Q&A with Bob Polomski 20

Extension Consumer Horticulturist, Clemson University

What is your opinion of zoysia as a turfgrass?

—S.R., Cherry Hill, NJ

Answer: Zoysia was introduced to North America from Japan in the 1890s. The cultivar ‘Meyer’, which is sold as ‘Amazoy’ in many newspaper and magazine advertisements, was developed and released by the USDA and the U.S. Golf Association in 1951. ‘Emerald’ was released in 1955 and is finer-textured than ‘Meyer’.

Zoysia is a low, slow-growing, warm-season grass whose stems creep above- and belowground to form a dense lawn that resists weed invasion. It is one of the most cold tolerant of the warm-season grasses, performing well in full sun or partial shade. Its slow growth rate means zoysia requires less mowing. It also tolerates low fertility and requires less water than most cool-season grasses.

However, zoysia has several drawbacks. First, zoysia is the first grass to go dormant in the fall, usually around mid-October, and the last one to come out of dormancy in mid-May. And when zoysia is dormant, it's not green, but straw brown. Second, the better varieties of zoysia can only be established vegetatively. Although seeded cultivars are available, they are coarse-textured. The better ones must be established by sod, plugs, or sprigs. When planting plugs or sprigs, more than one growing season will be needed for complete coverage. Third, the tough, wiry leaves of a zoysia lawn can be difficult to mow at the recommended height of 1/2 to 1 1/4 inches. Regular sharpening of the mower blade during the season will be necessary to maintain an attractive cut. Reel mowers do a better job than rotary ones. Finally, zoysia lawns are prone to developing thatch, which must be removed. Because the plants are slow growing, the turf will recover slowly from this treatment.

All in all, zoysia is a grass best adapted to an area that covers the middle of the eastern half of North America, eastward from mid-Kansas toward Washington, D.C. This is a difficult region in which to grow turfgrasses, because cool-season grasses are forced into summer dormancy by high temperatures and seasonal droughts, while the warm-season grasses sometimes suffer winterkill.

If you decide to try a lawn of zoysia, two-inch plugs should be planted on 6- to 12-inch centers in May or June. Thirty-six plugs can be cut from a square foot of sod. Water often enough so the plugs don't dry out. Deeper, more thorough watering can then be done as the roots begin to penetrate the soil. For more on zoysia, turn to page 36.

I've heard about the importance of deadheading lilac bushes after they bloom, but are there any other shrubs whose seed heads should be removed after flowering?

—A.P., Seven Hills, OH

Answer: Deadheading not only enhances the appearance of certain shrubs, but also improves next year's floral display by reducing the amount of energy spent on producing seeds. Lilacs, rhododendrons, mountain laurel (Kalmia latifolia), and andromeda (Pieris spp.) all benefit from deadheading. Researchers have found that in rhododendrons, for instance, deadheading leads to the production of larger shoots which, in turn, are more likely to support flower buds than the smaller shoots from which spent trusses have not been removed. Be careful as you remove the old flowers, so that you do not damage the emerging buds. Fingers are often sufficient to do the deadheading with rhododendrons, but you will need pruning shears to remove the flower heads of lilacs.

Some shrubs can be stimulated to produce a second flush of flowers by deadheading. These include spireas, butterfly bushes, and crape myrtles. The benefits of deadheading, in any case, are greatest if the procedure is done promptly after the flowers fade.

I planted a gunnera (C. manicata) tuber, and so far it only has three very small leaves. What can I do to ensure that it grows as big as it's supposed to?

—T.L., Olympia, WA

Answer: Gunneras, a group of 40 to 50 species of herbaceous or evergreen perennials native to the moist areas of southern Africa, Australasia, and South America, vary in size from small, mat-forming species such as G. magellanica to the mammoth Chilean rhubarb (G. manicata). When optimum growing conditions exist, Chilean rhubarb becomes a conversation piece with a “Jurassic Park” look; from its crown emerge four- to six-foot-long stiff, prickly petioles attached to four- to eight-foot-wide, umbrella-like leaves.

This plant prefers deep, rich, moist soils in sun or partial shade. In regions with cool summers, full sun is best. The plant is best sited near stream banks or on the margins of ponds or lakes, where the roots can venture into the moist soil. Apply a generous helping of rotted manure each spring and additional fertilizer two or three times during the growing season.

Chilean rhubarb can tolerate temperatures as low as 0°F with protection; however, it dislikes summer heat. Northern and coastal California, coastal Oregon, and western Washington are the best regions for growing this striking perennial successfully.

Why aren't members of the genus Ribes, and black currants in particular, more popular in the United States? I am from England, where black currants are a great favorite in the form of a cold drink that is about as ubiquitous as orange juice and lemonade. Black currant juice has extremely high vitamin C content, as well as a delicious flavor.

—D.B., Chocorua, NH

Answer: European black currants (Ribes nigrum) are native to northern Europe and north and central Asia. In the United States, they grow best in the cooler summers of the Northeast and the Pacific Northwest. Temperatures above 85°F cause injury to the leaves, often resulting in complete defoliation.

The 3/8-inch-diameter fruits are produced on one- and two-year-old wood, and are borne in long clusters called strigs. Black currants contain about four times as much vitamin C as citrus fruits. The fruits have a strong, “foxy” flavor, which limits their use as fresh fruits; however, they can be processed into juices, jams and jellies, syrups, pies, and other desserts. The leaves and buds are used as herbal medicines.

Black currants’ unpopularity in the United States stems from the fact that the plant is an alternate host for white pine blister rust (Cronartium ribicola). This fungal disease, which can kill white pines, western white pines, sugar pines, and other five-needled pines, was introduced from Europe and first reported in 1909 in Geneva, New York. By 1922, New Hampshire surveys showed areas with as much as 50% of pine stems infected.

Like most other rust diseases, white pine blister rust requires two completely different hosts to complete its life cycle. Black currant is most susceptible, red and white currants are less susceptible, while gooseberry is least susceptible to white pine blister rust. Several native North American ribes are also hosts. Although the disease does little harm to its ribes hosts, efforts to protect the forest industry soon led to calls for its eradication.

In 1917, New York declared black currant a public nuisance, and other states followed suit by banning both the sale and planting of any ribes. This ultimately led to a 1926 federal quarantine on the interstate movement of all currants and gooseberries. Meanwhile, an aggressive eradication program involving hand-pulling and herbicide application was begun. High school students, German and Italian prisoners of war, and members of the Civilian Conservation Corps all labored to “treat” great tracts of white pine acreage. However, the effectiveness of this eradication program has increasingly been called into doubt. Though blister rust infection is still common in many regions where cool temperatures and high levels of moisture prevail, the incidence of infection has declined dramatically.

Starting in 1966, individual states have been allowed to decide whether to allow ribes cultivation. It is now legal to grow certain disease-resistant varieties of ribes in New Hampshire, including the black currants ‘Consort’, ‘Crusader’, ‘Coronet’, and ‘Titania’; red currants ‘Rolan’, ‘Rubina’, and ‘Viking’; and ‘Jahns Prairie’ gooseberry. People who wish to grow them, however, should fill out and file a permit application with the New Hampshire Division of Forests and Lands (603-271-7858; www.nhdfl.com).

Several of my azaleas have swollen, thickened leaves. Can you tell me what's causing these malformed leaves and how it can be controlled?

—K.O., Norfolk, VA

Answer: Your azaleas appear to be infected with a fungal disease called azalea leaf and flower gall (Exobasidium vaccinii). Closely related species of this fungus produce similar galls on many other ericaceous plants, including arbutus, blueberry, camellia, cranberry, Labrador tea (Ledum spp.), leucothoe, and rhododendron.

The fungus overwinters in buds and attacks emerging azalea leaves (and occasionally flowers in the spring); older leaves are resistant to infection. The swollen, curled leaves turn pale green and then chalky white as spores are produced in late spring to early summer. Mature galls turn brown, dry, and fall to the ground. Wind or rain carries the spores to other leaf and flower buds, where they cause new infections.

Although this disease can reach alarming proportions, especially during a cool, wet spring, the damage is largely cosmetic. To prevent new infections, pick off the galls before they turn white and begin releasing their spores. Also, avoid wetting the leaves when watering. Several gall-resistant azalea cultivars exist, including ‘Coral Bells’, ‘Formosa’, ‘Glacier’, ‘Mrs. G. G. Gerbing’, ‘Nancy’, ‘New White’, ‘Pride of Summerville’, 'Sensation’, 'Sunglow’, ‘Treasure’, and ‘White Jade’.

Can you suggest some long-blooming plants for my water garden?

—P.C., Frederick, MD

Answer: Charles Thomas, chairman emeritus of Lilypons Water Gardens and founder of the International Waterlily and Water Gardening Society, suggests the following. For hardy water lilies, consider the white Nymphaea ‘Virginalis’—one of the first water lilies to come into flower and one of the very last to go dormant in the fall—and the salmon-pink N. ‘Colorado’. Two long-blooming, tropical day-blooming water lilies are N. ‘Blue Beauty’ and ‘Panama Pacific’. Some night-blooming tropical water liles are N. ‘Wood's White Knight’ and N. ‘Texas Shell Pink’. Tropical water lilies should begin to flower after three weeks of temperatures over 80°F.

Thomas's favorite long-blooming lotus is Nelumbo ‘Mrs. Perry D. Slocum’. It is often the first lotus to start flowering and the last one to stop. Rising above 20-inch-diameter leaves, ‘Mrs. Perry D. Slocum’ has large, double flowers that start out rosy red, change on the second day to pink mixed with yellow, and on the third day become creamy yellow white. (See photo, page 52.)

Thomas's choices for shallow-water perennials include pickerel weed (Pontederia cordata), with its blue-purple spike flowers that bloom all summer, and lizard's tail (Saururus cernuus), with very fragrant white blooms.

To encourage flowering during the growing season, your water garden should receive at least five to six hours of full sun each day. To keep your aquatic plants growing vigorously and blooming freely, fertilize them regularly during the summer months.

Can I plant a florist's hydrangea outdoors after the blooms fade?

—T.E., Harrisburg, IL

Answer: Florist's hydrangeas (H. macrophylla) are native to the temperate, maritime climate of Japan, on the Pacific side of Honshu Island. While crown hardy to USDA Zone 6, florist's hydrangeas cannot be expected to flower in your area without protection. Flowers are borne on last year's shoots, and these are often killed by cold winter temperatures.

These hydrangeas have become popular florist plants, forced into flower for holidays ranging from Valentine's Day to Memorial Day. The showy, sterile flowers that make up the rounded inflorescences may be white or various tints and intensities of pink or blue.

Hydrangea flower color depends on soil pH and the availability of aluminum ions. An acid pH (5.2 to 5.5), created by the addition of aluminum sulfate, results in a deep blue color. Pink to red cultivars need a pH above 6 and ample amounts of phosphorus, which competes with the aluminum ions.

For an extended floral display in your home, keep your potted hydrangea well watered, in a cool room (less than 70°F), and with ample bright, indirect light.