January 2002

Culture Tips for Regal Geranium By Karl Batschke

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.

Propagation

Regals are propagated from vegetative tip cuttings. A 2- to2 1/2-inch cutting should be stuck into sterile rooting media. Mist should beused 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 areeffective starting points. If VPD misting is not available, begin with 4-5 secondsof mist every five minutes and watch the crop for signs of wilt, reducing themist approximately 10 percent per day.

Regals will be calloused in 10-12 days at media temperaturesof 70-72° F. Rooting light levels are best kept below 2,500 ftc, but higherlight will be tolerated during winter months when air temperatures are below78° F. Transplanting can typically occur 4-6 weeks after stick.

Growing On

Any good, porous soil mix will be suitable for Regalproduction. Regals produce a fairly fine root system, which can be sensitive tooverwatering, so it is best to use a well-drained mix of high-quality peat and15-25 percent perlite. Water plants thoroughly at the start; Regal roots willnot tolerate high EC’s. A good soil starting pH would be 5.6-6.0 with ECof 0.75-1.0 mmhos.

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

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

Light levels are essential to quality Regal production. Itis important to maintain high (greater than 4,000 ftc) light during wintermonths, which helps develop greater petalage in the flowers and fuller, morecompact plants.

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

Bud initiation

Many growers choose to begin with bud-initiated liners fortheir 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 betweentotal 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 aredecreased, average temperature should be decreased as well. In other words,when Regals are budded under dark conditions, better quality will be achievedat lower night temperatures. As light levels increase, however, nighttemperatures 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.

Finishing

Today’s cultivars are ready to sell 7-10 weeks aftercompletion of bud initiation. Finishing Regals is nothing more than anextension of the budding process. The same conditions apply up until threeweeks before flower. Maintain 55° F night and 65° F day temperatureswith a RH of less than 70 percent. Light levels may go as high as 4,000 ftcuntil the plants begin to show color. At this time, light levels should bereduced 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 willmaintain 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° Fduring the last 3-4 weeks.

Production of a 4-inch, single, unpinched plant can be donewith 3-4 plants per square foot, and a 6-inch, single, unpinched plant requiresspacing of 12 inches. A 6 1/2-inch, pinched, single plant or three unpinchedplants, should be spaced at 15 inches on center. Other container sizes such as10- to 12-inch hanging baskets or patio pots are becoming increasingly popularas well.

Insects and Diseases

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

Botrytis can be devastating to a finished crop. Floweringplants should be kept in areas where good horizontal airflow is available, andwater can be kept off the flowers. Additionally, preventative Botrytis spraysused prior to first color will reduce the disease pressure and help ensure aproblem-free finish.

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

Growth Regulators

Cool nights, high light, moderate moisture and daytemperatures will easily produce outstanding-quality Regals. Current experiencesuggests two additional tools: Cycocel and Bonzi. Cycocel, used at 1,500-3,000ppm until bud elongation will reduce stretch. Also, growers are reporting goodsuccess with Bonzi at 8 ppm, used as often as weekly, until plants are showingcolor. Use growth regulators with caution and trial them in your growingenvironment before broad use is adopted.

Postharvest

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