Despite the odds, this spring has had one of Longenecker Horticultural Gardens’ best magnolia flower displays in recent memory. The first flowers began to open April 11, and within three days over 30 of the gardens’ eighty-plus magnolia specimens were in full bloom, thanks to record-breaking 80-plus-degree-F days. On April 16, however, that abruptly changed when snow and freezing temperatures returned.
Miraculously, the cold-sensitive open flowers suffered little to no damage. This was partly due to rain and wet snow covering and freezing on flowers. “Latent heat of fusion” is a term that describes the phenomenon of heat being released into the surrounding area during a phase change – such as water freezing (turning from liquid to solid). Fruit growers have taken advantage of this phenomenon for years to help protect open flowers and fruit during cold snaps. Water is applied to trees as the temperature drops below freezing, and as the water turns to ice it releases energy (heat) that helps protect the flower. Fortunately, the freezing temperatures here were short lived and followed by cool cloudy days that, like a refrigerator, helped preserve the open flowers and slow the opening of new buds.
In our region, spring frost damage to early-emerging magnolia flowers is not a new phenomenon, though climate change has increased the odds. Most of the dazzling early flowering magnolias that grow here are of Asiatic origin and evolved in Japan, Korea, and China. They are “precocious flowering,” meaning that they bloom before leaves emerge. The plants themselves may be cold hardy in our climate, but their precocious flowers, once open, are quite sensitive to cold damage.
Plant breeders have worked for decades to select precocious magnolias that bloom later in the spring to avoid cold damage. One of the first projects of this type was done at the U.S. National Arboretum in Washington, D.C., starting in 1955. That project produced eight hybrid magnolias: ‘Ann’, ‘Betty’, ‘Jane’, ‘Judy’, ‘Pinkie’, ‘Randy’, ‘Ricki’, and ‘Susan’ – known as the “Little Girl” series. They were selected to bloom two-to-four weeks later than the common star (M. stellata) and the saucer (M. × soulangeana) magnolias. The star and saucer were the most widely available precocious magnolias at the time, and they constitute the oldest types of magnolias in the Longenecker collection. The collection also includes six of the “Little Girl” series.
More recently, breeders have been crossing precocious Asiatic magnolias with the eastern North America native cucumber tree magnolia (M. acuminata), which is noted for its extreme cold hardiness (down to -40 degrees F) and very late emerging non-precocious flowers. Most of the yellow-flowered magnolias currently on the market are the result of crosses using the cucumber tree magnolia variety subcordata.
Dennis Ledvina (1939–2015), of Green Bay, Wisconsin, was a plant breeder who greatly advanced cold-hardy magnolias for northern climates. A former high school math teacher, Dennis became enamored with magnolias in the 1970s and went on to breed and introduce over twenty outstanding selections. Dennis donated many of his selections to the Longenecker magnolia collection, including his favorite, ‘Rose Marie’, which he named for his mother.
—David Stevens, Ed Hasselkus Curator, Longenecker Horticultural Gardens