Bright Ideas in the Works


Greenhouse Product News February 2010


Erik Runkle and Ryan Warner

The floriculture program at Michigan State University has a rich history of applied research, partnerships with industry leaders, dissemination of unbiased information, and industry, undergraduate and graduate education. With industry guidance and support, we continue to generate research-based information to advance floriculture in the United States and beyond. This article, the first of a six-part series focusing on floriculture research at MSU, summarizes lighting and plant growth regulator research to improve crop quality of containerized greenhouse crops.

Light provides the energy for plant growth (photosynthesis) and, in some crops, regulates the flowering process. We are fortunate at Michigan State to have 20 different climate-controlled greenhouse sections for floriculture production research. Within each greenhouse, and even on individual benches within a greenhouse, we can achieve various combinations of photoperiod, light intensity and temperature to further our understanding of how the environment influences crop timing and quality. We have determined the effects of daily light integral (DLI) and photoperiod on growth attributes (e.g., branching and stem thickness) and flowering characteristics (e.g., time to flower and flower number) of a wide range of bedding, potted and perennial ornamental crops. That research continues today as the need for efficient crops and reliable scheduling increases.

DLI Research

The DLI available to greenhouse crops varies considerably, depending on the time of year, location, cloud cover, crop spacing, glazing material, overhead obstructions, and use of supplemental lighting, whitewash and shade curtains. A recent focus of our research has been to quantify how the average DLI influences root and shoot growth of cuttings and seedlings as well as crops after transplant into their final containers.

Many bedding and perennial crops are propagated from December through March, when the day length is short and the angle of the sun is relatively low. During this time, the DLI is low, especially in the northern half of the United States and in Canada. We have shown that a low DLI during propagation can inhibit rooting of cuttings and seedlings. For example, cuttings of petunia rooted under a very low DLI took longer to become established compared with a higher DLI (Figure 2, page 29). Therefore, during the darkest periods of the year (December and January), supplemental lighting from high-pressure sodium (HPS) lamps can shorten liner production time.

Many Northern propagators of young plants have HPS lighting in only a part of their greenhouses. Greenhouse growers asked when lighting would have the biggest impact if they could provide lighting during only a portion of the plug stage.

In our research, we provided HPS lighting during different seedling stages; in general, lighting had the most benefit when the natural DLI was low and during the later part of the seedling stage. For example, we provided HPS lighting to petunia seedlings at two intensities (low [L], 20 foot-candles, or high [H], 630 foot-candles) from 6 a.m. to 10 p.m., when the outside light intensity was less than 900 foot-candles. When plugs received high light for only one-third of the production period (HLL, LHL or LLH, with each letter representing nine days), shoot mass was greatest when the high HPS lighting was provided during the last third (LLH) (Figure 3, page 30). Plant growth continued to increase with an increase in the duration of the lighting period.

In the past several years, we’ve also identified how DLI influences flowering and crop quality attributes on about 35 seed-propagated bedding plants. We have identified the “saturating DLI” with respect to flowering time, which is the DLI beyond which more light has little or no impact on timing of a particular species. This information has been published in articles and is being incorporated into the Virtual Grower computer program developed by the USDA.

Photoperiod Research

Flowering of a number of ornamental crops is influenced by day length. Our floriculture production group has identified the photoperiodic flowering response of hundreds of crops, most of which are tender or hardy herbaceous perennials (Figure 1, page 26). On some species, we’ve also identified the minimum number of days of a particular photoperiod required to induce flowering. With this information, growers can regulate the photoperiod to prevent or accelerate flowering, whichever is desired.

We have also performed several experiments to determine the pros and cons of different ways to deliver long days with different lamp types. Considerations include time to flower, flower number, flowering uniformity, stem elongation and operating costs to deliver the low-intensity lighting. With advances in light-emitting diode (LED) technology, we are planning to advance our understanding of how light quality (the color of light) influences flowering of ornamentals.

PGR Research

Regulation of plant height is a recurring challenge in the production of many containerized crops. PGRs are most commonly used to inhibit stem elongation, but responses depend in part on the crop, environment, a product’s active ingredient, and the method, concentration and volume of application. For these reasons, successful application of PGRs requires scientific information as much as grower skill, experience and intuition about future crop growth. We’ve performed a variety of experiments with PGRs on a range of floriculture crops during the young plant and finish stages. Experiments have focused on application methods (spray versus drench or liner dip), efficacy of different products at different rates, as well as identifying suggested starting rates for a particular product and crop. Summary information with photographs of representative plants in treatments is available online at We also work with PGR manufacturers to help evaluate experimental formulations or new active ingredients.

Other PGR Research

Several PGRs are used for purposes other than inhibiting stem extension. For example, we have identified how benzyladenine (BA) can be used to inhibit lower-leaf chlorosis or, in a more crop-specific case, promote flowering of phalaenopsis orchids. We have also identified suggested rates of products that contain gibberellic acid (GA), such as Fresco (Fine Americas), to overcome an overapplication of a growth retardant. For example, following a 10-ppm spray of Piccolo (an excessively high rate), as little as a 2.5-ppm spray of Fresco overcame the growth inhibition when applied 12 days later (Figure 4, page 32).

Recently, we have been performing research to help determine desirable application methods and concentrations of abscisic acid to improve the drought tolerance of floriculture crops. If a product could be applied that reduced plant water loss, crops would remain turgid longer, thus increasing their marketable period between waterings. We have worked with Valent BioSciences Corporation to investigate how ConTego, a product planned for release in 2010, could be used successfully by growers. Regardless of the chemical, the strategic use of each product is to increase crop quality or performance in an economical manner.

A Final Word

The research we perform is funded by greenhouse growers, allied trade companies, and state and national competitive granting agencies and endowments. In nearly all instances, projects are supported by multiple funding sources. As a result, industry funds are leveraged by additional resources to generate unbiased information with a goal of advancing the floriculture industry. Whether at MSU or another university, the importance of industry partnerships and support can not be overemphasized; without industry input, donations and leadership, we would not be successful in obtaining external funds. In addition, collaborations within and outside MSU are more important than ever as the number of university faculty solving floriculture industry–related problems continues to decrease.