Plant fertility as a factor in landscape design

When designing a landscape a lot of effort is put into plant selection.  Getting the right forms, colors and shapes, proper placement and repetition to establish rhythm and flow throughout the design are critical to its success.  Maintenance issues are often a secondary consideration. If the goal is to also support wildlife, use of native species and fruit and seed production is also of high importance.  An aspect of landscape plant selection that has a lasting effect on both maintenance and performance of the design is plant fertility.  That is the ability of the plants to reproduce.

Flowering plants have a variety of strategies for sexual reproduction.  Some species have perfect flowers, that is there are both male and female parts within the same flower.  Others have separate male and female flowers on the same plant, but in different locations.  These are referred to as monecious plants.  If the male and female flowers are borne on separate plants then they are referred to as being dioecious.  There are also shades of gray between these.  You can read more about the different flower types in this article from the Master Gardeners of Northern Virginia.

There is a strong bias for plants to encourage cross fertilization.  It improves the adaptability of species to evolve and cope with change.  Plants, even those with perfect flowers, have a variety of means to encourage cross fertilization:  In some, there are genetic mechanisms where pollen is unable to grow through the pistil and reach the ovary if they has the same genetic makeup.  In some there are physical barriers that keep pollen within a flower from reaching the pistil and there are timing mechanisms the pistils of a given plant are not receptive to pollen at the time that that plant is releasing its pollen.  In many species of oak, female flowers are concentrated near the top of the tree, while the males are lower down.  As the pollen is released it is carried by the wind where, hopefully, an updraft will carry it to the top of neighboring trees.

Even when self pollination is a viable option, seed production is generally higher when cross-pollination has occurred.  Pawpaw (Asimina triloba) and maypops (Passiflora incarnata) are two examples of North American fruits that benefit from this.

When considering plant fertility are two extreme ends of the spectrum.  For formal gardens and situations where consistency of appearance are of primary concern, minimizing plant fertility to minimize unplanned reproduction  is the goal.  If the plants are not producing fruits or viable seed then there won't be as many randomly placed plants popping up that need to be weeded out and there will be fewer fruits falling from the trees that require clean up.  

This semi-formal designed planting has a clearly defined placement of plants.

Conversely in landscapes intended to benefit wildlife with a less rigid, more naturalistic appearance, maximizing the ability of plants to produce fruits and seeds and to reproduce improves its function.  The composition of such a garden will change over time as more successful plants displace the weaker ones.

This parking lot island utilizes primarily native species adapted to dry conditions and
tolerant of summer heat.  Aside from the juniper and the chokeberry cultivars in the
back, all the perennials are wild type.  The installation here is 2 years old so
reseeding has not affected the appearance.

I live in a very naturalistic environment so for nearly every native species I plant I will make an effort to ensure that fertile seeds are produced.  The main reason for this is to support wildlife with natural sources of food.  However, there are a few cases where I will limit fertility such as using a non-native species or a North American species that is not a regional native or on those occasions where the properties of the cultivar are critical to the function of the design, particularly where size is concerned.  I these instances I would choose to use only a single cultivar (or gender).  

To the left, a freely reproducing/reseeding native garden bed.

In the photo above, the bed to the left consists of primarily native species and has been allowed to reseed naturally.  Over the course of 12 years, the dominant perennials have waxed and waned.  At first it was dominated by asters, mostly by Short's aster (Symphyotrichum shortii), then Phlox paniculata, then pale-leaved sunflower (Helianthus strumosus), and now there appears to be a return to aster dominance.  The 'October Skies' aromatic aster is the only one of that species in the bed and has remained in place for about 10 years with only two extra seedlings produced.

How to manage fertility...

Reproductive fertility in the landscape can be minimized by using single cultivars of a given species, using only one gender of dioecious species (often just males), choosing sterile cultivars or flowering plants with reduced numbers of fertile parts (double flowers), and having only a single individual of a species.

If you want to maximize fruit and seed production you should plant genetically diverse individuals (use seed grown plants, multiple wild type or a mixture of cultivars within a species), have both male and female partners of dioecious species, avoid cultivars with sterile flowers, plant mixed individuals of a species in proximity to increase cross pollination.

Back in the 1930's E. M. East of Harvard Univ. was compiling a list of plant genera, organized by family, and their propensity to self-pollinate (self compatibility).  Though not completed by the time of his death, this listing does give some clues about the ability of plants within a genus to self-pollinate.  Some notable genera that have multiple instances of self-incompatibility are: Trifoliums, perennial species of Festuca, and North American of Prunus (plums).  Species of Hamamelis, Fothergilla, Berberis and Spiraea are largely self-fertile.   

Examples of self-incompatible plants include many Asteraceae species.  Also apples, some cherries, and many grasses, e.g. Miscanthus, do not self-pollinate effectively.  Even though many of these have perfect flowers, that is their flowers have both male and female parts, there are genetic mechanisms (both mechanical and biological) in place that prevent or discourage self-pollination.  

Examples...

Examples of dioecious plants that you may want to use in your landscape are trees  fringetree,  persimmons, sassafras, tupelo, and willows, and shrubs such as bayberry, hollies, junipers, spicebush, and sumacs. In many cases male flowering plants produce showier flowers, with their larger pollen laden stamens.  Examples of these are spicebush, willows and fringetree.

The female fringetree, has a moderate f lowering display and will bear black berries
in early fall.  Male plants tend to be showier with somewhat larger and denser flowers.

Here are close ups of female and male spicebush flowers.  Female flowers have
a single style attached to the central ovary while the male flowers have up to
9 anthers loaded with pollen packed into each flower. 

The white catkins on these male pussywillows, Salix discolor, compete well with
forsythia in early spring.  Also, unlike forsythia flowers, the willow flowers are very
popular with the native bees.

Unfortunately, other than some ash trees and hollies, finding gender information from the retail nursery trade is pretty difficult.  This is understandable for seed grown wild type plants since the genders are not easily determined until the plant has matured, many years after the plants have sprouted.  For cultivars, plants reproduced asexually, the gender of the parent should be determinable and that information could easily be passed along.  Also be aware that any cultivars are generically identical to each other.  Planting 5 pots of 'October Skies' aromatic aster is the same as planting one as far as their genes are concerned.

The female flowers of this winterberry holly are dominated
 by the central ovary  surrounded by non-functional stamen. 

Male flowers have 4-6 stamen tipped with yellow pollen. 
Male flowers are usually in tight clusters.

One notable species of the Asteraceae family is Jerusalem artichoke (Helianthus tuberosus) also known as sunchokes, , native to the Northeast and Midwest.  In the wild these plants are aggressive spreaders, however single cultivars, readily available from mail order nurseries do not readily self pollinate, so spreading by seed is not a concern.  

Viburnums are another example. To get berries you need at least two individuals of the same species.  If you are shopping at retail nurseries that would mean buying two different cultivars.  If you have access to seed grown wild type plants then any two will do you. Or you could do a wild type and any cultivar, if you wish. For more on this check out my viburnum blog post from a few years back.

One situation that gets repeated time and again is the use of Ginkgo trees, Ginkgo biloba, as street trees.  These ancient trees, native to Asia, are tolerant of many stressful conditions making them a good choice as an urban street tree.  The caveat is that the fruit of the female trees has a large amount of butyric acid in them which smells of rancid butter or, to me, dog poop.  The fruit is a little less than an inch long and have a rather slimy coating.  Altogether stepping on one, on a beautiful fall day, can ruin an otherwise delightful experience.  For this reason, many towns that have planted a mixture of ginkgo have opted to remove their female ginkgo trees.  You can read more in this story from CBS News.  

'Gro-low' Aromatic sumac is a horizontally spreading shrub.  This female cultivar
 is wind pollinated in spring before the leaves come out.

Another example of the importance of knowing the genders of the plants used in your landscape is the Gro-Low cultivar of aromatic sumac, Rhus aromatica 'Gro-Low'.  Gro-low aromatic sumacs are all females.  The design intent with this cultivar is as a ground cover/soil stabilizer that maintains a height of 3' or less.  Planting males nearby could result in seedlings that do not have the same growth characteristics of the mother plant.  The result would be an irregular planting with a scattering of taller (6 or more feet) plants.  (I don't know of a low growing male cultivar.  But even then there is no assurance that the offspring will share that shorter stature.)  The flip side is that without a male present for pollination no berries will be produced to help feed the birds.

A case of what was thought of as a safe introduction of a self-sterile exotic is the Bradford pear, Pyrus calleryana 'Bradford'.  Initially this single cultivar was not able to fertilize itself and it largely stayed put.   After a few years, the poor characteristics, particularly the weak branches, became evident and new 'improved' cultivars of Pyrus calleryana were introduced.  These new introductions were able to cross pollinate with the large existing population Bradfords leading to an explosion of in viable seedlings and a serious invasive species.  A similar situation is brewing with Chinese silver grass, Miscanthus sinensis.

If allergies are an issue, use more female plants to reduce the amount of pollen produced for wind-pollinated species, particularly Junipers, and ash, maple and Oak trees.  Also reducing the amount of lawn grass, as well as other grasses, which are 100% wind pollinated, will reduce the amount of pollen in the air.  For more on this see The Allergy-Fighting Garden, by Thomas Leo Ogren. 

To read more about managing fertility in your landscape check out chapter 3 of  Designing Gardens with Flora of the American East by Carolyn Summers.