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Ask the ISU Experts

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Got gardening questions? Contact the Hortline at (515) 294-3108 (Monday - Friday; 10 a.m. - 12 noon and  1-4:30 p.m.) or send an e-mail to hortline@iastate.edu. For more gardening information visit us at Yard and Garden Online at www.yardandgarden.extension.iastate.edu



I have several grapevines in my backyard.  How do I prune them? 

Grapevines produce fruit clusters on the previous season’s growth. Before pruning, a grapevine may have 200 to 300 buds capable of producing fruit. If the vine is not pruned, the number of grape clusters would be excessive and the grapevine would be unable to ripen the large crop or produce adequate vegetative growth. 


The purpose of pruning is to obtain maximum yields of high quality grapes and to allow adequate vegetative growth for the following season. 


The most desirable time to prune grapevines is late winter or early spring. In Iowa, pruning can begin in late February and should be completed by early April. Grapevines pruned at this time of year may “bleed” heavily. However, the loss of sap will not harm the vines.  


To maximize crop yields, grapevines are trained to a specific system. The most common training system used by home gardeners is the four-cane Kniffin system. The four-cane Kniffin system is popular because of its simplicity. In a four-cane Kniffin system, the canes of the grapevine grow on two wires, one located 3 feet above the ground and the second 6 feet high. 


If using the four-cane Kniffin system, select four canes on the upper wire, two going in each direction. Also, select four canes on the lower wire. To aid identification, some gardeners tie brightly colored ribbons or strips of cloth on those canes they wish to retain. All remaining one-year-old canes should be completely removed. 


Going back to the upper wire, select two of the remaining four canes (one going in each direction). Prune these canes back to one or two buds. These short one or two bud canes are referred to as renewal spurs. The renewal spurs provide the shoots or canes that will produce next year’s crop. Prune the remaining two canes on the upper wire back to eight to thirteen buds. The number of buds left on the fruiting canes is determined by plant vigor. If the grapevine is vigorous, leave thirteen buds per cane. Leave only eight buds per cane if the grapevine possesses poor vigor. 


Prune the four canes on the lower wire the same as those on the upper wire. When pruning is complete, no more than 60 buds should remain on the grapevine. When counting the number of buds on the grapevine, include both the buds on the fruiting canes and those on the renewal spurs.


Tools required to prune grapevines include a hand shears, lopping shears and saw. Brightly colored ribbons or cloth strips can be used to identify fruiting canes and renewal spurs. 


What are the differences between hardneck and softneck garlic? 

Hardneck garlic varieties produce a flower stalk. Small aerial cloves (bulbils) usually form at the top of the flower stalk. Hardneck varieties are difficult to braid because of their hard flower stalks. They do not store well. 


Softneck varieties do not produce a flower stalk. They are easy to braid and can be stored for longer periods than hardneck varieties. 


I’ve been told that tiger lilies should not be planted near Asiatic lilies.  Is this true? 

Many lily enthusiasts don’t grow tiger lilies (Lilium tigrinum) because they are often infected with lily mosaic virus. Lily mosaic virus causes little harm to tiger lilies. Often, you can’t tell that they have the disease. However, aphids and other sap-feeding insects may carry the virus from the tiger lilies to other types. Many hybrid lilies infected with lily mosaic virus produce distorted foliage that is streaked or mottled. Also, infected plants produce fewer flowers and those that do form are often deformed. Lilies exhibiting lily mosaic virus symptoms should be promptly dug up and discarded. 


Contacts :

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

Jean McGuire, Continuing Education and Communication Services, (515) 294-7033, jmcguire@iastate.edu