ASHS 2024 Conference

Rooting of herbaceous perennial cuttings occurs year-round in greenhouses under photosynthetic daily light integrals (DLIs) ranging from ≈1–20 mol·m–2·d–1. The effects of DLI on rooting and cutting growth have not been quantified for many vegetatively propagated herbaceous perennials. As such, our objectives were to determine the effects of DLI on adventitious rooting and cutting quality of a popular herbaceous perennial, yarrow (Achillea sp.). Unrooted cuttings of yarrow (Achillea millefolium ‘Apricot Delight’) were received from a commercial cutting supplier and callused in a glass-glazed greenhouse for 5 d under ≈3.6 mol·m–2·d–1 at 24 °C root-zone and air temperatures. Upon callusing, cuttings were transferred to a rooting environment with 21 °C air temperature and 24 °C root-zone temperature set points. Cuttings were rooted under one of four different fixed-woven shadecloth providing ≈86%, 62%, or 26% shade or no shade (0%) thereby establishing mean DLIs ranging from 1.6 to 15.2 mol∙m‒2∙d‒1. Cutting growth and biomass accumulation evaluations were made 8, 11, and 14 d after transfer. In general, yarrow stem caliper, stem length, and leaf number were unaffected by increasing DLI; however, leaf, stem, root, and total biomass accumulation increased with increasing DLI though to different magnitudes. For example, as DLI increased from 1.6 to 15.2 mol∙m‒2∙d‒1, leaf, stem, root, and total biomass accumulation increased by 84%, 85%, 458%, and 93% at 8 days after transfer to 98%, 110%, 893%, and 128% at 14 days after transfer, respectively. When taken together, DLI should be properly managed to hasten and improve rooting and growth of yarrow cuttings during adventitious root development.