Responses of Growth and Secondary Metabolites in Fish Mint (Houttuynia cordata Thunb.) Cuttings to Far-Red Light

Fish mint (Houttuynia cordata Thunb.) is an aromatic herb used as food and medicine across Asia. We evaluated how far-red (FR) light influences growth and secondary metabolites in the non-flowering cultivar ‘BCV02’ propagated by cuttings. Seedlings were grown for 14 days under FR at 35, 50, and 70 μmol m−2 s−1 (as FR35, 50, and 70, respectively) or without FR (as control, CK). All FR treatments increased plant height but reduced the shoot/rhizome ratio. Total chlorophyll and carotenoid contents were unchanged, while the chlorophyll a/b ratio declined from 2.37 (CK) to 2.15 (FR70). In shoots, combined 3-, 4-, and 5-O-caffeoylquinic acids with rutin, hyperoside, isoquercitrin, and quercitrin reached 12.61–13.83 mg g−1 dry weight (DW) under FR treatments, exceeding CK (8.48 mg g−1 DW). However, in rhizomes, these secondary metabolite contents ranged 0.82–1.00 mg g−1 DW across all treatments. On a per-pot basis, the highest accumulated compounds (4.37 mg per pot) occurred at FR35. Overall, growth and secondary metabolite biosynthesis in fish mint cuttings respond differently to changes in FR treatments, with FR35 optimizing compound accumulation. Quercitrin in shoots was 0.09–0.20 mg g−1 DW and not quantifiable in rhizomes, potentially below pharmacopeial thresholds specified in the Taiwan Herbal Pharmacopeia and Hong Kong Chinese Materia Medica Standards. These results underscore the importance of aligning cultivar choice, light regime, and market specifications to secure both yield and quality of H. cordata.