Desirable cultivars, scientifically speaking

Desirable cultivars, scientifically speaking

Oak leaf hydrangea in full sun Photo by Karen Lawrence

Dr. Bob Gilbert – Columnist

Bob Gilbert

As the popularity for native plants increases, I am noticing more and more cultivars and hybrids becoming available originating from our natives.   Can they still be considered natives?  The answer is yes if two natives have been used for the cross.  To have a pure native garden you have to get comfortable with just a few terms.  To understand just a few horticultural terms, let’s cross pollenate two plants that are more or less alike that are in the same genus.   Let’s call them A and B for this discussion only.  Crosses between the two can be accomplished by hand using a Q-tip or camel’s hair brush.  If all the seedlings result into identical plants, they are called hybrids that I will call C.  The cross of A and B may be repeated time and time again and always get the same C results. This is often done because C has some distinguishing feature or some improvement from A and B.  Again, C is a hybrid and it is also given a commercial name.  A popular example is the kind of cross is between a Silver Maple Acer saccharinum and a Red Maple Acer rubrum.  The hybrid produced is called Autumn Blaze which has exceptionally brilliant red fall foliage.  To produce more of this particular hybrid, you can continue to make pollination crosses and plant the resultant seeds or make cutting from Cs.

A cultivar is a plant produced by crossing two compatible plants; this time let’s call them D and E.  None of the seedlings produced will be the same. The next time you cross these same D and E you get plants with different characteristics, not the same results. There is no duplication.  This can happen in nature as well in the nursery.  These singular variations are called cultivars.  The only way to duplicate cultivars is by rooting cuttings.  You can remember all this easily as a cultivar is a cultivated variety discovered in nature or is manmade.  In catalogs and texts cultivars names are surrounded by single ‘quotes’.  

Let’s pretend that you find a plant variety that has great blooms but it is not quite hardy for where you live.  So, a manual cross is made with the pollen of the plant with the desired flowers with a different but similar species that is hardier. With luck one resultant seedling will have both characteristics.  It might take numerous tries.  A cultivar is created.  Increasing its numbers can only be done from cuttings.  This is exactly how Knockout Roses evolved.  Someone just kept crossing and recrossing until the disease resistance and other desired features were incorporated into one seedling.  This took hundreds of crosses, all done in someone’s back yard!

 A sport is a variation discovered from a growth on a branch that is different. Cuttings are made from this morphological different segment and if it grows and remains stable it becomes a cultivar and given a commercial name. Many of the numerous conifer cultivars now available were rooted from sports.

Generally, pollen from different genera are not compatible.  But occasionally accidents happen and intergeneric hybrids are produced.  Leyland Cypress is an example.  Its scientific name is preceded by an X indicating an intergeneric cross, X Cupressocyparis leyandii.

Fruit trees are easy examples.  Take a seed from your favorite grocery store grapefruit and grow it to maturity.  If the resultant tree produces fruit, and it may not, it will be not be the same as the fruit from the store.  Grocery store fruits originated from a single tree whose fruit had some desirable commercial feature.  The grocery store grapefruits are identical because the trees were created from multiple cuttings from a choice tree not from seeds.  So, the grocery store grapefruit varieties are cultivars.  

 I like the story of the origin of the Granny Smith Apple.  Granny Smith’s family had an apple orchard.  She was in the habit of peeling and coring apples and throwing the trimmings out her kitchen window.  A seed from an unidentified apple core germinated and grew to produce apples we now know as Granny Smith.  This single tree became the source for hundreds of cuttings which are now grown world-wide.  Granny Smith, as well as her apple, are both cultivars.  One difference between them is that the apple was pollinated by a bee. 

There are many examples of horticultural cultivars.  Oakleaf Hydrangea or H. quercifolia has dozens.  Querus, by the way, means oak and folia means leaves. Hydrangea is derived from a Greek word, hydro, meaning water and angeion, meaning vessel, referring to the shape of the seed capsule.  It is native in North Carolina, Tennessee, Florida and Louisiana.  To help with identification, there are only two hydrangeas with cone- shaped flower clusters, Oakleaf and Pee Gee (PG) Hydrangea or Paniculata Grandiflora Hydrangea. Oakleaf is shrubby growing to 4 to 8 feet while PG can get to 10 to 20 feet and is more tree-like.   All the other vast numbers of hydrangeas with softball shaped white, pink or blue blooms are referred as mophead or bigleaf types.  A discussion and descriptions of the scores of hydrangea cultivars are best left to a text book.  Michael Dirr has published “Hydrangeas for American Gardens” which is well worth having in your gardening library.  

As an aside, the U.S. Hydrangea Society was founded in Atlanta by Penny McHenry.  She was an obsessed hydrangea fan who wore her hair in a mophead style.  No cone-shaped clusters on the top of her head.

In Hoover, Ala., on the outskirts of Birmingham a nurseryman Eddie Aldridge discovered a native Oak Leaf cultivar that he liked and named it “Snowflake.”  He eventually turned his 30-acre nursery over to the city of Hoover and it has been developed into a public garden, The Aldridge Garden.  Its main feature is hundreds of successfully growing Snowflake Oakleaf Hydrangeas all from cuttings from a single specimen.  It is a great plant with a 12”-15” long bloom cluster that has a double flowering appearance.  They are best grown in light shade. They are hardy from zones 5-9.  Fall colors of both the flowers and foliage are striking.  It is reported that stem borers are an infrequent problem with all hydrangeas.

Do not let horticultural and botanical terms intimidate you.  They are meant to explain, identify and help you make choices. If you want to grow only natives, check a plant’s origin.  If you find a cultivar you like, where did the original parents come from?  I would bet that it would be next to impossible to find a pure native Oak Leaf Hydrangea for sale.  Further, no one really knows the pollinating parents that produced Snowflake. It was found as a naturally occurring cultivar, most likely from natives? 

Dr. Bob Gilbert, now living in Franklin is cofounder of Smith Gilbert Gardens in Kennesaw Ga.

Karen Lawrence is a professional photographer of botanical subjects and wildlife is from Franklin, N.C.