By Connor Ryan, HF&G Rhododendron Collections Manager
One of my favorite parts of my job as Rhododendron Collections Manager is plant propagation – creating and growing new plants for our grounds and for our patrons. This is a routine part of the work all public gardens do. We propagate plants to display on our grounds, to preserve for conservation and research, and to share with fellow plant geeks.
There are two broad types of plant propagation: sexual and asexual. Sexual propagation is typically done with seeds, and each resulting plant is genetically unique. In asexual propagation, we make clones of plants – yes, cloning is a regular part of my job! Among the most common forms of asexual propagation is stem cuttings.
The products of sexual (pan of green seedlings) and asexual (against white background) Rhododendron propagation. Rhododendrons produce thousands of tiny seeds following pollination, leading to many unique plants if you sow them all. If you have a particularly interesting or unique plant, you can make copies of it through asexual propagation.
Stemcutting propagation is exactly what it sounds like – creating a new plant from a length of stem, typically about 4-6” long. The process is a lot like cooking. You follow a recipe – The Reference Manual of Woody Plant Propagation by Heuser and Dirr and Plant Propagation: Principles and Practice by Hartmann et al. are my go-to “cookbooks.” If you follow the recipe closely, you’ll have success. Usually this means collecting cuttings at the right time of year, applying the correct rate and formulation of a root-inducing hormone, using a porous but moist potting mix, and keeping the propagation area very humid. And waiting, often for a few months, for roots to form. To the right you can see a rooted stem cutting of a smooth hydrangea I love, ‘Hayes Starburst’, from this year. These are easy, and we’ll hopefully have some at a future plant sale!
But sometimes in cooking you have to create your own dish using the ingredients you have lying around the house. The same is true for rooting stem cuttings. There are some plants where there isn’t a recipe, so you have to make an educated guess. I use my knowledge of the science behind plant propagation and the propagation materials I have available to me, and just go for it. This year I tried to root cuttings from a Japanese azalea (Rhododendron wadanum) we have growing in one of our greenhouses at Holden’s David G. Leach Research Station. As far as I am aware, there are no published protocols for stem cuttings of this species. So I applied a rooting hormone (indole-3-butyric acid supplied through a commercial product called Hormodin) to the cut stems at a couple different rates, and the high rate worked. I got roots! Now I’ll start fertilizing the rooted cuttings to hopefully encourage them to produce new shoots.
Every year presents its own challenges, and my interests lead me to try propagating new plants each year. I’m no expert propagator, but I love the challenge of figuring out a protocol for propagating my favorite plants.
Connor Ryan is the Rhododendron Collections Manager at Holden Forests and Gardens . His primary responsibilities are overseeing the management of our David G. Leach Research Station and curating our Rhododendron collection. He has broad interests in woody plant ex-situ conservation, woody ornamental plant breeding, and plant propagation. Before coming to Holden, Connor earned a bachelor’s degree in Plant Science from Auburn University and a master’s degree in Plant Breeding, Genetics, and Genomics from the University of Georgia.