Lighting Effects on Black-eyed Susan By Meriam Karlsson and Jeff Werner

High-pressure sodium lamps and incandescent light are put to the test in this University of Alaska-Fairbanks research.

The Toto series was developed and introduced as anexceptionally short-growing black-eyed Susan (Rudbeckia hirta). The shortstature of 10-15 inches makes these plants an excellent choice for flowerbeds,interiorscapes and containers, with an expected production time of 12-16 weeksfrom seed to flower.

Continued breeding efforts have resulted in additional Totoselections that feature flowers in yellow and orange shades contrasted withdark brownish-black centers. The original cultivar Toto was re-named ‘TotoGold’ to signify the intense yellow petals. In addition to Toto Gold,selections with petals in clear yellow (‘Toto Lemon’) or very dark orange,fading into yellow at the tips of the petals (‘Toto Rustic’) are now offered.

Black-eyed Susan responds strongly to the type or quality ofincoming light. Natural light with a balanced wavelength distribution supportsgrowth and development optimally in most plants. Under conditions with highreliance on supplemental light that differs from daylight, modifications inplant growth and rate of flowering may be expected. The choice and use ofhigh-pressure sodium lamps for supplemental lighting is primarily based onenergy efficiency compared to other lamp type options.

The light of high-pressure sodium is concentrated to theyellow and orange wavelengths and limited in blue (short) and far-red (long)wavelengths. During periods of restricted natural light, small amounts of lightfrom regular incandescent bulbs improve the high-pressure sodium spectrum withadditional long wavelengths. We conducted this research to determine how muchincandescent light was needed and what the benefit of that light actually was.


High-pressure sodium lamps as the sole light source comparedto high-pressure sodium amended with limited amounts of incandescent light wereevaluated for the growth of the three Toto cultivars. The study was conductedduring restricted natural light and day lengths in a polycarbonate-coveredgreenhouse. The shortest natural day on December 21 in our location is threehours and 42 minutes between sunrise and sunset. The high-pressure sodium lampswere hung 4-5 feet above the plants and provided about 600-650 foot-candlesduring the 16 daily hours of supplemental light. In the amended treatments,incandescent bulbs provided 50 foot-candles throughout the 16-hour day.

Seed germinated at 64-72° F. Three weeks later, plantswere transplanted into 4-inch pots filled with Premier Pro-Mix BX. Six weeksfrom seeding, on November 6, plants had 7-8 expanded leaves, and theincandescent lamps were turned on over half of the plants. Temperature wasmaintained at 64-72° F. Plants were spaced at four pots per square foot andwatered once a day with fertilizer solutions of 100 ppm nitrogen using Peters’15-16-17.


Time from seed to flower was recorded as petals reflexed onthe first open flower and then again at three open flowers. There was nodifference in time to flower or response to light quality in this study among thethree Toto cultivars.

On average, the first open flower was observed 86 days fromseeding for plants in the incandescent enhanced environment. Flowering underhigh-pressure sodium as the sole supplemental source was, on average, eightdays slower at 94 days. Plants in the incandescent adjusted light had threeopen flowers one week after the initial flower. Nine or 10 additional days wererequired for two more flowers to open under high-pressure sodium alone.Subsequently, at the average time for three open flowers under the combinationof incandescent and high-pressure sodium, the first flower in the high-pressuresodium setting was just beginning to open.

Since stem elongation and plant height are expected toincrease under the conditions with lamps like incandescent bulbs that providehigh amounts of long wavelength radiation, these lights are usually avoided. Animportant plant characteristic to observe when using incandescent bulbs is,therefore, the amount of stem elongation. Although the average plant heightvaried slightly among the three cultivars in this study, the increase in plantheight due to the addition of incandescent bulbs was similar. Overall plantheight under high-pressure sodium alone averaged 91/2 inches for Toto Lemon,just over 10 inches for Toto Rustic and 11 inches for Toto Gold. All threecultivars grew approximately 11/2 inches taller by adding incandescent light(See Figure 1, page 40). Each plant had an average of four main branches and 14developed flowers and flower buds, regardless of cultivar and light quality.The diameter of the flowers was measured. Unexpectedly, the average flower sizewas 1/4 inch larger in environments with incandescent bulbs. The flower sizeincreased from about 21/2 to almost 3 inches for Toto Gold and Rustic, whilethe flowers of Toto Lemon increased to a diameter of 21/2 inches in theincandescent amended treatments (See Figure 2, page 40).

Applying the Results

If increased height is a concern for altering the lightspectrum to improve flower development with incandescent bulbs, techniques areavailable for managing overall plant appearance and quality. Close temperaturemonitoring, including the relationship between day and night (DIF), is commonlyused, and growth regulators also effectively control internode and stemelongation. In an earlier study on Toto, a foliar spray of B-Nine, Bonzi orSumagic successfully controlled height. The application rate for sprayapplication was 5,000 ppm active ingredient for B-Nine, 20 ppm for Bonzi and 10ppm for Sumagic. Application of growth regulator directly to the surface of themedium prior to planting is a promising, recently developed treatmenttechnique. Sumagic, at the rate of 10 ppm, applied with 0.054 fluid ounces ofsolution per 4-inch pot (about 12 sq. inches of surface area) was, in aprevious study, found to be most effective for height control. In contrast tofoliar sprays, the growth regulator media application did not slow overallplant development and flowering of Toto.

Pinching two weeks following transplant slightly reduced theoverall plant height of Toto in an earlier study. Although a pinch delayed theopening of the first flower, there was no delay at the time three flowers werefully developed compared to the intact control plants. The resulting moreuniform flower and plant development indicates pinching is a successfulcultural technique to produce high-quality, well-proportioned and balancedblack-eyed Susan for marketing.

The difference between high-pressure sodium and amendedhigh-pressure sodium light during production do point at the consequence andpotential for adding specific light qualities to improve overall plant growthand flowering. Increasing the level of high-pressure sodium light may overcomesome of the differences noted here and reduce the impact of adding incandescentlight.

The Toto series is suitable for a variety of plantings,relatively pest free and exceptionally weather and heat resistant in thelandscape. The newly added cultivars provide options for creating arrangementswith more variations, types and contrasts for indoor and outdoor installations.The black-eyed Susan series Toto fits nicely into the now-available range ofcompact species and cultivars for instant color, landscaping and interiordisplays.

Meriam Karlsson and Jeff Werner

Meriam Karlsson is professor and Jeff Werner is research associate at the University of Alaska-Fairbanks. They may be reached by phone at (907) 474-7005 or E-mail at [email protected]

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