Lighting Effects on Black-eyed Susan

August 22, 2003 - 07:43

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 an
exceptionally short-growing black-eyed Susan (Rudbeckia hirta). The short
stature of 10-15 inches makes these plants an excellent choice for flowerbeds,
interiorscapes and containers, with an expected production time of 12-16 weeks
from seed to flower.

Continued breeding efforts have resulted in additional Toto
selections that feature flowers in yellow and orange shades contrasted with
dark brownish-black centers. The original cultivar Toto was re-named 'Toto
Gold' 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 of
incoming light. Natural light with a balanced wavelength distribution supports
growth and development optimally in most plants. Under conditions with high
reliance on supplemental light that differs from daylight, modifications in
plant growth and rate of flowering may be expected. The choice and use of
high-pressure sodium lamps for supplemental lighting is primarily based on
energy efficiency compared to other lamp type options.

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


High-pressure sodium lamps as the sole light source compared
to high-pressure sodium amended with limited amounts of incandescent light were
evaluated for the growth of the three Toto cultivars. The study was conducted
during restricted natural light and day lengths in a polycarbonate-covered
greenhouse. The shortest natural day on December 21 in our location is three
hours and 42 minutes between sunrise and sunset. The high-pressure sodium lamps
were hung 4-5 feet above the plants and provided about 600-650 foot-candles
during 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, plants
were transplanted into 4-inch pots filled with Premier Pro-Mix BX. Six weeks
from seeding, on November 6, plants had 7-8 expanded leaves, and the
incandescent lamps were turned on over half of the plants. Temperature was
maintained at 64-72° F. Plants were spaced at four pots per square foot and
watered once a day with fertilizer solutions of 100 ppm nitrogen using Peters'


Time from seed to flower was recorded as petals reflexed on
the first open flower and then again at three open flowers. There was no
difference in time to flower or response to light quality in this study among the
three Toto cultivars.

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

Since stem elongation and plant height are expected to
increase under the conditions with lamps like incandescent bulbs that provide
high amounts of long wavelength radiation, these lights are usually avoided. An
important plant characteristic to observe when using incandescent bulbs is,
therefore, the amount of stem elongation. Although the average plant height
varied slightly among the three cultivars in this study, the increase in plant
height due to the addition of incandescent bulbs was similar. Overall plant
height 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 three
cultivars 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 14
developed flowers and flower buds, regardless of cultivar and light quality.
The diameter of the flowers was measured. Unexpectedly, the average flower size
was 1/4 inch larger in environments with incandescent bulbs. The flower size
increased from about 21/2 to almost 3 inches for Toto Gold and Rustic, while
the flowers of Toto Lemon increased to a diameter of 21/2 inches in the
incandescent amended treatments (See Figure 2, page 40).

Applying the Results

If increased height is a concern for altering the light
spectrum to improve flower development with incandescent bulbs, techniques are
available for managing overall plant appearance and quality. Close temperature
monitoring, including the relationship between day and night (DIF), is commonly
used, and growth regulators also effectively control internode and stem
elongation. In an earlier study on Toto, a foliar spray of B-Nine, Bonzi or
Sumagic successfully controlled height. The application rate for spray
application was 5,000 ppm active ingredient for B-Nine, 20 ppm for Bonzi and 10
ppm for Sumagic. Application of growth regulator directly to the surface of the
medium prior to planting is a promising, recently developed treatment
technique. Sumagic, at the rate of 10 ppm, applied with 0.054 fluid ounces of
solution per 4-inch pot (about 12 sq. inches of surface area) was, in a
previous study, found to be most effective for height control. In contrast to
foliar sprays, the growth regulator media application did not slow overall
plant development and flowering of Toto.

Pinching two weeks following transplant slightly reduced the
overall plant height of Toto in an earlier study. Although a pinch delayed the
opening of the first flower, there was no delay at the time three flowers were
fully developed compared to the intact control plants. The resulting more
uniform flower and plant development indicates pinching is a successful
cultural technique to produce high-quality, well-proportioned and balanced
black-eyed Susan for marketing.

The difference between high-pressure sodium and amended
high-pressure sodium light during production do point at the consequence and
potential for adding specific light qualities to improve overall plant growth
and flowering. Increasing the level of high-pressure sodium light may overcome
some of the differences noted here and reduce the impact of adding incandescent

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

About The Author

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

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