A grant to explore the Rhododendron

Protecting

Our Plant Health

The American Rhododendron Society awards a grant to explore Rhododendron microhabitats.

By Annie Rzepka, Director of Arboretum Horticulture

Holden Forests & Gardens commitment to rhododendron began in 1940 when Charles Dexter of Sandwich, Massachusetts, donated 250 rhododendrons to Holden Arboretum as a gift to Holden Arboretum’s first executive administrator Warren Corning. Over the last 80-plus years Holden has furthered this commitment to the genus Rhododendron by building our first

Rhododendron Garden in 1963, accepting the donation of the David G. Leach Research station property and Rhododendron collection in 1987, and constructing the five-acre Paine

Rhododendron Discovery Garden in 2013.

While HF&G has decades of rhododendron hybridization experience and a solid understanding of the cultivars that will persist in our region, in recent years we have noticed a decline in the health of our collection across our three core rhododendron gardens: Layer Garden, Leach Research Station and Rhododendron Discovery Garden.

Conditions vary greatly between the three garden areas and our rhododendron living collections exist in a diverse array of soil compositions and tree canopy light levels. We have long assumed that the soil traits in our garden beds, most notably drainage and canopy cover, have been the driving abiotic factors that influence the overall health and vigor of our collection, but diagnosing plant health problems is not straight forward. Many factors have the potential to impact the health of our rhododendron collection, including the inherent conditions of the site, pest and disease pressure, planting depth and root problems associated with improper nursery practices.

While we have made guesses at potential problems, we have not gathered empirical data to support these assumptions until now. Thanks to funding from the ARS Endowment Grant we have started to assess the basic factors that impact the health of our rhododendrons, including soil characteristics, macro and micronutrient compositions, soil drainage and light level gradients in our gardens so we may better understand how to care for and maintain these living collections.

As a museum, extensive long-term data sets exist for our accessioned rhododendron specimens, including provenance, height and condition. Last summer, using these existing data sets, we selected 43 targeted sampling locations focusing on beds containing Ironclad rhododendrons (Rhododendron catawbiense

and Rhododendron subgenus Hymenanthes section Ponticum). These rhododendron and their cultivars were selected because 1.) they provide parentage for a large portion of our existing collection; 2.) are present at several locations across our multiple campuses and; 3.) have historically demonstrated success within our region.

After we selected our sampling locations, we began collecting soil samples. Soil sampling was a relatively simple but rather timeconsuming process. Before each sample, mulch and leaf litter was removed. Using a soil probe, 10 samples were collected within each bed at a sample depth, which included the top 8 inches of soil. All 10 soil samples were combined into one bucket to get a composite soil sample for each bed tested. The combined samples were allowed to air dry on butcher paper at room temperature with no artificial heat. Once dried, the lumps were crushed to the size of wheat grains, mixed well and cleaned of roots and other large pieces of organic debris. Approximately one pint of the composite sample was placed into a soil sample bag associated with each collection site and mailed to Michigan State University’s Department of Plant, Soil and Microbial Sciences, Soil and Plant Nutrient Laboratory along with required soil submission forms.

In late fall of 2021, we obtained a full report for each bed detailing macro and micronutrient composition, pH factor, soil texture, cation exchange capacity, base saturation and percent organic matter. Initial results revealed that the soil pH was too high in 22 of our 43 sample locations and poor rhododendron condition seemed most highly correlated to a pH factor above 6.1. Elemental sulfur has been added to all the sample locations in the Rhododendron Discovery Garden. We are cautiously optimistic that this first step in the process will lead to improved health within the collection as lowered pH will allow for increased nitrogen and phosphate utilization as well as micronutrient uptake.

Our Logsdon Horticulture Intern, Eliza Tobin, has been working throughout 2022 to collect additional data on soil drainage and light level gradients at the targeted sampling locations. To determine soil drainage rate, Eliza conducts a percolation test at each of sampling locations. To begin, she digs a hole 12 deep by 12 inches wide. Then she scuffs the edges of the hole, removes all the loose soil material and fills the hole with water. This is done to prepare the sample site for the soil drainage percolation test the following day. When Eliza returns, she refills the hole with water and then begins to measure drainage rate by laying a straight edge across the top of the hole and using a yardstick to measure the water level. She continues to measure the water level every hour until the hole is empty, noting the number of inches the water level drops per hour. Ideal drainage occurs at a rate of 2” per hour, with readings between 1”- 3” generally included. Poor drainage occurs at a rate of less than 1” per hour, while fast drainage results when water drains at a rate of more than 4” per hour. Not all 43 tests have been completed, but initial results have revealed poor drainage in several locations. Once testing is completed, we will be able to begin to amend the soils and improve drainage to help minimize soil- pathogens, like Phytophthora, that are favored by wet conditions.

This August, all the micro-site information will be correlated, and we will have a better understanding of how these abiotic factors impact our rhododendron collections. We will continue to share our struggles (and successes!) in rhododendron cultivation and our approach for addressing rhododendron health problems in our gardens.

Conditions vary greatly between the three garden areas and our rhododendron living collections exist in a diverse array of soil compositions and tree canopy light levels.