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Dirty Fingernails: Closet time for your favorite poinsettia
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Dirty Fingernails: Closet time for your favorite poinsettia

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Poinsettia

Poinsettias are a bright holiday tradition.

Q: Is it time to put my poinsettia in a closet if I want it to bloom for Christmas?

A: It is. But remember that it goes into the closet — or gets covered with a box or heavy bag — only at night. The poinsettia needs a sunny window every day. In order to bloom, it must have nights longer than its days. The exact length of darkness is not critical, but most gardeners aim for 15-hour nights. And the darkness must be absolute. Even a little light for a few minutes will reset the poinsettia’s timer to zero and require starting over again.

As it prepares to bloom, the poinsettia will want more than its usual ration of water, and it will appreciate some fertilizer. The first visible change in its color will be that leaves near the stem tips turn darker green. Then they acquire dark red splotches and finally become the familiar Christmas red. Last of all, small yellow flowers appear.

Poinsettias belong to one of the biggest families of plants, the Euphorbias. They are named for Euphorbus; he was a Greek physician who served as doctor to royal courts and who married Antony and Cleopatra’s daughter. That may be only a factoid, but it helps to illustrate the sometimes-odd botanical names of plants.

The relatives of poinsettias number about 7,500 species, and they include a wide variety of plants — perennials, succulents, tropicals, spiny shrubs, trees. Several kinds of Euphorbias make good house plants because this group is mainly tropical and subtropical. However, one tough Euphorbia is a troublesome part of the Montana landscape: leafy spurge. It is currently on the Top Ten list of noxious weeds in western Montana.

Euphorbias other than poinsettia which are grown in homes include the African milk tree. It is neither a tree (although its growth is columnar) nor a cactus (although it is often mistaken for one). The plants grow as a group of vertical stems which eventually may be three feet tall. Stems are three-sided, triangular in cross-section, and they have thorns which grow in pairs. Between the thorns are small leaves shaped like teardrops. Look for this plant labeled as Euphorbia trigona.

Another thorny Euphorbia is the crown of thorns, officially E. milii. This plant, which grows as a creeping shrub in its native Madagascar, makes an excellent house plant if it is located where no one will brush against it accidentally. Its thorns are long and wicked. If happy in its surroundings, the crown of thorns blooms at all seasons. Its flowers, like those of all Euphorbias, are tiny. Also like its relatives, the insignificant flowers are surrounded by large and colorful bracts. They are poinsettia red, but small and shaped like the bell of a trumpet.

The pencil cactus, E. tirucalli, is a thornless plant which grows fans of pencil-shaped stems. Small leaves appear at the stem joints, flourish briefly, and die. Varieties of pencil cactus are available with green, yellow and red stems.

Less common succulent Euphorbias are the dragon bone cactus, E. lactea, and the basketball plant, E. obesa. The word “lactea” means milky, and it describes the milky sap common to all Euphorbias. If a poinsettia leaf is knocked off, a milky drop of sap will coagulate at the point of injury.

Euphorbia house plants are easy to grow as long as they have bright light and are allowed to dry between watering. Because their root systems are shallow, it is best to repot them only when they become top heavy and keep falling over. With some of the spiny Euphorbias, heavy gloves are necessary for repotting. When growing any Euphorbia as a house plant, expect it to need regular pruning to keep an attractive shape. These plants are not suitable for a gardener who cannot stand to prune a plant. It will look less and less attractive as it ages. The colored bracts and unusual flowers will grow sparse. The basal stems will become woody and brown.

Prune any Euphorbia by cutting stems at the spot where you would like to see them branch. New growth will start where you made the cut. Do not be surprised if growth does not begin for weeks, or even months. Euphorbias do not gallop away.

Cuttings easily turn into new plants. Let the cutting sit until the milky sap dries, then push the bottom end into potting soil. Give the cutting bright light, and do not cover it with a plastic bag. Keep the potting damp but never wet. It will take longer for a cutting to start growing now than it would next spring, but it will grow.

And yes, this method of growing from a cutting does apply to your poinsettia.

Q: What do you think about using paper for mulch?

A: I tried it for the first time three years ago, and I am more and more excited by it. One layer of cardboard is thick enough to create darkness underneath. For newspaper, I make a blanket of eight sheets. Even as the paper is preventing weed germination in the darkness it creates, it is beginning to break down. If left in place over the winter, the paper has half decayed by spring. I tear it with my fingers enough to plant a row of seeds or a transplant. By the end of the second summer, the paper has turned into soil.

Paper as a soil additive is a bonus. Underneath even half-decayed sheets of newspaper the soil is moist and crumbly — exactly the texture that plants love. Next spring, I am going to lay down sheets of paper between rows of peas and pole beans, where weeds have found a sanctuary in the past.

To keep newspaper from blowing away, I scatter a handful of dirt on strategic corners. If I were using newspaper in a decorative bed, I would top it with a thin layer of soil or compost, just enough to bury the written word. I have used cardboard mulch less often, but I know of a gardener who cuts pieces to fit between his perennial flowers and is delighted by the result.

Since water easily soaks through newspaper, there is no need to set up drip systems under paper mulch. Paper has all the moisture-retaining virtues of plastic mulches, while plastic cannot decay into soil.

Q: When I pulled my bean plants, some of the roots had little white bumps attached. Is that a disease I should worry about next year?

A: Those bumps are the opposite of a disease. They are the visible sign of a valuable natural process. One kind of soil bacterium called rhizobium can set up a partnership with beans, peas, and other legumes. When rhizobium bacteria take up residence on roots, the bumps form. While living on the roots, the rhizobium convert nitrogen, which plants cannot use, to nitrogen compounds which are fertilizer for plants.

You may have read that beans and peas need little fertilizer and actually can improve the fertility of tired-out soil. That is the result of the rhizobium colonies living on roots, in their bumpy white homes.

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