Superior succulents

© Jessica | adobe stock

In our three-part series on succulent production, we are sharing research-based information generated at Michigan State University on how environmental parameters can be managed to bulk up succulents that are notoriously slow to reach a marketable size. In part one, we explained how photoperiod and the photosynthetic daily light integral (DLI) could be manipulated to promote vegetative growth and flowering of some Echeveria cultivars.

It is well documented that the average daily temperature (ADT) and DLI that a plant is exposed to each day can interact and have a profound impact on photosynthesis, plant growth, morphology and development of crops such as herbaceous annuals and perennials. However, limited research-based production information is currently available on how environmental parameters interact to influence leaf unfolding, flower initiation and extension growth of succulents which are primarily grown for their foliage. Additionally, the difference between the day and the night temperature (DIF) can impact height, internode length, branching pattern and orientation, and flower stalk elongation. Therefore, in part two, we will show you how to manage day and night temperature, ADT and DLI to promote compact, yet rapid vegetative growth and enhance foliage color of several popular succulent genera.

Our research procedures

Shoot-tip cuttings of tree houseleek (Aeonium arboreum) ‘Nigrum’, round-leafed navel-wort (Cotyledon orbiculata) ‘Silver Peak’, Mexican hens and chicks (Echeveria) ‘Pollux’, kalanchoe (), panda plant (K. tomentosa), coral senecio (Senecio fulgens) ‘Amke’, and senecio (Senecio ficoides) ‘Mount Everest’ were received from a commercial breeder (Dümmen Orange), allowed to callus for two days and stuck in 18-01 trays filled with a substrate containing 52% peat, 41% perlite and 7% vermiculite. The trays were placed in a glass-glazed greenhouse with an average daily temperature (ADT) of 73 °F, 16-h photoperiod, and daily light integral (DLI) of 12 mol·m^?2·d^?1. Twenty-four hours after stick, a 100 ppm foliar application of a rooting hormone was applied to the cuttings. Depending on the genera and cultivar, the rooting period was anywhere from nine to 10 weeks. Plants were subsequently transplanted into 4.5-inch containers and moved to greenhouse compartments with day/night air temperature set points (12-h/12-h day/night) of 72/61, 77/66, 82/72, 88/77 or 93/82 °F (ADT of 66, 72, 77, 82 or 88 °F). Within each temperature treatment, three daily light integrals (DLIs) were created by placing fixed shade cloth providing ~50 or 30% shade or no shade over individual benches. Plants were grown under a 16-h photoperiod consisting of ambient daylight and supplemental lighting from high-pressure sodium (HPS) lamps when ambient light was low to maintain DLIs of 5, 8 and 12 mol·m^–2·d^–1.

What we learned

Generally, after nine to 13 weeks under the various day and night temperatures and DLIs, temperature had a larger impact on growth and development than DLI. Not surprisingly, the response to temperature was species-specific. For example, Aeonium ‘Nigrum’ (Fig. 1), Senecio ficoides and S. fulgens ‘Amke’ were of high quality and filled in the container at ADTs <68 °F. Although growth of S. fulgens ‘Amke’ was hastened at warmer temperatures, it had soft and spindly growth. Alternatively, time to produce a high-quality, compact, full and marketable potted Cotyledon ‘Silver Peak’ (Fig. 3), Echeveria ‘Pollux’ (Fig. 2), Kalanchoe tomentosa and K. villosa was hastened at ADTs between >66 and <77 °F. We also found that the higher the DLI, the better, but relatively low DLIs >8 mol·m^?2·d^?1 were sufficient to produce good quality plants. Higher DLIs generally resulted in more compact plants, increased branching, thicker stems and more vibrant foliage color. Therefore, depending on your location, supplemental lighting might be necessary to enhance the foliage color of your crop, increase the stem caliper and reduce unwanted stretch associated with lower DLIs.

While we did not see high mortality at the highest day and night temperatures of 93/82 °F (ADT of 88 °F), we did observe that leaf number, length and width, height, fresh and dry mass, and stem diameter of Aeonium, Echeveria, Senecio ficoides, and Kalanchoe were negatively impacted. This suggests that an ADT of 88 °F may be approaching the maximum temperature of these crops. However, the width and height of Senecio fulgens increased at high temperatures. However, this was not necessarily the plant architecture that a consumer would want.

Conclusions

Some of the temperatures utilized in this study were on the higher end of what most greenhouse growers would have their set points to show how the rate of development such as leaf unfolding is crop-specific and can be impacted by extreme temperatures. Our research provides evidence that the paradigm perpetuated by hobbyists indicating that all succulents require high temperatures (>77 °F) to hasten growth are untrue. Although many of the species in this study are native to warmer equatorial climates, Cotyledon, Echeveria, Kalanchoe, and Senecio in our study performed best at ADT =75 °F. Aeonium on the other hand is native to temperate regions of Europe and the Canary Islands and consequently requires cooler temperatures for optimal growth. Therefore, we categorized the crops in this article based on the ADT that led to the best overall growth that did not require the use of PGRs (Table 1). Aeonium arboretum and Senecio ficoides and S. fulgens were categorized as “Cool Temperature” crops that should be grown at ADT of 60 to 66 °F. On the other hand, Cotyledon orbiculata, Echeveria hybrid, Kalanchoe villiosa and K. tomentosa were categorized as “Moderate Temperature” crops that should be grown at ADTs of 66 to 75 °F. Given that most succulents use CAM photosynthesis, a positive DIF is recommended over a constant temperature. Although the interaction between ADT and DLI was low, we suggest targeting a minimum DLI between 8 to 12 mol·m^-2·d^-1.

Additional research on a wider range of day and night temperatures and succulent genera is necessary to elucidate the base, optimum and maximum temperatures for growth and development.

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. 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 and Dümmen Orange for plant material, The Blackmore Company for fertilizer, LS Svensson for shade cloth and East Jordan Plastics for containers.