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Q&A with Bob Polomski 28

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Extension Consumer Horticulturist, Clemson University

I like the look of bamboo, but I'm frightened about it spreading far and wide. Can you recommend any clumping bamboos that are hardy in Michigan? –L.Z., Sturgis, MI

Answer: Running bamboos can be thugs. These species have leptomorphic rhizomes—underground stems that reach great lengths and branch horizontally to colonize large areas. Buds along the length of the rhizome intermittently produce culms (stems). Many temperate-zone bamboos have this leptomorphic root system, making them highly invasive.

Clump-forming bamboos, in contrast, are often species of tropical or subtropical origin. They have short, thick, pachymorphic rhizomes. Buds on the rhizome grow upward and produce culms. Since each rhizome terminates in a culm, the clump expands fairly evenly. Even clump-forming bamboos can outgrow their allotted space, but when they do so, they are easily held in check by removing the new culms as they appear at the periphery of the clump.

Some clump-forming bamboos have proven hardy in temperate climates. The hardiest are species of Fargesia, which have been reported to survive temperatures as low as -20° F without leaf damage. These include the umbrella bamboo (F. murielae) and the blue fountain bamboo (F. nitida). There are several cultivars of the latter, including ‘McClure’, named after the great bamboo researcher. The dragon-head bamboo (F. dracocephala) is hardy to -10°F, while both F. robusta and F. utilis are hardy to 0°F. Native to the slopes of the Himalayan mountains, these Fargesia species like a cool, moist climate, but given the right conditions, they will form bushy clumps 10 to 20 feet tall.

I used to have a border of red phlox; now the flowers are shades of purple. Can you tell me what's happening? Is there anything I can do to the soil to prevent them from turning color? –R.H., Levittown, NY

Answer: Your garden phlox flowers are not changing color. The magenta-flowered phlox you have now are the offspring of your red-flowered plants. Phlox paniculata is a clump-forming herbaceous perennial that is native to stream banks, roadsides, low meadows, and moist woods from New York to Georgia, and west from Arkansas to Illinois. Wild plants typically have magenta or white flowers. Currently, there are over 100 cultivars and strains of garden phlox. Most of these selections will not come true from seed. For this reason, they must be vegetatively propagated by root or stem cuttings, by division, or by tissue culture. The novel color of cultivars typically results from recessive genes. Cross-pollination with a wild phlox typically introduces dominant genes that block the recessive genes, resulting in the “wild purple” hues that you have observed.

To prevent your red phlox from being overrun by offspring, remove the spent flower clusters so they don't self-sow. This will also encourage lateral shoot growth and the production of more flowers, provided you keep the plant well watered and fertilized.

On the other hand, self-seeding can lead to unexpected treasures. For example, the white-flowered phlox ‘David’, the Perennial Plant Association's Plant of the Year for 2002, was selected from a population of native, open-pollinated phlox from beds in the parking lot of the Brandywine Conservancy in Pennsylvania. This fragrant, long-blooming garden phlox is highly resistant to powdery mildew.

I'm in love with roses, but not with their thorns. I have seen thorn-free roses, but I don't know their names. Can you suggest some good varieties? –L.W., Folsom, OK

Answer: Fortunately, there are a number of roses that have smooth or nearly thornless stems, which can be planted near walkways or front porches without posing a hazard to those passing by.

Several old garden roses with the thornless trait are available, including the pink-flowered hybrid perpetual ‘Paul Neyron’ (1869) and ‘Reine des Violettes’ (1860), a recurrent bloomer bearing violet flowers. ‘Marie Pavie’ is a nearly thornless polyantha with cream flowers (1888). ‘Zephirine Drouhin’ (1868) is a thornless Bourbon rose with fragrant cerise-pink flowers.

The shrub rose ‘Nevada’ (1927) has creamy white flowers, and its sport ‘Marguerite Hilling’ (1959) has pink flowers. ‘Mortimer Sackler’ (2002), a David Austin shrub rose with rose-pink flowers, is almost thornless and can also be grown as a climber.

Modern fragrant hybrid teas that lack thorns include those in the Smooth series, developed in the 1980s: ‘Smooth Angel’ (cream-colored with apricot centers), ‘Smooth Lady’ (pink), ‘Smooth Prince’ (cerise red), ‘Smooth Satin’ (peach pink), and ‘Smooth Velvet’ (dark red).

My neighbor cut down a big tree-of-heaven that was growing close to the corner of my property. Now my yard is overrun with suckers sprouting from the tree's roots. How do I get rid of this tree? –I.B., Bethesda, MD

Answer: Called the “Brooklyn palm” by Betty Smith in A Tree Grows in Brooklyn, the tree-of-heaven, or Chinese sumac (Ailanthus altissima), is a rapid-growing, weak-wooded, deciduous tree that can add three to five feet per year and reach a height of 80 to 100 feet. This Asian native was introduced to the United States as food for a moth that produces a crude silk, but it is now naturalized across most of the country, especially in urban areas where it sprouts from city sidewalks and vacant lots. Each of its flower clusters bears hundreds of seeds; a single tree is capable of bearing up to 325,000 seeds. The tree can also send up suckers from the roots that extend up to 50 feet from the nearest stem. Unlike seedlings with their thin, branching roots, root suckers are connected to a thick, lateral root.

Repeatedly cutting all the stems to the ground will eventually starve the plant, but the cutting will encourage the production of sprouts from the stump and more root suckers, all of which must be dealt with. A more effective method is to apply a systemic herbicide, such as triclopyr (Brush-B-Gon) or glyphosate (Roundup). Apply a 2% solution of either product to the foliage from mid-June to September. Alternatively, you can apply a 50% solution of either herbicide to the surface of a stump within 15 minutes of cutting. Make the cut close to the ground and apply herbicide to the bark below the cut, as well as the cut itself.

Why doesn't my pomegranate set any fruit? –T.N., Longpoint, TX

Answer: Cultivated since ancient times for its glossy leaves, flowers, and fruit, pomegranate (Punica granatum) is a deciduous to semievergreen, twiggy, multistemmed shrubby tree or large shrub that can reach a height and width of 12 to 15 feet or more.

The one-and-a-half to three-inch long flowers are produced at the tips of the branches over several weeks in late spring. Fruiting cultivars bear round, three- to five-inch-wide, reddish green to violet fruits with a leathery rind. The fleshy seeds are enveloped in a juicy pulp, which may be sweet or tart, depending on the cultivar.

Pomegranates tolerate acid or alkaline soils and can be sited in full sun to part shade. However, the best growth occurs in regions with hot summers and cool winters. Pomegranates can be grown as far north as Washington County, Utah, and Washington, D.C., but cannot be expected to bear fruit there. Dormant pomegranates can tolerate temperatures down to 12°F; however, severe damage can occur from late-spring frosts when new growth emerges in the spring.

There are basically two kinds of pomegranates: cultivars that produce sweet or tart fruits and those that do not bear fruits at all. Some pomegranates have been selected for their showy flowers. However, some, like those with double flowers, fail to set fruit because the reproductive parts of the flower are malformed. Such ornamental pomegranates include ‘Alba’ (double white), ‘Mme. Legrelle’ (coral-red with white variegation), ‘Nochi Shibari’ (dark red), and ‘Tayosho’ (apricot).

One ornamental variety, the dwarf ‘Nana’ (one to three feet tall) produces single orange-red flowers and small, reddish purple, golf-ball-size fruits (though these are inedible).

Cultivars grown for their fruit include ‘Wonderful’, in cultivation since 1896, which bears orangish red flowers and greenish red to reddish orange fruit. ‘Grenada’, is a bud sport of ‘Wonderful’ with sweeter, darker fruit. ‘Ambrosia’ has a gold rind with a red blush, while the rind of ‘Sweet’ remains slightly greenish with a red blush when ripe.

From cuttings, a pomegranate will take two to three years to bear fruit, though it takes about seven years to reach its full bearing potential. Fruit drop commonly occurs during the first three to five years. Overwatering and encouraging excessive vegetative growth by overfertilizing or irrigating too much will also result in fruit drop.

Fruit is borne on short spurs on two- to three-year-old wood. Prune your pomegranate each year to encourage the production of new growth spurs. Remove any long, unbranched sprouts or suckers to channel reserves into bearing wood. H