Ways To Find Optimal Propagation Inputs for Difficult-To-Root Perennials
The Plant Select program, developed by Colorado State University (CSU) and the Denver Botanic Gardens, has many plant selections designed to thrive in the high plains and rocky mountain regions. These plants are selected to survive in a wide range of conditions and provide consumers with unique plant choices for their landscapes.
The goal of recent research at Colorado State University was to enhance the competitiveness of Plant Select perennials through customized propagation techniques by updating propagation protocols for growers. Based on a propagator meeting with Plant Select growers, we determined research was lacking in propagation techniques used to root cuttings of difficult perennials for greenhouse production.
Our goal was to look at hormones, application methods, irrigation, and root-zone heating temperatures in a combination to create optimal rooting percentages during production. We strived for 80% rooting or higher of cuttings in under four weeks. The Plant Select perennials focused on in our experiments were Epilobium canum subsp. garrettii ‘PWWG01S’ (Zauschneria garrettii ‘PWWG01S’), Osteospermum sp., Pterecephalus depressus, and Salvia pachyphylla. These four perennial taxa are identified by growers as having poor rooting percentages and low production rates.
Research Focused on Hormone Treatment, Irrigation, and Root Zone Heating
During multiple months at the CSU Horticulture Center, different combinations of rooting hormones, application methods, irrigation, and root zone heating temperatures were tested. For each experiment, cuttings were collected as specified in current propagation protocols. Once cuttings were treated for each experiment, they were placed in Jiffy Performa plug trays. Cuttings were then placed on a propagation bench to root for four weeks. Starting week 2, cuttings were recorded for number of visible roots seen growing out of the plug media. Photos were taken each week to give a visual progression of rooted plugs.
Experiment 1 focused on hormone treatment and root zone heating temperatures.
A Dip N Grow (IBA 1.0% and NAA 0.5%) solution was applied to cuttings at 1,000 ppm and 500 ppm. The application method was a quick dip of 30 seconds at the basal tissue of the cutting. Cuttings were rooted on a propagation bench with a root zone temperature of either 75°F or 68°F. The root zone temperature was created by two Redi-Heat Heavy Duty Propagation mats. Temperatures were determined by selecting a high and low temperature for the optimal range of rooting perennial cuttings.
Osteospermum species treated with Dip N Grow (500 ppm) by an immersion application in Week 2 of rooting on our propagation bench.
Photo courtesy of Colorado State University
Experiment 2 focused on hormone treatments and irrigation methods. The hormone applied during experiment 2 was Dip N Grow (IBA 1.0% and NAA 0.5%). Dip N Grow was applied at a concentration of 1,000 ppm and 500 ppm.
Two irrigation systems were used to provide moisture to cuttings during this experiment. The overhead irrigation system was located at CSU. Our overhead irrigation system was rebuilt with Dramm Pin-Perfect mist nozzles to provide a uniform water application.
Due to limited greenhouse space at CSU, we also used propagation space at a local greenhouse with an established fogging system. The fogging system was located at Gulley Greenhouse in Fort Collins, CO. Gulley Greenhouse has an entire greenhouse bay dedicated to fogging for cuttings in propagation.
Environment Variability Matters
While keeping cuttings moist during propagation is extremely important, experiment 2 taught many lessons about variabilities in greenhouse systems. It was quickly learned that data collected from either location had a wide variety of environmental factors that affected cutting propagation.
Beyond the environmental differences of each location, variations occurred in each plant taxa. The plant taxa variations were due to moisture differences required for rooting. Salvia pachyphylla tested in this experiment responded poorly in fogging by rotting off at soil level in the second week. Upon discussion with other growers, this salvia variety performs best when given less water and lower humidity.
Experiment 2 also provided more insight on the importance of communication with other growers in problem-solving irrigation systems and propagation. While one irrigation system may work for your company, another system may work best for your partner company. Also, trial testing different plant taxa can lead to finding the optimal irrigation level or irrigation timing.
Propagation Recommendations
After discussion on results of experiment 1 at a Plant Select propagator meeting in October 2019, a new experiment with an additional hormone was determined. Experiment 3 focused on hormone application and hormone treatment. Two different application methods for hormone treatments were tested. Cuttings were given either a quick dip exposing the basal tissue of a cutting for 30 seconds or an immersion of the entire cutting for 3 minutes in a hormone treatment solution. Two hormones were used separately during experiment 3. The two hormones used were Dip N Grow (500 ppm) and Hortus IBA (IBA 20% at 500 ppm). Hortus IBA was a requested addition to propagation experiments, as many of the Plant Select growers had moved to this product with a higher active ingredient of IBA. Cuttings for experiment 3 were placed on a root zone temperature of 75°F based on data collected from experiment 1.
Based on the data collected from April 2019 to April 2020, multiple recommendations were made for perennials propagation protocols of three plant taxa. Recommendations have not been discussed for Salvia pachyphylla as stock plants were prone to heavy infestation of powdery mildew causing poor vegetative growth.
Our recommendations made for protocols were:
- Osteospermum species prefer an immersion application with Dip N Grow at 500 ppm at a 75°F root zone heating temperature. When given these inputs, 100% rooting of all cuttings was seen in week 2.
- Pterocephalus depressus prefer quick dip application with either hormone at 500 ppm at a 75°F root zone heating temperature. When given these inputs, 95% rooting of all cuttings was seen in week 3.
- Epilobium canum subsp. garrettii ‘PWWG01S’ prefer quick dip or immersion applications with either hormone at 500 ppm with a root zone heating temperature in the range of 75°F to 68°F. When given these inputs, 100% rooting of all cuttings was seen in week 3.
These updated protocols have been placed on the Plant Select website for propagator use. The research completed on cutting propagation techniques could be applied to other taxa of perennials. From our standpoint as researchers of perennials for production, cuttings can handle a wide range of inputs. With the correct combination of inputs, increased rooting percentages can be achieved.
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