Advantages and Disadvantages to Asexual Propagation of Plants. Recent experiments with sheep have brought the debate over cloning animals back to the front burner. Most horticulturists are probably looking on in amusement. Plants have been cloned for centuries. If there ever was a moral debate over cloning plants, it hasn't resurfaced in my lifetime. I figure animal cloning will someday be considered in the same light. I see no reasons in Judeo-Christian heritage or modern science why cloning animals would be different from cloning plants. (Both have something to say about cloning humans, but that is a different debate.)
There are several ways to clone plants. Collectively these methods are generally called asexual propagation instead of cloning. The "a" means opposite. So asexual propagation is the opposite of sexual propagation. Most of the woody ornamentals sold by local nurseries are cloned by cuttings. Hollies and azaleas use tip cuttings. Roses use hardwood cuttings. Layering is another method of cloning. Grafting is used on many ornamental trees that gives a cloned top but a sexually reproduced root system. On dwarf apple trees there will be a cloned root system along with a different clone for the top.
The main advantage to cloning plants is quality control.
You know the quality of the plant you are going to get. For example, the Japanese maple known as "Bloodgood" is going to keep its red color in the summer. All of the different plants with this name should be clones. There probably are a few accidentally mislabeled plants called "Bloodgood" and there may have been some slight mutations even in asexual production. Other than those few we can expect all of them to keep their color. As always, the environment will play a part. A "Bloodgood" Japanese maple planted in shade will not do as well as one in partial sun.
One of the reasons for higher cost of certain plants is due to the fact they are grafted or cloned for quality. Japanese maples are grafted for leaf shape or color. Other reasons for asexual propagation include the quantity or quality of the flower, the quality or quantity of the fruit or the sex of the plant. There are times when we want the male plant. Asparagus, ginkgo and hollies grown for foliage are examples of this. Other times we want the female like on hollies grown for fruit. Most fruit trees are grafted for better fruit production. Some, like pecans, have known disease resistance along with known fruit production.
Notice that I have been careful to say known quality instead of better quality. When a tree is grafted, it doesn't mean the top is always better than the seedling rootstock. The bottom part could be better but the odds are against it. The top was chosen as the best out of 1000's or 10,000's of plants. The odds against a seedling apple being as good or better than the newer apples being selected is estimated at 10,000,000 to one. This is why I occasionally slip up and say a cloned plant has better quality.
If the graft breaks or dies on a fruit tree with a seedling rootstock I will recommend against growing the rootstock unless you don't need the space or time taken up by this gamble.
Another reason for cloning a plant is that it can be cheaper to grow a plant from a cutting than from a seed. This may be because the seed has very complex dormancy requirements. As an example, cotoneaster seed has a complicated double dormancy requiring warm stratification or a 90 minute acid bath plus cool stratification. Nobody really bothers because cotoneaster can easily be cloned. Another reason to clone is slow seedling growth. For example, camellias take several years to grow from seed.
There are disadvantages also. In addition there are some unexpected things that happen over time.
The main disadvantage to cloning is loss of diversity. Scientists agree that diversity is highly important in plants for disease resistance. The classic example of loss of diversity is the corn leaf blight in the early 70's. Most of Americas corn crop didn't make it. The leafspot on redtips was probably due to lack of diversity. I haven't seen any research that supports this but there are photinia species with resistance. Loss of diversity is a price we are paying even if we don't see insect and disease problems on the horizon.
Another implication of cloning is cross pollination. If a plant needs cross pollination, another tree of the same clone will not do the job. I thought about this one day when I saw a person buy four red delicious apple trees. Unless this person has neighbors with other apple trees they are going to be disappointed. To insure proper cross pollination, you must have different clones.
While a cutting is suppose to be an exact clone sometimes there is a subtle difference. For example, juniper cuttings from low lateral branches make a smaller plant. After five successive generations you can make a regular juniper into a slow growing dwarf. You can also make a dwarf conifer into a regular plant by always selecting the top branches. Climbing roses are selected the same way. This is the way they get the same rose cultivar as a climber and as a bush. I wonder if the same thing is happening on pear trees. I see some that want to grow straight up. They will eventually fill out. I can't explain why the dwarf or "climbing" effect takes place.
Another thing that takes place is mutations. While sexual recombination of genetic material accounts for most of the variation we see in nature, mutations account for a few. I was in one landscape in Carriage Downs where I noticed a few leaves on a sweet gum tree were mottled light green. This was on one limb out of hundreds of small limbs. The mottling was very low key and not worth propagating but this was an example of how things change in the growing tips. If a person had been cloning sweet gum from this tree they would have wound up with genetic differences. These mutations are related to the number of growing tips. A plant that has been heavily cloned has a greater change of mutations because there are a greater number of tips.
The other thing you have to keep in mind when cloning plants is the difference between adult and juvenile tissue.
Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center. Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/
Contact me at David_Goforth@ncsu.edu. Reviewed 2007.