Short Day Lengths Can Mitigate Excessive Stem Elongation and Promote Flowering of Echeveria Cultivars Under Low and Moderate Daily Light Integrals

Echeveria spp. (Mexican hens and chicks) are among the most popular genera of succulents sold because they are compact and form attractive, symmetrical rosettes with brightly colored, fleshy, broad, iridescent leaves, as well as large, showy inflorescences. However, they are slow-growing, and flower induction protocols are not widely available. Therefore, the objectives of this study were (1) to determine if photoperiod and the photosynthetic daily light integral (DLI) can be manipulated to promote rapid growth and leaf expansion without excessive extension growth of several cultivars of Echeveria and (2) to establish the critical photoperiod for flower induction. Cuttings of E. spp. and hybrids ‘Apus’, ‘Canadian’, ‘Elegans Blue’, ‘Jade Point’, and ‘Topsy Turvy’ were received from a commercial breeder and grown in a greenhouse at 20 °C for 5 weeks. Photoperiods were created using a truncated 9 h short day (SD) or a SD extended to 10, 11, 13, 15, 16 h or a 4 h night-interruption (NI), using light-emitting diode (LED) lamps providing a total photon flux density of ≈2 μmol·m−2·s−1 of red (R) + white (W) + far-red (FR) radiation. DLIs of 4.8 and 12.8 mol·m−2·d−1 were maintained with and without shade cloth and supplemental lighting. Photoperiod and DLI interacted to influence the final height of E. ‘Canadian’, ‘Elegans Blue’, and ‘Jade Point’; plants were tallest under photoperiods > 13 h and low DLI. Similar trends were observed for growth index and average plant diameter. No clear trend was observed for leaf unfolding or leaf length across DLI or photoperiod treatments. Flower initiation of E. ‘Apus’ and ‘Jade Point’ was highest under a DLI of 12.8 mol·m−2·d−1. Additionally, E. ‘Jade Point’ only developed inflorescences under day lengths ≤ 11 h, indicating an obligate SD response. Our results suggest that growers should maintain DLIs > 10 mol·m−2·d−1 and SD conditions to promote flower initiation of the Echeveria cultivars tested. Such conditions would prevent excessive stem elongation and encourage flowering, increasing crop quality and marketability.