Indumentum and Tomentum

Many rhododendrons have felt-like coatings on the top or bottom of the leaves. Composed of small hairs, the coatings can be white, tan, reddish brown or dark brown colored. See photos for illustrative examples.

     R. proteoides      R. 'Sir Charles Lemon'

Indumentum (Latin, literally: "garment") is a coating of hairs on the undersides of a leaf. Tomentum is a coating of hairs on the top surface of leaves. Stems and flowers can also be hairy, and this is generally referred as “tomentose”. A plant surface with any kind of hair is said to be “pubescent”.

Indumentum forms a protective, woolly layer that sheds water and/or provides leaf protection. During cold, dry weather, the hairy indumentum that covers the leaf's underside becomes an insulating shield. Some of the plant hairs hold water and absorb it to provide the plant with moisture in times of drought. During times of heavy rain, the hairs are used by the plant to transpire excess water from the surface of the leaf. Indumentum also provides some protection from insect damage.

R. smirnowii      R. roxieanum

Indumentum types include: Hirsute (hairy, shaggy, long-haired), Pilose (long soft hairs), Villous (shaggy), Stellate (radiating in a star-shape), Scabrous (small projections rough to the touch), and Scurfy (very rough to the touch).

Cardiocrinum giganteum

This summer I've had lots of nice things in bloom, but the plant that made my summer was Cardiocrinum giganteum, as my plant finally bloomed after six years. It was lovely to see and worth the wait.

The first time I saw Cardiocrinum in bloom was in a New Zealand rhododendron garden where it was growing as a companion plant. Well, everything grows in New Zealand, so there were seedlings coming up everywhere as well as having plants in all stages of growth throughout the garden. I remember tip-toeing along a path trying to avoid stepping on plants when the garden's owner said not to worry, they were something of a weed for him. The next time I saw the plant was in its native habitat in the Himalayas. Again, just a spectacular thing and it stuck in my memory as one of those "wouldn't it be nice to try and grow". So, when I was given a one-year old seedling in the spring of 2014 I planted it immediately.

The site I chose gets morning sun, but is shady by early afternoon. Soil is on the heavy side and holds moisture fairly well, only requiring watering towards the end of summer when drought really takes hold. The first couple of years, I put some copper strips around the leaf perimeter to deter any slugs, but by year 3, the plant was strong enough to take care of itself.

Cardiocrinum giganteum

 

As you can tell from the species name, Cardiocrinum giganteum eventually gets big. The heart-shaped leaves are held on a central upright stem. The stem dies down in the fall and each spring, the new stem grows taller and holds more leaves than the year before. This year, the stem emerged, set out some leaves, as per usual, then it took off and grew vertically fast. There were times I imagined I could actually see it growing. At about 8ft, flower buds formed at the stem apex in June with flowering in July.

Flowers are white flushed with red in the center, trumpet-shaped and spectacular. My plant only had about a dozen flowers, but I've seen pictures where an individual flower stalk has up to 20 or so. Apparently, the bulb can set offsets, so I'll leave everything alone until next summer to see if I'm lucky enough to get some new plants. However, this is one of those plants that blooms once, sets seed and then the old bulb dies. So, I will try to collect some seed and once ripened, I'll try starting some new plants.

Cultivar definition

Cultivar is a hybrid word constructed from cultivate (from Latin cultus, to care for or cultivate) and variety (from Latin varietas, absence of monotony). A cultivar is a plant type that has been propagated to show specific characteristics reliably over time.

Crossbreeding or hybridizing is the most common method for creating cultivars. Hybridizing two plants results in a large variety of genetic recombinations, called a "grex" for short. More commonly, the results are called "sister seedlings".

Usually the hybridizer wants a consistent set of characteristics to be repeated over a long period of time. Seeds will not do very well, though some seeds, particularly for vegetable gardening, are called cultivars. For plants like Rhododendrons, we want to be sure each plant has the exact characteristics the hybridizer chose.

To get them, they can be propagated several ways. The easiest for the amateur is to grow cuttings. By taking shoots of a desired plant and raising them in a soil mixture until they are safely rooted, it is possible to get a fairly large number of plants that are alike.

A more difficult method that can produce huge numbers of identical offspring is meristem culture. In meristem culture, unspecialized cells from buds are shaken apart and grown separately to produce entire identical plants. This is definitely not a method for the amateur. but it used by many nurserymen to get hundreds of one specific cultivar at the same time.

Another way to get a desired cultivar is by grafting, although the method is seldom used anymore. The method gets good, healthy plants that bloom well, but there is a major drawback. Rhododendron ponticum from Central Europe previously was used as the stock plant for grafting rhododendrons. Eventually the stock plant has a tendency to produce shoots from below the graft, slowly out-competing the grafted plant. The same problem occurs with grafted roses and a number of other grafted plants.

There are some other ways to get a cultivar, including air layering or ground layering, but generally the easiest way for most of us to get a particular desired cultivar is to buy it from a nursery.

Rhododendron thomsonii portrait

Joseph Hooker found R. thomsonii in 1849 while exploring in Sikkim, India. Other plant collectors subsequently found this rhododendron species in Tibet, Burma, and Nepal.

R. thomsonii grows to be a small tree up to about 20 ft. tall in its native habitat, but it is unlikely to get more than half that size in cultivation. Typical height in ten years is about four to five feet.

R. thomsonii has attractive foliage. The leaves are two to four inches long, rounded or orbicular shaped. The new growth has a quite noticeable blue cast that becomes greener as it matures, eventually becoming dark green on the top of the leaf and lighter-colored on the underside.

On older plants, the reddish smooth bark begins to flake or peel and makes a beautiful contrast with the mixed brown, fawn, and pink new bark being exposed by the old peeling bark. This bark-flaking characteristic occurs not only on the trunk of the plant but occurs on the larger branches as well.

R. thomsonii is slow to flower. Flowers are 2-3" across, bell-shaped held in a loose truss of about 3-10 deep red flowers that have a contrasting large waxy cup-shaped calyx. This calyx can be whitish-green, yellowish-buff or greenish scarlet. The plant blooms in April.

R. thomsonii does best if planted in a location protected from the afternoon sun and late frosts.

Rhododendron 'Naselle'

Imagine corollas of rose-pink with yellow centers and orange spots formed into large trusses on a nicely rounded rhododendron. This is 'Naselle,' which is named after both a town and a river in the state of Washington. By the way, the word Naselle was derived from a Chinookan tribe named Nisal.

The cross was hybridized by Jim Elliot in 1987. The Award of Excellence was given to 'Naselle' in 1992.

Description of 'Naselle': The new leaves are pale maroon. When they mature, the leaves are medium green and elliptic. Each large truss holds up to 15 beautiful corollas of rose-pink with yellow centers and orange spots, and it has been described as one of the most striking of the 'Lem's Cameo' hybrids. This hybrid is well-branched and looks rounded in shape. It will grow into a four foot high by four feet (1.2 x 1.2 m) wide plant in ten years, and can tolerate temperatures to 0 to -5°F (-18 to -21°C). With respect to its optimal placement in a garden, it does well with filtered sunlight, but enough sunlight to produce those wonderful blooms.

Looking Forward to Spring

Let's see... it's the middle of March, 2020, and everyone is trying to cope with the coronovirus pandemic. In my area, garden club meetings, plant sales and spring flower shows have all been cancelled. And, of course, the annual American Rhododendron Society Convention, scheduled for late April, has been cancelled too. These are all things I look forward to every spring.

But, there's still lots to anticipate. I've already seen Rhododendron 'Seta', 'Cilpinense', 'Cornell Pink' and 'Christmas Cheer' (see blog; 'Early Blooming Rhododendrons') in bloom at my house, and it looks like 'Mary Fleming' will open on the next warm, sunny day. Rhododendron buds are swelling and so are the lovely companion plants like Erythroniums, Trilliums, and Bloodroot.

I am lucky enough to have a warm, protected deck and I like to bring containers up to enjoy from my kitchen window. I mix and match the display depending on the state of blooms, and it usually ends up being a riot of colour. The picture below is from my deck in mid April, 2019. I'm already looking forward to this year's display as I expect it to be every bit as bright and cheerful. So, since we're all staying home for awhile, we can putter around the garden and enjoy our own gardens' private spring shows.

 

 
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