The root production method, or RPM, was developed to enhance the survivability of hard mast producing trees on challenging sites. Field planting of RPM seedlings has been utilized primarily in agricultural floodplains throughout the Midwest. The survival and growth of wetland species including several oak species produced utilizing the RPM growing methodology were evaluated for survival, growth rates and natural regeneration compared to sister seedling produced using traditional field grown and bare-rooted practices.
The Challenge of Hardwood Regeneration
It is difficult to regenerate hardwoods such as oak, and pecan on floodplains because competing vegetation, flooding, and animal damage can disrupt seedling growth. Additionally, slow growth rates are an inherent characteristic of oak, hickory, and pecan production, further complicating efforts to regenerate these species. The average age of an oak tree to produce acorns is 20 plus years. The proliferation of roots produced in the RPM growing methodology induces precocious early mast production thus providing natural regeneration of oak seedlings as many as 15 years earlier than traditional produced seedlings.
RPM and Seedling Growth Success
Forrest Keeling’s patented root production method is a nursery culture process to grow seedlings with dense, fibrous root systems in large containers (11-19 liter). Using an example of an oak, encouraging quicker growth with increased vitality begins with acorn selection. Acorns are gathered from floodplains and then graded and sized using an aspirator and gravity table. Only the largest, heaviest seeds are used to produce RPM seedlings.
Following the selection of acorns, they are placed in mesh-bottomed trays filled with composed rice hull, pine bark, and sand medium. Next, slow release fertilizer, a wetting agent, and micronutrients are added to the soil medium, and trays are enclosed in plastic to maintain proper humidity. Next, trays of acorns are moved to a heated greenhouse – a process that typically takes place in early February to initiate germination.
Process after Emergence in Spring
Approximately 1-2 months after emergence – when seedlings have completed their first shoot flush, they are then transplanted into individual plastic (10cm deep) bottomless containers and placed on a base of wire and wood-framed benches in the greenhouse in order to permit continued air root pruning.
Following this step, the seedlings are then graded based on height, stem caliper, and root development. In most cases, only the largest 50% of seedlings will continue through the RPM process. In early May, seedlings are transplanted once more. This time into 11 or 19-liter plastic containers, and placed outside under mist irrigation for 48 hours. This allows them to become acclimated to an outdoor environment. Shallow containers are used to concentrate root growth in the upper 15-20cm soil surface. The larger containers are used for the seedlings, which will spend a second year growing in the nursery.
Final Stages of RPM Development
Once the seedlings have spent one to two seasons in the nursery, they will have developed large root systems with basal stems exceeding 2.0 cm in diameter and growth of more than 1.5 meters in height.
Additional growth details regarding weight were also recorded. Dey and others (referenced in the first paragraph of this article) reported root dry weight of pin oak and swamp white oak seedlings grown with RPM averaged 117 and 101 grams for 11 and 19-meter plants, respectively. For comparison, root dry weight of 1+0 bareroot oak seedlings averaged 18 grams.
When compared using root volume by water displacement, RPM oak seedlings showed clear dominance. Nineteen-liter swamp white oak and pin oak RPM seedlings averaged 252 and 22 ml root volume, respectively. In contrast, root volume of swamp white oak and pin oak bareroot seedlings averaged 33 and 26 ml, respectively.
Experience Faster Growth and Stronger Natives for Your Nursery
Decades of research and experimentation have given us the revolutionary RPM advantage when it comes to native growth and restoration. Your nursery deserves the best the industry has to offer. We strive to provide you with the natives your customers want.
Find additional results of these studies in the following journal articles:
THE ROLE OF LARGE CONTAINER SEEDLINGS IN AFFORESTING OAKS IN BOTTOMLANDS
Daniel C. Dey, John M. Kabrick, Michael Gold
COMPARISON OF BARE ROOT AND RPM SEEDLING PRODUCTION TECHNOLOGIES: IMPLICATIONS FOR AGROFORESTRY
Larry D. Godsey’, John P. Dwyer, W. Dusty Walter, and Harold ‘Gene’ Garrett