Culture Tips for Regal Geranium

January 11, 2002 - 14:36

With improvements in blooming, flower size, color and habit, Regals are profitable plants for growers and a favorite of consumers.

Regal geraniums (Pelargonium x domesticum) have been popular garden and patio plants since hybridization of this class began in the 1830s. The class combines the genetics of as many as nine pelargonium species. After introduction into the United States in the late 1800s, Regals became well-liked and were commonly called ‘Martha Washington’ geraniums after a popular variety of the day.


Regals are propagated from vegetative tip cuttings. A 2- to
2 1/2-inch cutting should be stuck into sterile rooting media. Mist should be
used to reduce desiccation for the first 10-12 days. Vapor Pressure Deficit
(VPD) targets of 120 mB/min. on day 1 ramped to 1,200 mB/min. on day 10 are
effective starting points. If VPD misting is not available, begin with 4-5 seconds
of mist every five minutes and watch the crop for signs of wilt, reducing the
mist approximately 10 percent per day.

Regals will be calloused in 10-12 days at media temperatures
of 70-72° F. Rooting light levels are best kept below 2,500 ftc, but higher
light will be tolerated during winter months when air temperatures are below
78° F. Transplanting can typically occur 4-6 weeks after stick.

Growing On

Any good, porous soil mix will be suitable for Regal
production. Regals produce a fairly fine root system, which can be sensitive to
overwatering, so it is best to use a well-drained mix of high-quality peat and
15-25 percent perlite. Water plants thoroughly at the start; Regal roots will
not tolerate high EC’s. A good soil starting pH would be 5.6-6.0 with EC
of 0.75-1.0 mmhos.

Newly planted liners will root aggressively, with new roots
visible at the edge of the container 7-10 days after planting.

Fertilize to maintain a soil EC of 0.9-1.1 with a balanced
fertilizer. Regals respond well to calcium/magnesium-type fertilizers
alternated with 20-10-20 applied at 150 ppm.

Light levels are essential to quality Regal production. It
is important to maintain high (greater than 4,000 ftc) light during winter
months, which helps develop greater petalage in the flowers and fuller, more
compact plants.

Starting temperatures for newly established, non-precooled
plants should be 65° F night temperature and 72° F day temperature. If
pinching is to be part of the program, it can be done two weeks after
transplant. Once plants are established (in 1-2 weeks), or pinched plants have
new shoots 1/2-inch in length, the bud initiation phase can begin.

Bud initiation

Many growers choose to begin with bud-initiated liners for
their finished crop, which allows for quick finish times (7-10 weeks). However,
Regal bud initiation is not a difficult task.

Lighting. The budding process includes extended-day lighting. This can be achieved with “mum” lighting from 5 a.m.
until daylight and 4 p.m. until 8 p.m. There is a strong relationship between
total irradiance (amount of accumulated light) and flower quality. Essentially,
the more light, the more petals in the flower.

Temperature. Regals will flower with night temperatures between 45 and 58° F. Most growers find night temperatures of 54-55° F to offer a good balance between flower quality and crop length, since cooler night temperatures lengthen days to flower. As average light levels are
decreased, average temperature should be decreased as well. In other words,
when Regals are budded under dark conditions, better quality will be achieved
at lower night temperatures. As light levels increase, however, night
temperatures can also increase, up to a maximum of 58° F at 4,000 ftc.

Humidity and moisture stress. A Regal grown under high humidity and no moisture stress will happily remain vegetative, even under proper temperatures and light. Ideally, humidity levels should be less than 70 percent in Regal production. Even bud-initiated liners can revert to vegetative when humidity is kept too high. The bud initiation phase of production will normally take four weeks. At that time, the finishing phase can begin.


Today’s cultivars are ready to sell 7-10 weeks after
completion of bud initiation. Finishing Regals is nothing more than an
extension of the budding process. The same conditions apply up until three
weeks before flower. Maintain 55° F night and 65° F day temperatures
with a RH of less than 70 percent. Light levels may go as high as 4,000 ftc
until the plants begin to show color. At this time, light levels should be
reduced to 2,500 ftc to reduce leaf temperature and extend flower longevity.
Fertilizer levels should not exceed 1.1 mmhos, and a pH of 5.6-6.0 will
maintain adequate micronutrient availability.

Once the flower buds have developed and begun to elongate,
finished plant size can be increased by raising night temperatures to 62° F
during the last 3-4 weeks.

Production of a 4-inch, single, unpinched plant can be done
with 3-4 plants per square foot, and a 6-inch, single, unpinched plant requires
spacing of 12 inches. A 6 1/2-inch, pinched, single plant or three unpinched
plants, should be spaced at 15 inches on center. Other container sizes such as
10- to 12-inch hanging baskets or patio pots are becoming increasingly popular
as well. 

Insects and Diseases

The most significant malady of Regal geraniums is greenhouse
whitefly; Regals are a good indicator plant for whitefly scouting. Yellow
sticky cards should be used to monitor adult populations during production.
Marathon applied during the vegetative growing stage has been an effective
control. Other products such as Pyrigro, Orthene/Tame and Plantfume 103 are
also effective. In any case, it is important to achieve control during the
nonflowering phase because Regal flowers are sensitive to most chemical sprays.

Botrytis can be devastating to a finished crop. Flowering
plants should be kept in areas where good horizontal airflow is available, and
water can be kept off the flowers. Additionally, preventative Botrytis sprays
used prior to first color will reduce the disease pressure and help ensure a
problem-free finish.

As with all pelargonium species, Regals can be host to
bacterial blight (Xanthomonas c. pelargonii). Although without symptoms in
Regals, bacterial blight can be spread from infected Regals to other geranium
species. Culture indexing is the most effective method of ensuring the
integrity of a Regal crop. Carrying stock of a “favorite” old Regal
variety may also mean carrying an undetected and unwanted pathogen.

Growth Regulators

Cool nights, high light, moderate moisture and day
temperatures will easily produce outstanding-quality Regals. Current experience
suggests two additional tools: Cycocel and Bonzi. Cycocel, used at 1,500-3,000
ppm until bud elongation will reduce stretch. Also, growers are reporting good
success with Bonzi at 8 ppm, used as often as weekly, until plants are showing
color. Use growth regulators with caution and trial them in your growing
environment before broad use is adopted.


Regals that are ahead of schedule can be held in a cold
greenhouse at temperatures as low as 40° F for up to three weeks.
Displaying Regals in a covered, outdoor area will intensify the vivid flower
colors and dramatically increase shelf life. Consumers should expect 6-8 weeks
of heavy flowering from a well-grown Regal, provided they keep them in a cool,
bright location. Because of their sensitivity to ethylene, Regals should be
considered a local market plant and shipped when one-third of the flowers have
begun to open. It is possible to box and ship Regals at the “cracking
color” stage in refrigerated trucks; however, greenhouse finishing
facilities are then necessary to complete the process.

About The Author

Karl Batschke is director of North American production for Oglevee, Ltd., Connellsville, Pa. He may be reached by phone at (724) 628-8360 or via E-mail at

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