Dutch iris forcing guide for Florida

Material Information

Dutch iris forcing guide for Florida
Uniform Title:
Florida Cooperative Extension Service Circular 539
Rogers, Marlin N.
Tjia, Benny Oen Swie
University of Florida -- Florida Cooperative Extension Service -- Institute of Food and Agricultural Sciences
Place of Publication:
Gainesville, Fla.
Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida,
Publication Date:


Subjects / Keywords:
Agriculture ( LCSH )
Farm life ( LCSH )
Farming ( LCSH )
University of Florida. ( LCSH )
Agriculture -- Florida ( LCSH )
Farm life -- Florida ( LCSH )
Plant bulbs ( jstor )
Flowers ( jstor )
Flowering ( jstor )
Spatial Coverage:
North America -- United States of America -- Florida


Florida Historical Agriculture and Rural Life

Record Information

Source Institution:
Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location:
Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management:
All rights reserved, Board of Trustees of the University of Florida
Resource Identifier:
027254551 ( ALEPH )
09858155 ( OCLC )


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Circular 539

Dutch Iris

Forcing Guide for Florida

M. N. Rogers and B. Tjia


\;1 07 1983

Florida Cooperative Extension Service /
institute of Food and Agricultural Sciences /

University of Florida, Gainesville
John T. Woeste, Dean for Extension





Trades names, where used in this publication, are for the purpose of
providing specific information. Use of brand names does not imply endorse-
ment of products by IFAS, nor does it imply disapproval of products not

Marlin N. Rogers and B. Tjia*
Many spring-flowering, commercially grown bulbous plants origi-
nated in the Mediterranean area and have evolved with special physio-
logical adaptations to survive in climates with long, hot, dry summers
and cool, mild winters. Leaves grown in early spring produce carboh-
ydrates that are stored in large, succulent bulb scales (modified leaves)
to sustain the next growth cycle of the plant. The foliage matures and
dies (greatly reducing moisture stress), as hot, dry weather begins, and
bulbs remain underground. Eventually, flower buds are initiated in the
apical meristem of the bulbs, followed by elongation of flower stalks
and flower opening. New leaves arise during this stage to produce
storage carbohydrates for the next growth cycle.
Some bulbous plants (tulip, narcissus and hyacinth) store enough
carbohydrates to complete their life cycle without further need of
additional photosynthetic activity. Other bulbs such as Dutch iris and
most lilies require active photosynthesis during the forcing period to
complete floral development.
Different bulbs have basically similar environmental requirements,
but exact growth cycles and responses of different species vary. Floral
initiation (the change in the apical meristem from production of leaves
to production of embryonic flower buds) occurs during the initial
warm growth period in tulips, narcissi, and hyacinths. The Dutch iris,
on the other hand, initiates flower buds after the cool period (winter),
and in the Easter lily floral initiation occurs even later, as the warm
forcing period begins (Fig. 1).
Dutch iris bulbs for forcing are produced primarily in the coastal
areas of Oregon and Washington, and in the Netherlands. For early
forcing, American grown bulbs are superior to Dutch grown. In
northern Europe, Dutch irises are forced on a year-round schedule,
but in the warm climate of the southern United States, the production
season extends approximately from December to May. Excessively
high temperatures of late spring, summer, and fall in this area are
detrimental to the production of high quality, long-lasting flowers.
Forcing Dutch irises during mid-winter in areas of low winter light
intensity is also not successful, but this is no problem in Florida.
Culture of bulbous plants can be divided into 3 phases: (1) the
bulb-development phase, (2) the programming phase, and (3) the
forcing (greenhouse) phase. The mild winter climate of Florida per-
mits the forcing phase to be carried out in greenhouse structures,
*Visiting Professor of Horticulture, University of Missouri, Columbia, Missouri and
Associate Professor in Ornamental Horticulture and Extension Floriculture Specialist,
Ornamental Horticulture Department, IFAS, University of Florida 32611.


(Summer) (Winter) (Spring)

Tulip Dutch Easter
Hyacinth Iris Lily

Fig. 1. Time of flower bud initiation (FBI) within the annual development cycle
for the major bulb crops.

polypropylene shade houses, or open field areas with a minimum of
protection. Growers forcing Dutch irises outdoors will find it more
difficult to pinpoint the harvest of given crops to specific flowering
dates than those forcing the crop in a controlled environment green-
house. But for continued weekly flower production, excellent results
can be achieved outdoors.

Bulb Development Phase
A major cause of variation in forcing performance is differences in
maturity of bulbs at time of harvest. There is no absolute test to
determine maturity in a Dutch iris bulb before digging. Stuart, et a!
studied this problem in detail and found that the main controlling
factor in bulb maturity is the amount of heat the bulbs received during
midsummer. Bulbs were slow to mature following cold, wet summers
and early forcing performance was erratic. After hot summers, forcing
was easier and flowering was uniform. Stuart, et al found that at least
300 degree days (base temperature 600F)l after June 20 were required
to produce bulbs with an acceptable degree of maturity. Most bulb

1The number of degree days (base temperature 600F) received by a bulb are
calculated by summing the products of the number of days times the average
temperature minus base temperature for each day after June20. If the average
temperature forJune 21 was700F, and for June 22 was 680F: (1 day x [70 60]) +
(1 day x [68 60]) = (1 day x 100F) + (1 day x 80F) = 10 + 8 degree days
accumulated. If the average temperature for the day is below the base
temperature of 600F, no degree days are accumulated for that day.

producers now closely monitor this factor as a guide to the proper
bulb digging date, which normally occurs about July 25 in average
Delayed maturity of iris bulbs resulting from cold, wet summers can
be partially overcome by heat curing them immediately after digging.
The same treatment is used in programming bulbs for earliest possible
flowering. Current commercial practice is to provide 10 days curing at
Programming Phase
The low temperature programming treatments needed by bulbs
such as hyacinths and narcissi to promote physiological and anatomi-
cal development of flower buds are given after the bulbs have been
potted or flatted in a growing medium. Bulbous iris are occasionally
programmed in the same way, but more often the cold temperature
treatments needed are provided while the bulbs are still in bags or
trays before planting. The exact temperature sequences and durations
are given in controlled temperature refrigerators.
Programming treatments for Dutch iris vary depending upon the
desired timing of flowering crops: (1) early flowering (November to
January), (2) normal season flowering (February to March), or (3) late
season flowering (after normal season).
Programming for early flowering. Plant heat-treated, precooled
bulbs. These are bulbs which received 10 days of curing at 90F imme-
diately after digging. This is often followed by a stabilizing period or
end treatment of 2 to 3 weeks at 65 to 700F. This temperature can be
obtained during the shipping period as bulbs move from production
areas to jobbers or distributors for forcing. This treatment accelerates
bulb development for early forcing. At this time the bulb is still vegeta-
tive and continues to initiate new leaves in the apical meristem.
Precooling may be performed either by the jobber-distributor or by
the forcer. The bulbs are held at 550F for 6 to 9 weeks. This causes
termination of leaf formation and promotes flower bud initiation in
the bulb's meristem. Flower bud formation begins 4 to 5 weeks after
the start of the cold storage treatment. The bulbs normally begin to
sprout as precooling nears completion, and planting should begin as
promptly as possible. For early flowering, only the cultivars Wedge-
wood and Ideal are recommended.
Programming for normal season forcing. Heat-treated bulbs are not
necessary. Shed-cured bulbs (held at 65 to 70F after digging) are
normally used. Bulbs remain at these temperatures from the time of
digging until about November 1, when precooling begins. The
method of handling the bulbs and the length of precooling treatment
given is identical to that used for early forcing.

Programming for late season flowering. Retarded bulbs are used for
late season forcing or year-round flowering in areas with mild summer
temperatures. Retarded bulbs are programmed by placing them at
high temperatures (900F) as soon as received by jobber-distributors (or
by growers, if they are doing their own programming). Respiration at
this temperature is at the lowest possible level, with the best possible
conservation of stored carbohydrates for long term storage. This is
followed by an end treatment of 2 weeks at about 650F, and 6 to 8
weeks of precooling before planting. Growers should not attempt to
flower Dutch iris after April in south Florida, or after Mother's Day in
northern parts of the state because of high prevailing forcing tempera-
tures (Tables 1 and 2).

Forcing Phase (Greenhouse)
The growing medium should be steamed or chemically treated with
methyl bromide or Vapam before planting to prevent problems from
weeds or soil-borne fungi. Planting is accomplished by gently pressing
bulbs into the growing medium in flats or in prepared beds until the
tip of each bulb is flush with the soil surface.
Three key environmental factors affect the success of iris forcing:
temperature, light intensity, and soil moisture. Night temperatures
should range from 50 to 600F for best results. Top size bulbs can be
forced at slightly higher temperatures than smaller ones. If tempera-
tures are too high in relation to light levels, flower blasting (abortion or
deformation of the developing flower) may occur. Dutch iris forcing in
Florida can be carried out in greenhouse structures, lightly shaded
(25%) polypropylene structures, or open field growing areas. Timing in
a temperature-controlled greenhouse at 55 to 600F night temperature
is quite predictable 60 to 65 days from planting of the bulbs to
harvesting of buds at the commercial harvesting stage (Fig. 2). The time
span in the field in Florida will vary from 60 to80 days, depending upon
weather conditions. Low temperature injury occurs only rarely, since
Dutch iris can tolerate temperatures to 270F for the first few weeks
after planting without serious harm.
In contrast to tulip, narcissus, and hyacinth, Dutch iris requires high
light levels and good photosynthetic activity during the forcing period
to prevent blasting. Bulbs should not be planted so close together that
mutual shading of leaves of adjacent plants occurs. Six to 9 square
inches of space should be provided per plant.
Dutch irises are sensitive to moisture stress, and blasting of flowers
can occur if the plants are subjected to short term drying. Plants in flats
in northern greenhouses are watered daily until free drainage occurs.
Similar watering practices should be used when the plants are being

Florida Planting Date Approximate Flowering Date Approximate Days
Bulb Condition (Weekly Interval) Based on Average Green- to Flower
North Central South house Temperature of 58-60F Greenhouse Field
Special Pre-cooled Oct. 4 Dec. 6 63-65 60-70
Special Pre-cooled Oct. 11 Dec. 13 63-65 60-70
Pre-cooled Oct. 18* Oct. 18* Dec. 20 63-65 70-80
Pre-cooled Oct. 25* Oct. 25* Dec. 27 63-65 70-80
Pre-cooled Nov. 1* Nov. 1* Nov. 1* Jan. 3 63-65 -
Pre-cooled Nov. 8* Nov. 8* Nov. 8* Jan. 10 63-65 -
Pre-cooled Nov. 15* Nov. 15* Nov. 15* Jan. 17 63-65 -
Pre-cooled or retarded Nov. 22* Nov. 22* Nov. 22* Jan. 24 63-65 -
Retarded Nov. 29* Nov. 29* Nov. 29* Jan. 30 63-65 -
Retarded Dec. 6* Dec. 6* Dec. 6* Feb. 7 63-65 -
Retarded Dec. 13* Dec. 13* Dec. 13* Feb. 14 63-65 -
Retarded Dec. 20* Dec. 20* Dec. 20* Feb. 21 60-65 -
Retarded Dec. 27* Dec. 27* Dec. 27* Feb. 27 60-65 -
Retarded Jan. 3* Jan. 3* Jan. 3* March 6 60-65 60-70
Retarded Jan. 10* Jan. 10 Jan. 10 March 13 60-65 60-70
Retarded Jan. 17 Jan. 17 Jan. 17 March 20 60-65 60-70
Retarded Jan. 24 Jan. 24 Jan. 24 March 27 60-65 60-70
Retarded Jan. 31 Jan. 31 April 2 60-62 60-70
Retarded Feb. 7 Feb. 7 April 8 60-62 60-70
Retarded Feb. 14 Feb. 14 April 14 60-62 60-70
Retarded Feb. 21 Feb. 21 April 20 60-62 60-70
Retarded Feb. 28 Feb. 28 April 26 60-62 60-70
Retarded March 7 -May 3 60-62 60-70
*Under greenhouse conditions only.

Table 2. Forcing Schedules of Pre-Treated Blue Ribbon Iris

Florida Planting Approximate Flowering Date
Date Based on Average Green- Approximate Days
Bulb Condition (Weekly Interval) houseTemperature of 58-60F to Flower
North Central South Greenhouse Field
Pre-cooled Nov. 8* Nov. 8* Nov. 8* Jan. 20 62-73 -
Pre-cooled Nov. 15* Nov. 15* Nov. 15* Jan. 24 60-70 -
Pre-cooled Nov. 22* Nov. 22* Nov. 22* Jan. 31 60-70 -
Pre-cooled Nov. 29* Nov. 29* Nov. 29* Feb. 7 60-70 -
Pre-cooled Dec. 6* Dec. 6* Dec. 6* Feb. 14 60-70 -
Pre-cooled Dec. 13* Dec. 13* Dec. 13* Feb. 21 60-70 -
Pre-cooled or retarded Dec. 20* Dec. 20* Dec. 20* March 9 68-79 80
Retarded Dec. 27* Dec. 27* Dec. 27* March 15 68-79 80
Retarded Jan. 3* Jan. 3* Jan. 3 March 21 68-78 80
Retarded Jan. 10* Jan. 10 Jan. 10 March 26 66-76 80
Retarded Jan. 17* Jan. 17 Jan. 17 March 31 64-74 75
Retarded Jan. 24 Jan. 24 April 3 62-70 75
Retarded Jan. 31 Jan. 31 April 10 60-68 75
Retarded Feb. 7 April 15 60-66 75
Retarded Feb. 14 April 20 58-64 70
Retarded Feb. 21 April 26 58-63 70
*Under greenhouse conditions only.

Fig. 2. The right stage to cut iris flower, when color is barely showing (right).
When half of flower bud has slipped out from the spathe, flower will
open within 3 to 5 hrs and is considered too open to cut and ship
(middle). When flower petals begin to unfurl (left) the entire flower will
open within 3 hrs and is considered too late to cut for shipment.

grown in sandy Florida soils. It is essentially impossible to overwater
the plants during forcing, but very easy not to water them enough.
If the bulbs were adequately fertilized during production, further
fertilization during forcing is not of critical importance since the bulbs
normally carry sufficient nutrient reserves to complete their flower
development. Soils in the medium range of fertility (Table 3) and
soluble salts, with a pH near 7.0, are suitable. Stem topple (collapse of
the flowering stem below the flower) has been reported as a result of
calcium deficiency. However, it can be prevented by weekly feedings
with a 600 ppm calcium nitrate solution (1 oz per 2 gallons of water).

Blindness is a failure of flowers to develop from flowering-sized
bulbs. These bulbs form only 3 leaves, rather than the normal 5.
Blindness often occurs after a cold, wet production period, or it may
be due to use of small bulbs, or improper curing and storage
Blasting, the production of malformed or aborted flowers, results
from improper environmental conditions during forcing. Moisture

Table 3. Medium range soil test values suitable for Dutch iris production in
MgO in
Lb per
P2O, in Lb per Acre K20 in Lb per Acre Acre
Sandy Loamy Mineral Organic
Loam Fine Sand Sand Muck Soil Soil
41-60 61-100 81-140 81-160 91-150 151-300 101-200

stress, high night temperatures, or light levels too low for optimum
photosynthetic activity commonly result in blasting.
Penicillium blue mold can cause bulb decay and poor forcing
results. The fungus enters through wounds, so bulbs should always be
handled carefully. Bulbs planted in cold wet soils (at less than 58 to
600F) are more likely to become infected than those planted in warmer
situations. A preplanting 30-minute soak treatment with Benlate and
Truban at 2 and 1 tablespoons of each per gallon of warm water,
respectively, should provide some protection.
Other disease problems such as mosaic and Fusarium basal rot, can
occur, but most of these must be controlled at the bulb production
phase. Forcers who use healthy planting stock should rarely expe-
rience problems from these diseases.
Aphids are the most common insect problem and respond to nor-
mal preventative treatments.

Harvesting and Marketing
Flowers are cut after color begins to show (Fig. 2), but before petals
and sepals unfurl. This reduces injury during handling and shipment.
They are bunched in groups of 10. Buds open quickly from this stage at
warm temperatures, so it is almost essential that refrigerated storage
space be available for holding the flowers until they can be moved into
sales outlets. Like gladiolus, iris flowers are opened for use after they
reach the retail outlet.
Flowers can be held in dry pack storage at 310F for a few days if
necessary. But since their useful shelf life is not long even under ideal
conditions such holding should be avoided if possible. Flower preser-
vative solutions have not given consistent benefits so deionized water
is currently recommended as a holding solution.

This publication was promulgated at a cost of $744.25, or 41
cents per copy, to provide ornamental growers with a forcing
guide for irises. 11-1. 8M-82.

Tefertller, director, In cooperation with the United States Department IAS
of Agriculture, publishes this Information to further the purpose of the
May 8 and June 30, 1914 Acts of Congress; and is authorized to pro-
vide research, educational Information and other services only to Indi-
viduals and institutions that function without regard to race, color, sex or national ori-
gin. Single copies of Extension publications (excluding 4-H and Youth publications) are
available free to Florida residents from County Extension Offices. Information on bulk
rates or copies for out-of-state purchasers is available from C. M. HInton, Publications
Distribution Center, IFAS Building 664, University of Florida, Gainesvllle, Florida
32611. Before publicizing this publication, editors should contact this address to deter-
mine availability.