May 2019 Plant Profile: Rhododendron edgeworthii
I was walking around the grounds at the Center for Urban Horticulture last week looking for a plant to feature in the May edition of our Plant Profiles. While walking through the Fragrance Garden a really cool-looking rhododendron caught my eye, just about to bloom. Little did I know just how cool this rhododendron was until I started researching it! Rhododendron edgeworthii is a species rhododendron and belongs to the lepidote (scaly leaved) group. In fact it was the glossy dark green leaves that are heavily puckered (bullate) along with the dark pink buds that drew my eye to it.
Rhododendron edgeworthii is a showstopper year round. The blooms are borne on trusses of three to four. The flowers are large, funnel shaped, and delightfully fragrant. Flowers range from white to white tinged red to white tinged pink. The mature bark is smooth, glossy, and peeling. The underside of the leaves and young stems have a tawny colored woolly indumentum making them feel very soft. For me the indumentum and dark green puckered leaves are what set this apart from many rhododendrons.
This species was discovered by J.D. Hooker in the Himalayas in 1849. It was named for M.P. Edgeworth who was a commissioner with the Bengal Civil Service. It is native to NE India, Tibet, Bhutan, Myanmar, and SW China. It grows in a wide range of habitats including cliffs, rocky fields, deciduous and mixed forests and is typically found at elevations of 6000 to 13,000 feet. Rhododendron edgeworthii is also an epiphyte, which means that it is a plant that grows on another plant without taking any nutrients from that plant. The thing that blows my mind is that these rhodies are often found growing at the tops of tall trees or in snags! I have personally never seen anything like that.
Rhododendron edgeworthii is a good candidate for the Pacific Northwest garden. It prefers a partially shaded site as it can get leggy in full shade and burn in full sun. In its native habitat it can grow to 12 feet but in cultivation it is typically three to four feet tall. It needs very well-draining soil and because of its epiphytic nature it is quite at home growing on a rotting log. It is hardy to zone 8b (around 10 degrees Fahrenheit). It also works well as a container plant. Orchid growing medium would be the best potting medium in this situation.
Come by in the early weeks of May to see this lovely gem in bloom! Three plants are located at the Center for Urban Horticulture in the Fragrance Garden, next to the little stone fountain.
Family: Ericaceae
Genus: Rhododendron
Species: edgeworthii
Origin: Himalayas, NE India, Bhutan, Myanmar, SW China
Height and spread: 3-4 feet
Bloom time: Mid spring
Location: Seattle Garden Club Fragrance Garden at the Center for Urban Horticulture
Hi, Margeaux. Annie may wish to add to this information, but there is an article from the Journal of the American Rhododendron Society with a general description of those hairs (trichomes) and their function: “The term trichome is probably preferable to the word hair in plants because plant hairs do not arise from multi-celled subsurface follicles as they do in mammals but are modified surface cells. Scales don’t resemble other plant hairs, but that is what they are. In some plants other than rhododendrons, olives for example, they are usually called peltate (Latin, peltatus , a shield) hairs. Trichomes come in various shapes and have a number of distinct functions. Many aromatic leaves such as rosemary, sage and geranium carry their scent inside swollen hairs that burst open to release the smell. Nettles have hairs containing an irritant. Some succulents such as ice plant have expandable surface vesicles (Latin, vesicula , a small bladder) for storage of water in dry conditions. The tiniest structures that absorb water and nutrients for higher plants, the root hairs, are trichomes. The trichomes that we rhododendron enthusiasts are most familiar with are those we term indumentum (Latin: hair covering). Plants living at high altitude and in deserts often have silvery hairs on their surfaces for sun protection and water retention. Indumentum is a general botanical term for hairs on leaves and stems, along with numerous other terms (a page and a half in Stern’s Botanical Latin ). Applied to the genus Rhododendron many of us tend use the term restrictively to mean coverings of soft hairs of various kinds on the undersides of leaves. They might repel insects also. Whether indumentum on rhododendron leaves does any of these things is an interesting question.” [source: https://scholar.lib.vt.edu/ejournals/JARS/v62n3/v62n3-palmer.htm ]
Hi Annie, I was wondering if the function of the wooly indumentum is understood? Thanks!