Using PGRs on Spring Bulbs

May 7, 2002 - 09:42

Proper cultivar selection, length of cooling and forcing temperature can all affect plant heightand forcing temperature can all affect plant height.

Flower bulbs comprise a significant proportion of spring
production in many North American greenhouses. Their quick greenhouse time,
close spacing and “green” reputation (little fertilization
required, limited pest problems) are all attractive to growers. In North
America, many of the crops and cultivars are too tall when grown in 4- or
6-inch pots, and this is one of the issues we are addressing at Cornell

In many cases, the real issue for growth regulation of spring bulbs does not come during production but during postharvest. This is
clearly seen above with the miniature ‘Tête-à-Tête’ daffodil as it appears during marketing and after seven days in a typical interior (consumer) environment at 70° F. Similar exuberant growth is seen in many hyacinth cultivars. Thus, spring bulb growth regulation is often a case of PGR application in the greenhouse for growth control in the low light and warm interior environment.


Factors Affecting Height

Cultivar. Effective
height management in bulb crops depends on proper cultivar selection. Most
tulips were originally selected for use as cut flowers. Therefore, for
effective use as potted plants, PGRs are generally needed. There is a constant
(and increasing) stream of new tulip cultivars being introduced, with
increasing emphasis on cultivars adapted to pot culture without the need for
PGRs. Across the range of bulb genera, there are numerous other examples of
smaller cultivars available for pot forcing: Dwarf lilies, dahlias, daffodils
and amaryllis are but a few.

Length of cold. Tulips,
daffodils and hyacinths all share a common characteristic: The longer the
length of cooling (and Á rooting) before forcing, the taller the plant
ultimately will be. Therefore, not exposing plants to excessive lengths of cold
can achieve a significant degree of height control. The obvious problem here is
that growers want to plant their entire bulb crop at one time. While efficient
at the start, this method leads to excessive height and possible lost quality
for the last crops of the year. Cooling bulbs to the proper length would
require bulbs to be delivered at differing times or for proper temperatures to
be applied at each grower’s facility. Labor scheduling can be an issue as

Generally speaking, warmer forcing temperatures promote greater overall
elongation of bulb crops. A good example can be seen in pot freesia, where
George Wulster showed that increasing temperature from 50-68° F doubled
plant height.


Plant Growth Regulators

With most spring bulbs, the primary PGR application is by
media drench. This is especially true for tulips, and to date, the major
products used have been ancymidol (A-Rest) and paclobutrazole (Bonzi).
Hyacinths and daffodils are an exception, and foliar sprays with ethephon
(Florel) are the standard treatments used. While an important product for
Easter and hybrid lilies, Sumagic has not emerged as a major PGR on
rooting-room bulb crops even though it is an effective product in many cases.

A few words on media drenches are in order. The optimum
drench completely saturates the media with solution and allows less than 10
percent of the applied volume to leach. This is especially true for bulb crops
where the vast majority of the roots are in the bottom third of the root ball.
Typically, drenches are applied in two or four ounces of volume for 4- and
6-inch pots, respectively. Recipes are given in Table 1 on the right. To ensure
even solution dispersal, plants should be watered 24 or fewer hours before
applying the drench. Drenching into an overly dry media may lead to unevenness
from channeling, or at the very least, less PGR effect as the material may not
penetrate to the bottom third of the pot where the roots are.

The rate ranges, products and application methods are given
below for the major spring crops. Additionally, the Holland Bulb Forcer’s
Guide is an important reference for forcing bulbs, and the industry owes a
Á great debt of gratitude to Gus de Hertogh for its conception and
refinement over the years. It is an essential reference for anyone producing
bulbs commercially and is highly recommended. Specific use rates by cultivar
and forcing period can be found there.


Height Control Methods

Tulip. A key point
is that the longer a tulip is cooled (especially while it is rooted), the
taller it will be, all other things being equal. Thus, within the confines of
labor, facilities and management limitations, avoiding excessive cooling can
provide a good degree of height control. A second key point is that growers
should constantly check their crops in the rooting room and promptly reduce
temperatures to 32-33° F after all plants are rooted. Reducing temperature
reduces shoot elongation in the cooler and may also help to control unneeded
root growth that can lead to problems with Trichoderma fungus. The difference
between 32-33° F and 40-42° F is enormous for bulb stem elongation over
a 6-8 week period.

The typical treatment for tulips is to apply a Bonzi or
A-Rest drench within 1-2 days of moving into the greenhouse. Rates vary from
0.125-0.5 mg per pot of A-Rest and approximately 0.5-2 mg per pot Bonzi,
depending on cultivar and time of year. Delaying the drench will give
significantly less growth control, as much of the PGR effect is concentrated in
the bottom internode of the plant. Conversely, Bonzi and Sumagic are both
effective in reducing the elongation of the top internode and can provide good
height control in the postproduction environment.

Experimentally, pre-plant bulb soaks in Bonzi or Sumagic are
effective on tulips. While not recommended as a potted plant, certain dip
treatments are practically capable of tailoring ‘Apeldoorn’ into a
usable, 4-inch plant. Another interesting find is that even with severe height
reduction, there is essentially no delay in flowering, nor any reduction in
flower size. Also, we have achieved effective height control by drenching with
Bonzi or Sumagic immediately after planting and prior to cooling.

Hyacinth. While most
hyacinths flower within the “aesthetic ratio,” their very heavy
flower stalks often topple over. Cultivar selection can help avoid this
problem. Aside from cultivar selection, Florel is commonly used for height
control in hyacinths and results in shorter, stockier flower stalks that are
more resistant to toppling. The safe window for spraying Florel is short; it
should be sprayed when the plants are 3-4 inches tall, but the flowers must not
show full color at spraying. Some cultivars require a second application (2-3
days after the first) to keep the flower stalks sufficiently short; be certain
to consider this in relation to the timing of the first spray. If flowers are
open, Florel can cause premature senescence. Florel should be sprayed to run
off onto well-watered plants. To avoid water on the leaves of flowers at night,
a late morning to early afternoon application is recommended. Guidelines for
preparation of Florel solutions are given below.

We have been experimenting with bulb dips and pre- or
post-cooling drenches of Bonzi or Sumagic. Bonzi or Sumagic pre-plant dips were
effective in controlling height of prepared ‘Anna Marie’ hyacinths.
In these experiments, you can see that leaf length was drastically reduced, as
was stem length below the flowers. While individual flower size was not
affected, the length of the inflorescence 
was reduced, leading to a tighter appearance. Our trials exploring
cultivar effects and alternative application methods continue.

Narcissus. Many
potted daffodil cultivars are sprayed with 1,000-2,000 ppm Florel when the
leaves and/or flower stems are 3-4 inches long in the greenhouse. With Á
some cultivars, and especially for late crops where plants have received
excessive cold weeks, a second spray 2-3 days later is suggested. Data for many
cultivars can be found in the Forcer’s Guide.

One cultivar that has no height control recommendation in
the guide is Tête-à-Tête, the most highly used potted
daffodil in North America. As mentioned above, this cultivar is susceptible to
excessive elongation from the moment the first flowers open. In the
postproduction phase, we have seen positive results from Florel sprays (1,000
ppm) applied in the greenhouse at a stage when leaves are substantially
unfolded but before buds are readily visible. We have also seen positive
results with Bonzi applications, either as pre-plant dips, pre-cooling drenches
or as drenches 1-3 days after placement in the greenhouse. All three methods
with Bonzi have given positive results over a 2-year period. At present,
however, we still consider these to be experimental treatments and look forward
to providing more concrete recommendations in the future.

Muscari (grape hyacinth). Grape hyacinth is a highly desirable plant with wonderfully fragrant
flowers. Unfortunately, the leaves grow too long and detract from the plant.
This is because Muscari leaf growth begins at the end of summer. If you have
grape hyacinths perennialized in your garden, you will see a lush crop of
leaves emerge in early fall that persist through the winter.

Typically, growers plant grape hyacinths in the fall,
placing them into the rooting room for 14-plus weeks, depending on the
marketing date. In the rooting room, leaf growth is retarded by low
temperature, especially if the cooler is at or close to 32° F. Upon moving
to the greenhouse, leaf growth is very rapid and much more vigorous than
flower-stem growth. Consequently, flowers are often buried in the foliage. We
have not worked on a growth regulator solution to this problem but do know that
“late planting” is an effective method of producing a high-quality
plant with shorter leaves.

Late planting is simply planting the bulbs after a
substantial part of the cooling phase is complete. Thus, bulbs would be cooled
dry (unplanted), using the regular rooting room. Bulbs are planted only after a
significant proportion (perhaps 70-75 percent) of the cold weeks have elapsed.
Then, bulbs are planted and cooled at 40-45° F for the last 25-30 percent
(3-4 weeks) of cold. It is critical to give the proper total length of cold (before
and after planting), as the bulb perceives cold in both stages. Delayed
planting has no effect on flowering date or quality but does give shorter,
less-rank leaves and markedly improves plant quality.


Thanks are expressed to the Dutch Exporters Association for
Flowerbulbs and Nursery Stock, SePRO, Uniroyal and Valent USA for financial and
material assistance with this research. Also, thanks to Cornell’s Barbara
Stewart and the three Dutch interns involved with these projects: Dirk Warmerdam,
Jeffrey Wagemaker and Pieter Heemskerk.

About The Author

Bill Miller is professor of flowerbulb and greenhouse crop physiology in the Department of Horticulture, Cornell University, Ithaca, N.Y. He can be reached by phone at (607) 255-1799 or E-mail at

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