Arborists certainly do much more than cut down trees. In fact, most arborists make a career of arboriculture because they enjoy trees so much. Michael Hart, one of the most prominent arborists on Long Island in New York, has actually just returned from his annual winter pilgrimage to commune with the Big Trees in Sequoia National Park.

Nurserymen likewise do much more than merely grow trees. There are actually only a few trees at Bay Laurel Nursery in Scotts Valley among the acres of rhododendrons, azaleas and camellias. Of these, most are small trees such as dogwoods and Japanese maples. There simply is not much demand for real trees, relative to other nursery stock. This is because only one or a few trees are planted in most urban gardens during the same many years in which several smaller plants are installed.

Just as arborists are dedicated to trees, nurserymen can be overindulgent with propagation, particularly if it involves exotic species or cultivars. Fortunately, nursery production necessitates propagation of large numbers of new plants. The rhododendrons, azaleas and camellias are grown from cuttings because so many can be obtained from each of the stock plants that supply the cuttings.

Propagation by cutting is so effective among some species that it is more practical than other forms of propagation. However, not everyone is equipped with a heated and "misted" greenhouse necessary to root species like rhododendrons, hollies or junipers. In the garden, such species can be rooted more easily by layering.

Layering is the procedure by which stems can be rooted while still attached to the parent plant. Long stems near to the ground can be bent downward so that a portion can be buried while the distal portion (tip) remains above the surface. The buried portion roots as a cutting would. The exposed distal portion becomes the new plant. Rooting is promoted by cutting partly through the underside of the buried portion of stem. (A cut on the underside of the stem is held open as the tip is bent upward.)

The portion of stem that is buried should be rather substantial and sufficiently deep so that newly developing roots do not become desiccated. Some species root more reliably if a rooting hormone is applied to the open cut on the underside of the stem. Once rooted, new plants can be dug and placed elsewhere in the garden. Layering is certainly not as productive as propagation by cutting, but most of us only want a few copies of any particular plant anyway.

Tops of some tall and scrawny houseplants can be air-layered by wrapping the partially cut or gouged stem in moist sphagnum moss contained within sealed plastic wrap. This plastic wrap should be tied tightly above and below the mass of sphagnum rooting media. When newly rooted upper portions are pruned off and potted, the remaining lower portion of most houseplants that are conducive to air-layering will generate new adventitious buds lower on the stems. The lower portion can then become a shorter and densely foliated plant.

Houseplant of the Week: Dracaena marginata

Dracaena marginata is one of the classic houseplants that becomes a candidate for air-layering as it matures and slowly becomes as high as 8 feet. It is generally much smaller, with a few stems that are each topped by foliar tufts. The strap-like leaves are as long as 18 inches but are narrower than half an inch. Leaves have a dark red margin. Leaves of the cultivar "Tricolor" also have a narrow pale yellow strip.

Dracaena marginata likes bright but indirect sunlight. Direct exposure may burn foliage. Irrigation should not be so frequent that the potting soil remains continually moist. Over-irrigation is actually the most common problem associated with this species.

Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.