Thursday, November 28, 2013

Finding a Mate for My Plants

Berries of Winterberry Holly persist into well into winter
 when they provide late season food for birds.

Shortly after we moved into to our new place I identified a nice looking Winterberry Holly (Ilex verticillata) growing near the front door.  It was mid fall and I didn't see any berries on the shrub.  This meant that either it was a male plant or that there were no compatible males within 100 feet to provide pollen.  The bright red berries of Winterberry are its main ornamental and ecological feature. 

Female flowers have a large central ovary surrounded by undeveloped
stamen-like structures (lacking anthers).  This plant was blooming in early June.

I had to wait until the following June to get a good look at the flowers.  After careful examination it turned out that this plant was in fact a female.  The solution was to find a suitable mate, that is one with a similar bloom time.  I found a male cultivar at a nearby nursery called ‘Jim Dandy’.  I sat him next to the established female for a few days while I located a nearby spot to plant him.  

Male flowers have well developed stamen and a very small ovary-like center.
The male plant seemed to attract smaller sized insects.

By September I noticed a few of red berries on the plant.  Success!  I hope to see more berries next year as there will be a much longer time for pollination to occur. 

These red berries were formed by mid-August.  

One of my goals in designing a landscape is its habitat value.  That is, what does the landscape gives back to wildlife in the form of food and shelter.  So when I select plants I look for ones that produce flowers, fruits and/or seeds that wildlife can use.  When selecting plants from a commercial nursery many of them are cultivars, which are genetically identical.  This becomes an issue in the habitat garden if the plants are single sexed (dioecious).  It is also a problem for plants that have both male and female flowers on the same plant (monoecious) or have perfect flowers (both male and female parts in the same flower) if the plants are not self fertile. 

All this plant fertility can have a down side.  In a formal garden production of viable fruits and seeds may lead to increased weeding and spreading of plants out of their designed boundaries.  Also, some consider that the mess that falling fruits create outweighs providing food for birds and other wildlife.  Personally I think of the landscape as a dynamic thing that changes over time.  In the designed landscape plants need to be kept under some degree of control, but I really enjoy seeing native species spreading to new areas where they are happier than in my initial design.

Here are the flowers of a female persimmon.  They have large ovary structures.
Male flowers are narrow at the base.
My experience with the Winterberry Holly has heightened my awareness of dioecious plants.  I want to generate as much natural food for wildlife as possible.  Also I would like to grow some ‘wild’ fruits and berries.  Persimmons (Diospyros virginiana) are dioecious.  My established persimmon is a female, but I have not seen any fruit on it.  I have brought in four more new wild-type plants, at least one of these should be male.  I don’t know of any named male cultivars of persimmon; however there is one called ‘Meader’ that is self-fertile. 

In general female plants need at least one pollen source (male) close enough that pollen can be transferred.  This transfer can be by wind (grasses and many trees) or by insects (vectors) most flowering plants.  For native hollies, like Winterberry, a male should be located with in 50 ft for effective pollination, though pollination over greater distances (100-200 ft) may be possible. 

Female Box Elders are covered with seed pods (samara) in the fall.
I noticed some squirrels eating them right off the tree.

Some other dioecious plants that I have are Box Elders (Acer negundo), Junipers, Tupelos (Nyssa sylvatica), Yews and Goatsbeard (Aruncus diocus). 

Some plants with perfect flowers are not self-fertile.  This may not be convenient for a habitat gardener with limited space for lots of plants. However, it works out on the larger scale to provide the greater genetic diversity adaptation to change and continuation of the species.  I have posted a number of times about the difficulty of getting Viburnums to bear fruit when only a single cultivar was planted.

There are some practical situations where limiting plant fertility and reproduction is called for.  In urban landscapes Ginkgos are very successful street trees.  The fruits, however, are loaded with butyric acid and smell of dog poo when stepped on.  For this reason most cultivars in the trade are males.  In my last post about Jerusalem Artichokes I noted that these are not self-fertile.  By only using only a single cultivar one limits the risk that this somewhat aggressive native will spread by seed. 

Another issue for some in urban and suburban landscapes is pollen allergies.  In many cases these are due to the (over) use of male cultivars.  Males don’t produce messy fruits, but they still send out their pollen.  This results in a type of air pollution that effects sensitive individuals.  While searching for information about dioecious plants I came across an excellent resource book by Thomas Ogren titled Allergy-Free Gardening.  This book contains a lengthy list of dioecous plants and the sex of individual cultivars.  There is also an allergy rating for a wide variety plants including monoecious plants and those with perfect flowers.  I did not realize that many maples, such as Red and Silver Maples, came as separate male and female plants.  Since I am more interested in facilitating plant fertility for enhancing habitat value I see using this book differently than the author intended.  But, if I had a client with particular allergy problems this book would be an excellent resource for plant selection to design a low-allergen garden.

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