Using PGRs on Spring Bulbs By Bill Miller

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 springproduction in many North American greenhouses. Their quick greenhouse time,close spacing and “green” reputation (little fertilizationrequired, limited pest problems) are all attractive to growers. In NorthAmerica, many of the crops and cultivars are too tall when grown in 4- or6-inch pots, and this is one of the issues we are addressing at CornellUniversity.

In many cases, the real issue for growth regulation of spring bulbs does not come during production but during postharvest. This isclearly 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. Effectiveheight management in bulb crops depends on proper cultivar selection. Mosttulips were originally selected for use as cut flowers. Therefore, foreffective use as potted plants, PGRs are generally needed. There is a constant(and increasing) stream of new tulip cultivars being introduced, withincreasing emphasis on cultivars adapted to pot culture without the need forPGRs. Across the range of bulb genera, there are numerous other examples ofsmaller cultivars available for pot forcing: Dwarf lilies, dahlias, daffodilsand amaryllis are but a few.

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

Temperature.Generally speaking, warmer forcing temperatures promote greater overallelongation of bulb crops. A good example can be seen in pot freesia, whereGeorge Wulster showed that increasing temperature from 50-68° F doubledplant height.


Plant Growth Regulators

With most spring bulbs, the primary PGR application is bymedia drench. This is especially true for tulips, and to date, the majorproducts 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 forEaster and hybrid lilies, Sumagic has not emerged as a major PGR onrooting-room bulb crops even though it is an effective product in many cases.

A few words on media drenches are in order. The optimumdrench completely saturates the media with solution and allows less than 10percent of the applied volume to leach. This is especially true for bulb cropswhere 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- and6-inch pots, respectively. Recipes are given in Table 1 on the right. To ensureeven solution dispersal, plants should be watered 24 or fewer hours beforeapplying the drench. Drenching into an overly dry media may lead to unevennessfrom channeling, or at the very least, less PGR effect as the material may notpenetrate to the bottom third of the pot where the roots are.

The rate ranges, products and application methods are givenbelow for the major spring crops. Additionally, the Holland Bulb Forcer’sGuide is an important reference for forcing bulbs, and the industry owes aÁ great debt of gratitude to Gus de Hertogh for its conception andrefinement over the years. It is an essential reference for anyone producingbulbs commercially and is highly recommended. Specific use rates by cultivarand forcing period can be found there.


Height Control Methods

Tulip. A key pointis that the longer a tulip is cooled (especially while it is rooted), thetaller it will be, all other things being equal. Thus, within the confines oflabor, facilities and management limitations, avoiding excessive cooling canprovide a good degree of height control. A second key point is that growersshould constantly check their crops in the rooting room and promptly reducetemperatures to 32-33° F after all plants are rooted. Reducing temperaturereduces shoot elongation in the cooler and may also help to control unneededroot growth that can lead to problems with Trichoderma fungus. The differencebetween 32-33° F and 40-42° F is enormous for bulb stem elongation overa 6-8 week period.

The typical treatment for tulips is to apply a Bonzi orA-Rest drench within 1-2 days of moving into the greenhouse. Rates vary from0.125-0.5 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 givesignificantly less growth control, as much of the PGR effect is concentrated inthe bottom internode of the plant. Conversely, Bonzi and Sumagic are botheffective in reducing the elongation of the top internode and can provide goodheight control in the postproduction environment.

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

Hyacinth. While mosthyacinths flower within the “aesthetic ratio,” their very heavyflower stalks often topple over. Cultivar selection can help avoid thisproblem. Aside from cultivar selection, Florel is commonly used for heightcontrol in hyacinths and results in shorter, stockier flower stalks that aremore resistant to toppling. The safe window for spraying Florel is short; itshould be sprayed when the plants are 3-4 inches tall, but the flowers must notshow full color at spraying. Some cultivars require a second application (2-3days after the first) to keep the flower stalks sufficiently short; be certainto consider this in relation to the timing of the first spray. If flowers areopen, Florel can cause premature senescence. Florel should be sprayed to runoff onto well-watered plants. To avoid water on the leaves of flowers at night,a late morning to early afternoon application is recommended. Guidelines forpreparation of Florel solutions are given below.

We have been experimenting with bulb dips and pre- orpost-cooling drenches of Bonzi or Sumagic. Bonzi or Sumagic pre-plant dips wereeffective in controlling height of prepared ‘Anna Marie’ hyacinths.In these experiments, you can see that leaf length was drastically reduced, aswas stem length below the flowers. While individual flower size was notaffected, the length of the inflorescence was reduced, leading to a tighter appearance. Our trials exploringcultivar effects and alternative application methods continue.

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

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

Muscari (grape hyacinth). Grape hyacinth is a highly desirable plant with wonderfully fragrantflowers. 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 havegrape hyacinths perennialized in your garden, you will see a lush crop ofleaves 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 themarketing date. In the rooting room, leaf growth is retarded by lowtemperature, especially if the cooler is at or close to 32° F. Upon movingto the greenhouse, leaf growth is very rapid and much more vigorous thanflower-stem growth. Consequently, flowers are often buried in the foliage. Wehave not worked on a growth regulator solution to this problem but do know that”late planting” is an effective method of producing a high-qualityplant with shorter leaves.

Late planting is simply planting the bulbs after asubstantial part of the cooling phase is complete. Thus, bulbs would be cooleddry (unplanted), using the regular rooting room. Bulbs are planted only after asignificant 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 (beforeand after planting), as the bulb perceives cold in both stages. Delayedplanting 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 forFlowerbulbs and Nursery Stock, SePRO, Uniroyal and Valent USA for financial andmaterial assistance with this research. Also, thanks to Cornell’s BarbaraStewart and the three Dutch interns involved with these projects: Dirk Warmerdam,Jeffrey Wagemaker and Pieter Heemskerk.

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Bill Miller

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 [email protected]

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