Dealing with Cultivars and Clones, something to consider for Wildlife Gardeners: A Garden GOOPS

It’s not exactly the first of the month, but it is my first posting written in June, so Joene asked me to share some of my garden GOOPS (see Joene's Garden Oops). Just in the past week the following situation came to light and I would say it qualifies as a GOOP.

A few years ago I bought 2 Viburnum nudum ‘Winterthur’ for the attractive summer and fall foliage and the promise of multicolored berries turning into treats for the birds. So far I have not seen more than 1 or 2 sad little berries. What I learned recently (pointed out by Ellen at Turkey Hill Brook Farm) was that many Viburnums, including V. nudum are self-sterile. This means to get viable fruit it needs a pollen source that is genetically dissimilar but from the same (or compatible) species. So in my case a different cultivar of V. nudum, or the straight species will be in order. As luck would have it, I found a species form just the other day, so I snapped it up. Now I just have to find a nearby place for it.

This situation is better known when dealing with blueberries, where at least 2 genetically distinct cultivars, with similar bloom times, are planted in proximity in order to get a good yield of fruit. Since many blueberries are labeled as early-, mid- or late-season blooming selections, it is easier to match up the right plants.

For the garden planner, a similar situation occurs with plants that are dioecious, that is, having male and female flowers on separate plants. This is well known with hollies. Since the red berries on the female plants of many hollies are an important attribute, male counterparts for each of the female cultivars have been identified with similar bloom times.

Most cultivars are reproduced by some asexual means, be it cuttings, grafts, divisions or tissue culture. This ensures that every plant you get performs the same, since they are all genetically equivalent. What you get with a such a cultivar is consistency and reliability of performance. This includes size and shape, bloom time and color, as well as disease resistance. For edible plants, like apples and blueberries, you could include properties of the produce, like size and taste. From a design point of view cultivars can make a more effective composition where consistent size and color are important factors. Also the reduced fertility of some cultivars means you don’t have to deal with ‘nuisance’ seedlings and squishy fruits. 

Even plants that are not labeled as cultivars may have been reproduced asexually.  It is much faster to grow a mature plant by cuttings and divisions, than it is from seed, but there are consequences.  What you lose with these genetically identical plants is diversity. Over a series of generations, if the plants can reproduce, you decrease the gene pool and the ability of the population to adapt to change.

In working with plants that are introductions, not native to an area, this reduced fertility can be seen as a way of reducing the invasive potential of this new plant in the environment. For example, for many years the Bradford pear was a sterile clone. Even overused as it was, it did not spread by natural means. Recent cultivars, with improved branch strength over earlier forms, have inadvertently developed increased fertility which may set the stage for an invasion.

For the gardener who wants to maximize the diversity and wildlife value of their gardens by producing fruits and viable seeds, knowing which plants are dioicous (separate male and female plants) and which are self-sterile will be an important guide in the selection of plant materials. For those lucky enough to be living near wild populations, or having neighbors with similar, but genetically distinct, plantings there may be diversity enough. However for the isolated wildlife gardener, surrounded by a suburban monoculture of lawns and Bradford pears, you may be on your own in creating your own ark of diversity. I am always one the lookout for seed-grown plants and open-pollinated seeds as a means of broadening your local gene pool.

If you like like to read more on Viburnum pollination, check out the discussion at Dave's Garden.