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Extension Consumer Horticulturist, Clemson University

I have a winter home in Georgia, and I am fascinated by the Spanish moss that drapes the branches of many of the live oak trees there. Can you tell me more about it?—E.C., by e-mail

Answer: Spanish moss (Tillandsia usneoides) belongs to a large genus of about 550 mostly epiphytic species in the bromeliad family (Bromeliaceae). Spanish moss is native to the southern United States (southeast Virginia to Florida and west to Texas and Mexico; USDA Zones 8-11). Lacking roots, this “air plant” relies almost entirely on atmospheric moisture and rainfall for sustenance. The limbs of its host tree (or telephone pole or clothesline) serve only to provide support.

The long, slender, grayish green stems and leaves of Spanish moss can reach 20 feet. They are covered with dense trichomes, or hair structures, that act like reservoirs, capturing moisture and nutrients. Three-petaled pale blue or chartreuse flowers are borne singly in the leaf axils in late spring and early summer. Their musky fragrance attracts the moths that pollinate them. The small fruit capsules that result split when ripe to release seeds that are outfitted with silky hairs. Dispersal is mainly by wind. Reproduction also occurs by vegetative offsets that are broken off and transported by wind or animals.

Historically, Spanish moss has been harvested and baled for use as livestock feed, garden mulch, mortar reinforcement, packing material, and mattress stuffing. It was also used as stuffing in the seats of Henry Ford's Model T cars.

Visitors seeing live oak trees heavily draped with Spanish moss frequently ask if the plants are harming the tree. While Spanish moss is not a parasite in the way that mistletoe is, it can affect its host. First there is the shear weight of the moss, which can sometimes cause weak limbs to break. A subtler effect has been termed nutrient piracy. By intercepting wind- and rain-borne dust before it reaches the ground, Spanish moss may capture nutrients that might otherwise go to feed the host plant. Such a cost is a minor one, however, and not a reason to worry about the Spanish moss that gives the Deep South so much of its character.

I'm worried about adding diseased sycamore leaves to my compost pile. Can you give me some advice? —L.L., by e-mail

Answer: In order to kill pathogenic fungi, a compost pile has to reach a temperature of at least DOT and maintain it for three successive days. This can only be obtained in a so-called fast compost heap, where a careful blend of carbon-rich and nitrogen-rich materials are placed in a large pile that is carefully managed and routinely turned.

Most home compost piles are examples of slow composting, where whatever waste material is available gets piled in one place and is occasionally, if ever, turned. These slow compost piles rarely generate heat sufficient to kill anything.

What, then, should a home gardener do with diseased material? In extreme cases, such as fire blight on pear, burning the infected branches in a wood stove may be the simplest option. You might want to bury the anthracnose-infected leaves of the sycamore, knowing that soil-dwelling microbes will safely break down the remains over time. You might go ahead and make compost with mildewed phlox, but use it in another part of the garden than your perennial border. Or you might decide not to worry about what goes into your compost. A surer way of protecting plants from disease is to pay attention to selection and culture. Choose disease-resistant varieties and raise them well. You will find that you have few plant disposal problems to worry about.

Can you tell me the effects of having ivy grow on the side of houses? Is it a threat to wood siding or brick and mortar?—W.L., Cliffside Park, NJ

Answer: Both English ivy (Hedera helix) and Boston ivy (Parthenocissus tricuspidata) are commonly used to decorate the walls of buildings. English ivy clings by little rootlets; Boston ivy, by sets of adhesive discs. Creeping fig (Ficus pumila), climbing hydrangea (Hydrangea anomala subsp. petiolaris), and Japanese hydrangea (Schizophragma hydrangeoides) also join in the ascent.

All of these simply attach themselves to the surface of the structure. Nevertheless, these vines have repeatedly faced charges that they are responsible for loosening mortar, dislodging bricks, and defacing woodwork and causing it to decay. These charges typically arise when an institution, such as a college, announces its intent to strip all the ivy from the walls of a building.

The truth of the matter is that these plants are largely innocent. A paper published in the Journal of Arboriculture by Danish scientist Palle Kristofferson (“Climbing Plants on Walls—Advantages and Disadvantages”) reports that vines' damage to brickwork is negligible. Kristofferson also found that the degree of dampness of vine-covered walls was no greater (and in several cases, much less) than that of bare walls.

Furthermore, the paper points out, the vines afford a savings by insulating walls, keeping them cooler in summer and warmer in winter. By reducing daily temperature fluctuations—in effect, acting as a mulch—the vines may be protecting mortared walls from the thermal tensions that lead to masonry cracking.

Yes, the vines can dislodge gutters and downspouts, lift roof tiles, and leave marks on painted woodwork, but routine pruning can prevent such damage. All brick and mortar or stone and mortar walls will periodically need repointing, but this is not the fault of the vines. When the masonry needs attention, the vines can be pruned to the ground—and then allowed to climb back up to their rightful places.

Some cultivars of flowering plants, such as daylilies and bearded iris, are advertised as “repeat bloomers.” During a single year, will a reblooming plant produce more flowers than a cultivar that flowers only once a year? I have never seen one of these so-called remontant plants in full bloom. —S.S., Greenwood, SC

Answer: Provided that they are given the right cultural and environmental conditions, a reblooming, or remontant, plant will produce flowers more than once or continuously over a growing season. They will not, however, exhibit the “full bloom” typically associated with a plant that is in bloom for only a short period.

The precise number of blooms obtained from a cultivar of each will depend both on the genetics and the relative vigor of the plants in question.

In the case of a number of woody plants—including forsythia, crab apples, magnolias, and viburnums, which form buds in the present year and open them the next—you may notice a few blooms in late fall. While this phenomenon is not completely understood, it appears linked to stress from drought, high temperatures, and even sudden defoliation (such as from hurricanes). When conditions improve, some buds open without the chilling that is usually necessary to overcome dormancy.

Once opened, however, these buds wilt not rebloom next spring. In this case, the buds you have are the flowers you get, now or later.

When a Christmas tree is cut down, is it still alive? If it is, how long can it live without its roots?—T.E., by e-mail

Answer: A cut Christmas tree is very much alive, despite having been severed from its roots. Its longevity depends on how long the tissues are kept hydrated. Over time water is lost as it transpires through the needles. Eastern red cedar and Atlantic white cedar dry out rapidly, while fraser fir and noble fir lose water much more slowly.

Water can be replaced through the cut end of the trunk. To facilitate this, make a fresh cut an inch from the end of the trunk before you put your tree in a water-filled tree stand. Experiments with additives, such as floral preservative, have shown they provide no benefit over pure water. The tree stand must never be allowed to go dry.

How long a cut tree survives depends on the freshness of the tree when purchased and how long it is kept moist. One family (the editor's) left a recently cut balsam fir long after the holidays in a cool room with ample water, and it actually put out several inches of fresh needles in early March. H