Producing succulents at the speed of light

According to USDA statistics from 1997 to 2019, the wholesale value of cacti and succulents increased by 218% from $22 to $70 million. However, unlike most other floriculture crops, succulents are slow-growing and require unique growing conditions as they are native to Africa, Mexico, Central America and equatorial latitudes where they have adapted to arid environments with limited water availability.

Many plants classified as succulents come from the stonecrop family Crassulaceae and include the following genera: Tree house leek (Aeonium), round-leafed navel-wort (Cotyledon), jade plant (Crassula), Mexican hens and chicks (Echeveria), kalanchoe (Kalanchoe), jewel plant (Pachyveria), stonecrop (Sedum) and houseleek (Sempervivum). Additionally, nearly all species in the Crassulaceae family utilize a form of photosynthesis called Crassulacean acid metabolism (CAM). CAM photosynthesis is generally characterized by stomatal (pores on leaves that control the rate of gas exchange) closure during the day and opening during the night to accumulate carbon dioxide to reduce water loss.

Overall, there is very little knowledge and literature regarding greenhouse production of succulents. Therefore, in 2019, we began a research program to investigate if temperature, photoperiod, daily light integral (DLI) and vernalization could hasten growth and flowering and shorten production time of potted succulents. In this three-part series, we will share the results of our on-going research.

Echeveria is one of the most popular hybridized succulent genera produced as a potted plant due to its compact and symmetrical rosette habit. Additional attributes include its brightly colored, fleshy and broad iridescent leaves and large and showy inflorescences with a wide range of flower colors. Its only downside is that it is very slow-growing and can take as long as six to eight months to become a marketable plant from an unrooted cutting if grown under suboptimal conditions. Therefore, in part one of this series, we will share how photoperiod and the DLI can be manipulated to promote rapid growth and leaf expansion without excessive stem elongation and potentially induce flowering of several cultivars of Echeveria.

In a preliminary study, we grew several succulent genera under 9-, 12-, 14- or 16-h photoperiods and found that the diameter of Echeveria expanded significantly under long days when the DLI and temperature were high. For example, after 12 weeks under a high DLI of 23 mol·m^–2·d^–1 and an average daily temperature (ADT) of 80 °F (summer conditions), the width of Echeveria ‘Domingo’ plants in 6-inch containers under a 9-h photoperiod was 6.8 inches and 8.7 inches under a 16-h photoperiod (Fig. 1). This increase in plant size reduced production time by approximately three to four weeks. Additionally, we did not observe excessive stem elongation under any daylength as a result of day-extension (DE) lighting. Why is this important? Compact succulents with larger leaves and plant diameter result in a larger potted plant and a reduction in production time.

These results led us to further investigate if utilizing low-intensity screw-in light-emitting diode (LED) fixtures (i.e., flowering lamps) to provide long day lengths as either DE or night interruption (NI) lighting would result in larger plants as the FR light would lead to rapid leaf expansion under much lower DLIs that are typical during winter and early spring greenhouse production.

Our research procedures

Shoot-tip cuttings of Echeveria ‘Apus’, ‘Canadian’, ‘Elegans Blue’, ‘Jade Point’ and ‘Topsy Turvy’ were received from a commercial breeder, stuck and received a foliar application of a rooting hormone 24 hours later. They were then allowed to root for six weeks at 68 °F and under a 16-h photoperiod. Plants of each cultivar were then placed in a greenhouse with an ADT of 68 °F under one of two DLIs (5 and 13 mol·m^-2·d^-1) that each contained seven photoperiod treatments: a 9-h short day (SD) or a 9-h SD extended by 1 [10 h; (5 to 6 p.m.)], 2 [11 h; (5 to 7 p.m.)], 4 [13 h; (5 to 9 p.m.)], 6 [15 h; (5 to 11 p.m.)], or 7 hours [16 h photoperiod; (5 to 12 a.m.)] were created with screw-in LED lamps containing red + white + far-red diodes (Arize Greenhouse Pro; General Electric, Boston, MA) on each bench. A NI treatment was also included, and it consisted of a 9-h day with a 4-h NI from 10:00 p.m. to 2:00 a.m.

What we learned

Echeveria ‘Canadian’, ‘Elegans Blue’ and ‘Jade Point’ exhibited excessive stem elongation (stretch) when grown under a low DLI of 5 mol·m^?2·d^?1 and daylengths =13-h. For example, the tallest ‘Elegans Blue’ plants were those grown under 15- or 16-h photoperiods and a low DLI (Fig. 2). While DLI and daylength interacted to affect the diameter of ‘Apus’, ‘Elegan Blue’ and ‘Jade Point’, there were no clear trends that would be commercially significant. These results were not surprising as plants grown low DLIs and high ratios of FR light (from the low-intensity LEDs) can lead to leaf expansion and stem elongation due to the shade avoidance response. Unfortunately, under the low and moderate DLIs provided in this study, leaf expansion and thus an increase in plant width did not occur as it had under the high DLI in our preliminary study.

Inflorescence initiation in response to daylength and DLI was cultivar-specific. For example, the highest percentage of ‘Apus’, ‘Elegans Blue’ and ‘Jade Point’ that developed an inflorescence was under photoperiods =11-h and a moderate DLI and no plants developed inflorescences under photoperiods =13-h (Fig. 3). ‘Apus’ developed inflorescences under a moderate DLI, no inflorescences under a low DLI. However, ‘Canadian’ developed the greatest number of inflorescences per plant under 15- and 16-h photoperiods and flowering occurred under all photoperiods.

Conclusions

To stimulate compact vegetative growth and leaf expansion of most Echeveria, plants should be grown under natural day lengths in the summer with DE lighting (depending on your location) to provide a 16-h photoperiod and DLIs >15 mol·m^?2·d^?1. Ensure that the low intensity LEDs provide at least 2 to 3 µmol·m^?2·s^?1 of light. Under lower DLIs, do not use low intensity lighting that contains FR light, as it will lead to excessive stem elongation. In the winter, the higher the DLI you can afford to provide with supplemental lighting, the better the overall quality of the crop.

Horticulturally, rapid, uniform and complete flowering is essential to meet market dates. Although we determined that some Echeveria cultivars initiated inflorescences under 9- to 11-h photoperiods, it was never more than 50% of plants. Therefore, we are currently conducting studies to determine if Echeveria has a juvenile period before it is receptive to environmental parameters and if it flowers in response to vernalization, applications of gibberellins or changes in daylength.

Roberto Lopez is an associate professor and controlled environment/floriculture extension specialist and Anthony Soster is a former graduate student in the department of horticulture at Michigan State University.

Authors' note: The authors gratefully acknowledge the Western Michigan Greenhouse Association, the Metro Detroit Flower Growers Association, The Horticulture Research Institute (HRI) and the Fred C. Gloeckner Foundation for funding, Dümmen Orange for plant material, The Blackmore Company for fertilizer, LS Svensson for shade cloth, and East Jordan Plastics for containers.