WHY DO SOME PLANTS FLOWER BEFORE THEY LEAF OUT?

Thursday, March 5, 2015

All New Orleanians know that the late winter-very early spring transition is the time when a number of our favored urban horticultural species and more rural native trees flower brilliantly.  For the most part their flowering occurs before they produce leaves.

Classic examples we see in  February and March include Callery Pears (Pyrus calleryana; several cultivars are used locally), Redbud (Cercis canadensis), Drummond Red Maple (Acer rubrum var. drummondii), several species of native plums (Prunus sp.), Pecan (Carya illinoinensis), Live Oaks (Quercus virginiana), and more.

Our most obvious early flowering horticultural tree is “Japanese Magnolia,” a common name actually used for two species of Asian origin:  1) Saucer Magnolia, Magnolia x soulangiana (often misspelled as M. x soulangeana), a hybrid derived most commonly from a combination of M. liliiflora and M. denudata, and having dark pink to white flowers with shorter petals giving them a cup-shaped flower; and 2) pure Magnolia liliiflora (Lily or Tulip Magnolia), which has deeper purple and longer petals forming a more pitcher-shaped flower.  Both of these descriptions were shared by the gentleman botanist Dr. Mac Alford, of the University of Southern Mississippi.

There are other related species used in horticulture, but these are our commonly used species.

We normally see plants begin their vegetative growth each season, become well established, then put energy into their most important job - reproduction.  This makes sense, because this common approach establishes the plant, which may have died back during winter, and thus provides the nutrition basis to redirect energy to the huge task of producing seeds.

Over the winter these species reach differing levels of dormancy.  They have, however, adequate stores of carbohydrates that will be important in launching their spring growth.  Since reproduction is so important, the adaptive strategy of pre-leafing flowering is to use those energy stores to produce flowers that provide the seeds.

Additionally, the production of flowers requires sunlight, just as does the production of leaves.  Early flowering ensures that the leaves will not block the sunlight from the flower buds.  Once the flowers have done their jobs, they fall and leaves appear.

The grand strategy is that these plants put all food reserves into reproduction before all the other plants begin to grow, then spend the rest of the growing season focusing on growth and storing food for the winter.

Next time we have a hurricane (heaven forbid) or very strong fall storm, note that some trees have a survival response by either flowering out of season (Japanese Magnolias) or producing new leaf growth (Bald Cypress, Taxodium distichum).  These are simple survival responses, but important enough to the species that do this to make them risk using valued energy stores.  Why take the risk?  Possibly to ensure they produce a new seed crop or have new leaves for boosting food stores late in their growth seasons.

L.S.U. AgCenter Consumer Horticulture Specialist Dr. Dan Gill has written about the Japanese Magnolia-hurricane relationship in his column in The Times-Picayune.  He notes that the plant’s new buds have formed by fall, and a storm blows the leaves off, exposing the buds to direct sunlight.  The fall Japanese Magnolia flowers are paler than the winter/spring flowers since the heat destroys many of the purple pigments.  Sadly, new buds will not form after a fall flowering, so the intensity of the next winter/spring bloom will be diminished.

Such is Nature!

Callery Pear, Pyrus calleryana, blooming on Parish Road, Chalmette, LA, on March 12, 2011.  Photo by Bob Thomas.

Magnolia liliiflora, showing more purple and elongated petals, blooming on Broadway Street, New Orleans, February 12, 2015.  Photo by Bob Thomas. 

Redbud, Cercis canadensis, flowering in New Orleans on February 18, 2011. Photo by Bob Thomas.

Saucer Magnolia, Magnolia x soulangiana, blooming on St. Charles Avenue, New Orleans, February 2013.  Photo by Bob Thomas.

Live Oak, Quercus virginiana, Girard Playground, Metairie, La., March 17, 2011.  Note how thinly leafed this tree appears. Following the most recent leaf-fall, the obvious catkins hosting the male flowers and the less obvious female flowers appear for reproduction, followed by another leafing.  Photo by Bob Thomas.