L. Clarence Towe
Walhalla, South Carolina
For various reasons, most gardeners will eventually have a shrub they would like to propagate. There are numerous ways this can be done, the two most common methods being by rooting cuttings under glass or mist and by grafting. The problem with both of these methods is the equipment needed to do them properly. The following techniques of low-volume propagation are not new, but are modifications of proven methods that may be of assistance to those who lack the time to experiment with more traditional methods. Their primary advantage is that they can produce rooted plants in a relatively short period of time.
Rocks and Rings
Years ago a method called "stooling" was successfully used to propagate Exbury azaleas. This is a simple procedure that involves building a wooden fence (stool) around the base of a plant, which is then filled with a suitable rooting medium. If kept moist the lower limbs that are covered by the medium will eventually root. How quickly depends on the rootability of the plant and the moisture content and fertility of the medium.
Deciduous azaleas are especially good candidates for this method of propagation, which can be used on plants growing in the garden or in the woods. Instead of taking the time to build a wooden fence, a heavy-duty PVC nursery container makes a quick fix. Figure 1 shows a well-established division from a yellow Rhododendron arborescens hybrid found in the woods several years ago. As can be seen in the photo, the area around the base of the plant has been cleared of debris and leveled. With offset metal shears, a 9-inch (22.5 cm) wide ring was cut from the top of a heavy-duty 7-gallon (32 liter) nursery container that measures 15 inches (37.5 cm) in diameter. How wide the ring should be depends on how close to the ground the limbs are. Since the ring was too small to fit over the plant, it was cut again so it could be opened and placed around the base of the plant like a collar.
Figure 1. Ringed yellow R. arborescens hybrid.
Photo by L. Clarence Towe
The cut was overlapped 1 inch (2.5 cm) and rejoined with three small screws and the ring was then secured to the ground by driving large 12-inch (30 cm) nails through the handles. The ring is now ready to be filled with ground pine bark. The medium should be kept moist with a high-phosphate 10-60-10 fertilizer mixed at a rate of one teaspoon per gallon (4.5 liters) of water. Ideally, watering with fertilizer should occur once a week. In addition to promoting rapid root formation inside the ring, the plant also benefits from the excess water and fertilizer that drains through the mix.
If watered regularly limbs up to ¼ inch (0.5 cm) in diameter can root in a few months. If layered in April the ring should be removed and limbs checked for roots in September. If no roots are present, the ring should be replaced and checked again the following April. When sufficiently rooted, a limb should be cut off with shears just below the rootball and potted until ready to be planted in a permanent location. If all the limbs are rooted they can be cut off and the ring and mix cleared away from the stumps. Within two weeks or so the stumps should begin to sprout to form a new plant. If all goes well, the plant will regrow and can be re-ringed at a later date.
Where it is not convenient to ring a plant with a container, such as a selection found in the woods, the area around the plant should be cleared to bare soil as for the method above. The plant should then be ringed to the appropriate height with large rocks or decaying limbs and the inside area filled with nearby soil and leaf mold. Granular fertilizer should then be liberally added to the top of the soil and the entire area covered with leaves. If water is located nearby, drench the soil thoroughly. Return later and remove any limbs that are rooted. Depending on rainfall and other factors, this method may take two years.
When rooting all the limbs on a plant is not desirable, rooting a single limb is fairly simple. In Figure 2, a small heavy-duty PVC container has been cut from the top to and through one of the bottom drain holes. A limb is then bent down and adjusted so that it enters the cut drain hole and exits the container at the top. The container can then be moved along the limb to the proper location. Overlap the cut and attach with two screws to a wooden stake driven into the ground at least 6 inches (15 cm). The container should then be filled with an appropriate medium and watered weekly with liquid fertilizer as above. A rock or half brick placed on top of the pot may help to hold the limb in place.
Figure 2. Pot layers on an R. viscosum hybrid.
Photo by L. Clarence Towe
If the limb to be rooted is too brittle to bend down to the container, as with large-leaf rhododendrons, it may be necessary to resort to high-rise pot-layering. To do this a wooden stake is driven into the ground near the limb to be rooted. The container is cut from top to bottom and is stretched open so the limb enters the drain hole and exits the top as above. After adjusting for height, the container should be attached to the stake with two decking screws at least 1.5 inches (4 cm) in length. Since a container filled with wet bark is heavy, the stake should be at least 1 inch (2.5 cm) thick by 2 inches (5 cm) wide and be driven into the ground 8 inches (20 cm) or more. The screws should also be backed by two large-diameter washers with small holes. Drilling a hole in a penny is usually quicker than finding a washer of the right size.
Follow the Leader
Roots on deciduous azaleas found in the woods are very efficient but do not grow in a pattern conducive to digging the entire plant. When a seedling germinates in dry soil, a taproot forms to give it stability, but it immediately begins to develop lateral roots to obtain moisture. As these laterals grow outward from the plant, they follow their noses toward moisture, diving deeper during dry weather and coming near the surface during wet weather. As they encounter obstacles, such as roots and rocks, they go over, around and under them with little effort. As can be imagined, this makes for an irregular, flat root system spread over a wide area.
If the base of an azalea is cleared of debris, the large laterals can be seen at or just below the surface of the ground. With a little archeological finesse, a lateral can be followed out from its attachment to the plant to the point that it branches off into ¼-inch (0.5 cm) roots. At this point these pencil-size roots, along with the smaller feeder roots, can be cut from the plant with shears and dug with a sharp spade, keeping a large rootball attached if at all possible. This rootball should then be put in an oversized container in a vertical position with the cut ends of the roots pointing upward. The space around the rootball should be filled with ground bark and watered and fertilized weekly. If the soil falls off the roots during digging, wrap them in a wet towel until they can be potted. Within three weeks or so the cut end of a root should begin to produce tiny red bumps which will grow into small limbs. This method of root-pruning produces plants more slowly than the other methods but is useful in certain situations.
Clarence Towe is a member of the Azalea Chapter.