Archive | September 2014

When is the best time to prune spireas?

Spireas are deciduous, multistemmed shrubs that have been cultivated in the South since the mid-1800s.

When you prune a spirea depends on whether the flowers occur on last year’s growth or on current season’s shoots.

Spring-flowering spireas bloom on last year’s wood and should be pruned when their flowers fade. This will encourage new growth that will mature over the summer and fall and bear next year’s flowers.

Spring-blooming spireas include baby’s breath spirea, bridalwreath spirea, double Reeves spirea, and Vanhoutte spirea.

To prune them, first remove any dead or crossing branches. Then, thin out one-fifth to one-third of the oldest branches to the ground. Shorten long, lanky branches by cutting back to a side branch or bud oriented away from the center of the shrub. Finally, tip back a few of the branches to encourage branching from below the cut to create a full-looking display.

Spireas that bloom on current season’s growth should be pruned in late winter before budbreak. They include Billiard spirea, bumald spirea, and Japanese spirea and its cultivars and hybrids.

Thin out the oldest shoots at ground level to reduce overcrowding. Cut them back by two-thirds of their height or close to the ground to encourage the production of many young shoots that will bloom later in the season. To maintain an informal structural framework, stagger your pruning cuts.

During the growing season, removing the spent flower heads will promote continuous flowering, especially in Shibori Japanese spirea and bumald spirea.


spanish moss

spanish moss spartanburg

Spanish moss hanging from a branch on the campus of Spartanburg Technical College in Spartanburg, SC (USDA cold hardiness zone 8A).


Crepe mrytle in Gainsesville, FL festooned with Spanish moss. Credit: T. Polomski.

In my travels throughout the South I’ve been fascinated by the Spanish moss that drapes the branches of many trees, notably live oaks and crepe myrtles. Spanish moss (Tillandsia usneoides) belongs to a large genus of about 550, mostly epiphytic,  species in the bromeliad family (Bromeliaceae). Spanish moss (USDA zones 8-11) is native from the southern U.S. (southeast Virginia to Florida and west to Texas and Mexico).  This “air plant” lacks roots, relying almost entirely on atmospheric moisture and rainfall for sustenance. The limbs of its host tree (or telephone wire 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 are covered with dense trichomes that act like reservoirs to capture moisture and nutrients.

Three-petaled pale blue or chartreuse flowers are borne singly in the leaf axils in late spring and early summer and are pollinated by moths drawn to the musky fragrance. 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 in a number of ways. 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 can reach 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 it’s character.



Insecticidal soap vs. dishwashing liquid

On the radio last week a caller read the label of an insecticidal soap over the phone, and wondered about the ingredient: “potassium salts of fatty acids?” The caller also wanted to know if dishwashing liquid could be used as a substitute for insecticidal soap.

I explained to the caller that these potassium salts of fatty acids are created by adding  potassium hydroxide to fatty acids obtained either from  animal fat or in plant oils. The resulting “soap salts” are most effective in controlling  soft-bodied pests such as aphids, scale and mealybug crawlers, thrips, whiteflies, and spider mites. Generally, they have little effect on beetles and other hardbodied insects (an exception being cockroaches). The soaps must come into direct contact with the pest to be effective.  The soap penetrates the outer cuticle of the insect’s body and dissolves or disrupts the cellular membranes causing dehydration and death.  Soaps can also block the spiracles or breathing pores in the insect’s body which interferes with respiration. In some cases soaps may also act as an insect growth regulator, affecting the metabolism of cells and metamorphosis.

Certain common dishwashing liquids and laundry detergents when mixed with water have also shown insecticidal and miticidal properties. When applied to an assortment of vegetable crops, Palmolive®, Dawn®, Joy®, Ivory®, and Dove®, for example, have effectively reduced populations of whitefly , aphids, and spider mites. However, dishwashing and laundry detergents are not labeled as insecticides. Although they may be insecticidal, they are chemically different from  the registered insecticidal soaps.  Furthermore they may prove phytotoxic, causing injury by dissolving the waxy cuticle of the plant’s leaf surface. I told that caller that it’s better to save these soaps or the washing of clothes and cleaning of dishes for which they were designed.