**4. Conclusions**

With its outstanding biomass as well as terpenoid efficacy, the broad-spectrum LED system represents a strong competitor to the conventionally used HPS and FL lighting systems in greenhouses under naturally insufficient light conditions as investigated in this study. Marketable *Thymus vulgaris* L. can be achieved faster and thus more often when replacing HPS and FL light sources with the tested LED system, ultimately resulting in greater revenues at simultaneously highly reduced production costs for greenhouse growers. The comparatively high initial capital costs of LEDs which have delayed their establishment in the past are decreasing [59]. Based on our results, combined with the typically low maintenance and long operating lifetime [13,15,63,64], the initial investment into LEDs should quickly become a source of profit for greenhouse growers. Additionally, different adaptive control approaches making use of the dimmability of LEDs [35,65] can further decrease the power consumptions and help to achieve consistent growth rates at a daily and seasonal level as shown by [60,66]. Our results suggest that an implementation of a broad-spectrum LED system in greenhouses could provide the possibility to cultivate a greater variety of crops with greater DLI-requirements under naturally insufficient light levels as conventional lighting systems are capable of today. Further, the broad-spectrum LED system could extend greenhouse production seasons, which are currently constrained by low supplemental DLIs, and allow a year-round production of a wider variety of selected greenhouse crops then HPS and FL systems are able to at present. However, further trails with a variety of greenhouse crops need to be investigated to confirm the suggested applicability for a range of crops. Therefore, in prospective broad-spectrum LED studies, the crops' individual DLI requirements need to be incorporated and compared to commonly applied mono- and dichromatic LED light spectra at equal light intensities for advancing our knowledge on the impact of LED light spectra on morphological, physiological, and metabolic plant responses. In addition, more studies examining the impact of light qualities on terpenoid biosynthesis, content, and composition are needed to optimize the quality of aromatic plant species in the future.

**Author Contributions:** Conceptualization and project administration, J.M.T. and H.S.; methodology, H.S.; formal analysis and validation, J.M.T., H.S., and A.K.; investigation, J.M.T.; data curation, A.K.; Visualization, J.M.T.; writing—original draft preparation, J.M.T.; writing—review and editing, H.S. and A.K.; supervision, H.S.; funding acquisition and resources, H.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the European Innovation Partnership for Improvement of Agricultural Productivity and Sustainability (grant number: 204016000016/80168353) via the European Agricultural Fund for Rural Development.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author, A.K., upon reasonable request.

**Acknowledgments:** We thank Roland Buchhorn, Claudia Könecke, Heike Bäumer, Sabrina Pilz, and Fabian Hinze for cultivation assistance, René Grünwald for analytical assistance, and Marcus Müller from Humboldt University of Berlin for technical assistance. We further thank Torsten Meiners and Christoph Böttcher for their critical revision of the manuscript, and Matthias Melzig from Free University Berlin for his support.

**Conflicts of Interest:** The authors declare no conflict of interest.
