Growing Grandiosa

Dr. P. Allen Hammer

Grandiosa is an interspecific geranium (Pelargonium x domesticum x crispum) that exhibits “regal-looking” flowers but with significant production and garden performance differences when compared to regal geraniums. Grandiosa responds to a cool growth period; however, it doesn’t have a long cold requirement. This results in a shorter production period and earlier flowering.
Grandiosa roots easily in four weeks like a zonal geranium, is free branching and requires no pinching. Most importantly for consumers, Grandiosa geraniums flower throughout the summer—even in areas with hot summers.

Propagation
Grandiosa should be rooted with a 4-week schedule using the same environmental settings as you’d use with zonal geraniums, with the exception of reduced mist. This is because Grandiosa leaves don’t lose as much water as zonals. We recommend a rooting hormone powder treatment to improve uniformity of rooting.

Potting liners
Rooted liners should be potted in a well-drained root medium with a pH of 5.8 to 6.3. Water newly planted pots well until liners are established. Grow plants at 70F (21C) days and 65F (18 C) nights for one to two weeks after potting to speed root development, although this isn’t essential. Apply one spray application of chlormequat (Cycocel or Citadel) at 350 to 750 ppm once plants are established and are well-rooted in the pot. After plants establish themselves, the root medium should be kept in the dry to moist range. Don’t keep Grandiosa pots wet. We don’t recommend pinching, which will add two to three weeks to the schedule.

Cool growth period
A cool growing period of 50F to 54F (10C to 12C) day/night temperatures and reduced watering will improve flower display at finish. Extending the time at these temperatures will produce exceptional quality plants. The accompanying schedule shows the recommended minimum weeks of cool growth period for acceptable flower displays. Keep in mind that Grandiosas will grow and add bulk during the cool period, so watering and a light fertilizer (100 ppm N) application is required.

Warm finishing growth period
Increase temperature to 70F (21C) days and 65F (18C) nights to speed the crop’s finish time. Increase watering and fertilizer applications to maintain an EC of 1.2. You can add applications of chlormequat (Cycocel or Citadel) at 350 to 750 ppm to improve plant form depending on the environment. 

Dr. P. Allen Hammer is a retired professor of floriculture at Purdue University, Lafayette, Indiana, and is now in product development and support for Dümmen USA.