Rhododendron maximum:  a winter thermometer

As previously seen at http://www.NEWFS.org/vermont
by Thelma Hewitt and Victoria Herzberg; NEWFS photograph
DESCRIPTION:  Rhododendron maximum is at the northern edge of its range in Vermont, but there are still several
natural occurrences here.  As the name indicates, this is our largest Rhododendron, growing up to 15’ in our area, and up
to 30’ further south.  The dark green leaves of R. maximum are up to eight inches long, and the habit of the plant is
open and loose. Some growers have found the variety “roseum” to be slightly less cold hardy than the white flowered
maximums.  The normal dates of flowering are between June 28 and July 22, though they may be as late as August 12.

One of the most noticeable features of Rhododendrons is their habit of folding their leaves in cold weather to conserve moisture.
The tightness of the curl reflects the temperature, and at the frigid temperatures typically reached during Vermont winters, leaves are maximally
folded.  The small-leafed R. carolinianum, or Carolina Rhododendron, is a particularly good thermometer because at 29oF. it can be HEARD
crackling. R. carolinianum is not native to our immediate area, however it is a parent of many of the more hardy, small-leafed Rhododendrons
that many Vermonters grow (such as Balta, Laurie and PJM), some of which also make this distinctive sound under the right conditions.

CULTIVATION:  Like all Rhododendrons, R. maximum prefers moist, acid soil and excellent drainage.  Some shade,
especially as protection from late winter sun, is important to prevent leaf burn.  Since the plants are shallow rooted, they are unable to replace
moisture lost by exposure to wind and winter sun.

When planting container grown Rhododendrons, it is extremely important that any matted, pot-bound roots be cut.  Make
vertical slits in the root ball to increase soil penetration. Bound roots and/or lack of proper drainage are major causes of failure to thrive.

PROPAGATION: Rhododendrons can be challenging to propagate.  To germinate from seed:  Gather ripe seed when they are about to fall,
and sprinkle over sphagnum moss on a base of moist sand and peat moss. Spray gently, use bottom heat and place in shade.  Keep seedlings cool.
Expect some variation in characteristics from the parent plant. To clone the parent plant, you may use cuttings. A reasonable percentage of viable
plants can be obtained using hormones, fungicide and maintaining high humidity. For the ordinary home gardener, layering is perhaps the easiest,
most reliable method of clonal propagation.

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