Extension News

Ask the Iowa State University Extension Experts

11/28/2007

Is summersweet clethra a suitable landscape plant for Iowa? 
Although not widely planted, summersweet clethra (Clethra alnifolia) performs well in Iowa when grown in a favorable location. Summersweet clethra performs best in moist soils in partial shade. However, it can be successfully grown in full sun as long as it has ample moisture. Avoid hot, dry locations. 

Summersweet clethra produces small, fragrant flowers on 3- to 6-inch-long, bottlebrush-like spikes in mid- to late summer. Bees and butterflies find the flowers irresistible. The foliage of summersweet clethra is lustrous green in summer changing to pale yellow or golden brown in fall.  Plants generally grow 4 to 6 feet tall and have a similar spread. Widely grown varieties include ‘Rosea’ (light pink flowers fade to white), ‘Pink Spires’ (soft pink flowers), ‘Ruby Spice’ (deep pink flowers) and ‘Hummingbird’ (white flowers, 3 feet tall). 

How do you construct a grape trellis? 
Constructing a grape trellis is similar to constructing a farm fence.The trellis must be substantial enough to carry the weight of the vines plus a heavy crop during high winds. Basically, the trellis consists of two or three wires, one above the other, stretched tightly and secured to firmly-set posts. 

End posts serve as the anchor points as well as wire supports. End posts are generally 8 feet long, with a diameter of 4 inches, set approximately 2 feet deep in the soil. They may be braced in several ways. A common method is to set an extra post within a few feet of the end post. A heavy piece of wood or another post makes a good brace between the two end posts. Line posts are also 8 feet long, but with a diameter of 3 inches. They are set approximately 2 feet into the ground and spaced about 24 feet apart within the row. 

Use galvanized wire for the grape trellis. Galvanized wire is durable and does not cause serious wire chafing of young vines. Wire sizes commonly used include numbers 9, 10 or 11. Wires are secured to end posts in various ways. A common method is to wind the wire around the post once or twice and then twist the end several times around the wire as it is stretched to the next post.

Some gardeners use special devices to attach the wires to the end posts because they simplify tightening of the wires. These devices employ cranks that eliminate removing the wires from the end posts when tightening. Wires are fastened to the line posts with ordinary staples. Space the wires vertically according to the training system to be followed. For example, a 4-cane-Kniffin system would use two wires. One wire should be 3 feet above the ground and the second wire 6 feet above the ground. The 6-cane-Kniffin system uses three wires positioned 2, 4 and 6 feet above the ground. 

The best time to construct a grape trellis is during the first part of the growing season. Tying new shoots to the trellis wires allows for straight grapevine trunk development in future years. 

What are the symptoms of salt damage to trees and shrubs?   
To prevent accidents on slippery surfaces, deicing compounds are used by highway departments, businesses and homeowners to melt ice and snow on roadways, parking lots, sidewalks and driveways. Most deicing materials are salts that melt ice and snow by lowering the freezing point of water below 32 F. While deicing salts improve travel conditions, they can damage landscape plants. 

The symptoms of salt injury to deciduous trees and shrubs include stunted growth, marginal leaf scorch, early fall coloration and twig dieback. Accumulation of salt in the soil over several years may result in progressive decline and eventual death. 
 
Salt damage to evergreens results in yellowing or browning of the needles and twig dieback. Evergreens near heavily salted roadways are often damaged by salt spray. Spray damage is most severe on the side of the plant nearest the highway. 

The severity of salt damage depends on the amount of salt applied, soil type, amount of rainfall, direction of run-off, prevailing winds and condition and type of plant material. 

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Contacts :

Richard Jauron, Horticulture, (515) 294-1871, rjauron@iastate.edu

Jean McGuire, Extension Communications and Marketing, (515) 294-7033, jmcguire@iastate.edu