. and T.S. ; methodology, V.P., T.S. and A.M.; software, O.Z. ; validation, A.M., V.S. and M.S. ; formal analysis, investigation, V.P., T.S. and P.S. ; resources, A.M. and P.S. ; data curation, A.M., V.S. and M.S. ; writing--original draft preparation, V.P. and T.S. ; writing--review and editing, V.P., T.S. and O.Z. ; visualization, O.Z. ; supervision, V.P and T.S. ; project administration, V.P. and T.S. ; funding acquisition, V.P.