A plant can use two different methods to reproduce: sexual and asexual. Sexual reproduction occurs when the plant produces pollen, or male reproductive cells, that are then carried to the female reproductive organ, the stamen. Some plants self-fertilize, others cross-fertilize. Asexual reproduction can occur in a variety of different manners as well. Propagation, or development of another plant through a leaf, stem or other part of the plant, is one such method. They can also send out runners or stolons and reproduce in that manner. Some plants, such as the maternity plant, produce tiny versions of itself on its leaves. The clone then drops to the soil and roots.
No Genetic Variety
In asexual reproduction, the offspring are genetically identical to the parent. Blackberries, for example, sprout roots where a stem touches the ground. These roots support a new plant if the connecting stem is damaged or broken. The new plant is a clone of the original bush.
Plants that reproduce via underground runners or above-ground stolons produce a clone of itself at each node. An entire forest of quaking aspen trees--which use runners to asexually reproduce--can have the exact same genes. In fact, the San Francisco State University Department of Geography states that the oldest and largest aspen grove is in Southern Utah. "Pando" as it is called, covers 43 hectacres, has almost 50,000 clones and weighs over 13 million lbs.
Genetic variation is a good thing--it allows species to adapt to changing environments through naturally selecting for advantageous traits and against deleterious ones. If no genetic variability is present, such as with a parent plant and its clone, the plants will be less likely to survive environmental stressors and disease. If the climate of a quaking aspen grove, for example, were to change dramatically--like if the water source dried up--the entire grove would die. Sexual reproduction could possibly provide a mutation that reduced the tree's water requirements. This genetic variability would ensure at least a few individuals of the grove survived the change.
Asexual reproduction requires plant parts to drop off, to send out runners or stolons or to grow roots from contact with soil. Therefore, the clones are all in close proximity of the parent plant. This creates crowding that can work against the plant in an environment where the resources are limited. The parent and its clones compete for water, light, nutrients and space, which can result in a decrease in the health of all--parent and clone alike.