Making Coleus Better and Brighter

December 13, 2005 - 12:36

University of Florida’s coleus breeding program is creating varieties with lots of consumer appeal.

Coleus is a versatile annual that has been in the industry for a long time and is valued by many consumers for its brightly colored foliage, rapid growth rate and superior performance in landscapes and containers. Coleus hybrids were first introduced in England in 1862. Coleus quickly became popular, and plant breeders began a race to breed new coleus hybrids, with main developments centering on bright colors, unique leaf shapes and distinctive plant forms. After the initial fever subsided, coleus evolved into a seed-propagated bedding plant with low vigor and a relatively narrow range of foliage colors. In the last decade, coleus has been rejuvenated with the introduction of a large number of interesting vegetatively propagated varieties that have improved performance in containers and the landscape.

While there are many good, new varieties being introduced, there is a need for more diversity and potential in the genetics that is currently under utilized. The main objective of our new coleus breeding program at the University of Florida is to produce new coleus varieties with the following characteristics: 1) improved sun-tolerance with less fading/bleaching of foliage color, 2) increased plant vigor and branching, 3) late flowering, 4) brighter colors in trailing forms and 5) cut-flower forms. Coleus is also a good crop for students to learn the principles of plant breeding for ornamental crops.

Screening Methods

Several thousand coleus seedlings are grown and evaluated in our greenhouses in early spring. We rate them on a 1-5 scale (1=poor, 5=excellent) based on the following characteristics: bright and novel colors, consistency of color pattern, plant vigor and lateral branching. Only the highest-scoring seedlings are selected for further evaluations and are vegetatively propagated. At this stage, cuttings with poor rooting characteristics are eliminated from the trialing program. The remaining “elite” varieties are then planted at three field trial locations during the summer for further evaluations. In 2005, main field trials were conducted in the full sun in Citra, Fla. (hot, sun trial, average high temperature of 92° F), Richfield, N.C. (cooler trial, average temperature of 85° F), and under 30-percent shade in Gainesville, Fla. (hot, shade trial, average temperature of 92° F).

Color Fading

Many brightly colored coleus varieties grown in warm, sunny environments have foliage that either burns and becomes necrotic or transitions to a completely dull green or maroon. Undesirable to consumers, this color fading has been the most difficult problem for us to solve to date, but we have made significant progress in selecting varieties that do not fade.
Through the full sun and shade trials, we discovered that more than 80 percent of our elite varieties had foliage that transitioned to a dull maroon or green and less than 5 percent displayed burning and necrosis. Approximately 15 percent of the elite varieties had foliage that remained bright and consistent in all locations; most of these remain in the breeding program, and seeds are being produced from them for testing in our 2006 crop. By breeding only with varieties that do not fade, we can be assured of consistently bright colors as we select for other traits.

Vigor And Branching

In our program, developing a wide range of vigor in coleus has not been problematic. We noticed plants that were vigorous growers as seedlings continued to grow strong when mature, while slower-growing seedlings were less vigorous mature plants. This observation also paralleled how the plants did in the field, with those less-vigorous ones having a poor start in the field and growing slowly throughout the season in all three test locations. Although many of these weaker plants reached maturity in Florida, they were usually too weak to avoid being overtaken by weeds in North Carolina. The most vigorous varieties grew so large under Florida conditions that they would likely require pruning in the landscape, but these varieties performed well under the cooler conditions of North Carolina. Varieties best suited for growth in Florida often had inconsistent performance in North Carolina.

It also appears that developing highly branched coleus varieties is also achievable through our selection process. We found that seedlings showing highly branched plant architecture almost exclusively displayed this characteristic within the first eight weeks of seedling growth, thus providing an easy method for screening during the early stages of evaluation. These varieties continued to be highly branched throughout the season in all trials; thus, most Á elite varieties had branching patterns suitable for both landscape use and stock plant production.

Late Flowering

Coleus varieties that initiate flowering early are often not desirable for the landscape because flowers decrease a plant’s foliage quality, often slow plant growth and increase landscape maintenance. Therefore, we have identified several elite varieties that either flower late in the season or do not initiate flowers at all. A small number of varieties that had not flowered as of early November have also been selected, and many of these also retain excellent foliage color characteristics throughout the season. Unfortunately, very late flowering varieties may not be useful as parents in our breeding program; it would be difficult to collect seeds from them. Further research in this area would involve determining how to induce flowering in very late or non-flowering varieties to enable their use as breeding lines. Selecting for late flowering varieties in the future will allow for gradual gains to be made.

Trailing Varieties

Currently, there are several excellent coleus varieties with upright habits. However, there is a significant shortage of varieties with bright foliage and a trailing growth habit suitable for use as a ground cover or in a hanging basket. In an effort to breed new trailing types with bright colors, crosses were made between ‘Red Queen’ and ‘Sedona’. We chose these two varieties as parents because they flower continuously in the greenhouse. ‘Red Queen’ has the trailing habit we are looking for; ‘Sedona’ possesses the bright color we want to introduce.

Seedlings from these two varieties are fairly consistent. Self-pollination of ‘Red Queen’ mostly produces a trailing habit with dark red or green leaves, and seedlings from ‘Sedona’ have an upright habit with bright orange or yellow leaves.

We have been successful in producing F1 hybrids with confirmed characteristics. The F1s displayed hybrid vigor and had a qualitative distribution of the mottling, pink speckling and trailing characteristics. Since the mottling trait is found only in ‘Sedona’, and the pink speckling is found only in ‘Red Queen’, those seedlings that possessed both traits confirmed that the F1s were indeed hybrids. These F1 hybrids exhibited certain degrees of trailing, but most were still dark in color, with a slight handful having lighter foliage or appearing brighter due to mottling or speckling.

Cut Flower Production

Early in this project, we thought some of the elite varieties with a strong, upright growth habit and brightly colored foliage may have potential for use as a cut-foliage product. These plants can be quickly produced in beds or containers by methods used for cut-flower production and have foliage colors that match and accentuate a wide array of flower colors used by florists. Five varieties were propagated and grown at close spacing for seven weeks to produce long, straight stems suitable as cut foliage. Stems of all varieties were harvested at sunrise and placed in plastic vases filled with deionized water; they were then put into postharvest evaluation rooms. Some stems were first stored in boxes in the dark for two or four days at 50, 60 or 70° F. Stored stems were then moved from storage to vases. Stems were evaluated for two weeks for the postharvest characteristics of leaf wilting, leaf yellowing and general appearance and then compared with non-stored, control stems.

Cold storage of any stems below 50° F results in blackening of foliage due to cold injury — a reflection of the tropical nature of coleus. We also observed that all varieties, whether they were stored or not, immediately wilted in vases, remained wilted for at least five days after harvest and had poor visual appearance. After 6-8 days, all stems from all varieties and all treatments (except those stored at 50° F) produced adventitious roots in the vases, re-hydrated and lasted in the vases for up to six weeks without significant changes in visual quality.

We were able to reduce transpiration of cut coleus stems by 50 percent through the use of foliar anti-transpirants, but this was still not enough to significantly reduce the wilting observed in the first 5-7 days after harvest. The main point to note here is that if this 1-week wilting barrier can be overcome, coleus may prove to be a valuable cut-foliage product because coleus stems will actually last for quite a while in a vase, and a potential niche may be opened for cut-foliage coleus.

Field Day Evaluations

During University of Florida’s Floriculture Field Day in May, 126 of our elite varieties were showcased, and a survey was given to 52 participants (industry and master gardeners) in the show. The participants’ ages ranged from early 20s to late 60s, and an equal ratio of females to males participated in the survey. The results of the survey showed that 47 percent of the participants listed foliage color as the major attribute they preferred when choosing coleus. Additionally, 61 percent favor coleus with a mounded growth habit, and 21 percent prefer coleus with a highly branched growth habit. A majority of Á the participants expressed interest in seeing more trailing coleus types on the market, especially trailing types with brighter and novel foliage color. Many of the participants were impressed with the bright array of colors seen in the elite varieties, and many commented that they were looking forward to acquiring these newer coleus varieties when they became available. The statistics at the Field Day reinforced the idea that there is demand for newer and better coleus varieties.

Concluding Remarks

Our breeding program began two years ago with several open-pollinated coleus varieties. Since then, we have expanded tremendously and approximately 85 new coleus hybrids, all of which possess many of the desired characteristics we have identified, are in our program at the present time. Throughout our program, we have gained knowledge in trialing coleus and selecting varieties with desired characteristics preferred by production companies and consumers. The efficient selection process we have for screening thousands of coleus seedlings and the established field-trialing techniques should allow us to make continued progress to improve future hybrids. Our biggest challenges appear to be developing new trailing varieties with bright colors and varieties that hold their bright colors better in hot, full sun conditions. The necessary genetic variability appears to be present in coleus to accomplish these objectives.
Currently, three plant-marketing companies are evaluating several of the elite varieties for potential introduction in their product lines. There is even a strong probability that some of our new varieties will be in production this spring, which would be surprisingly fast for a new breeding program.

About The Author

Penny Nguyen is a PhD graduate student, Becky Hamilton is senior biologist and David Clark is associate professor in the Environmental Horticulture Department at University of Florida, Gainesville, Fla. Clark can be reached by E-mail at dclark@mail.ifas.ufl.edu.

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