Suzanne D. wrote:

Thanks very much for the information. I will look more into heading, and
will try to find out what types we have in order to prune them correctly
in the future.

One more question: a few of the plants have some suckers at the bottom.
Would it be okay to allow these to grow to eventually take the place of
the older plants, or should I eliminate these altogether?
--Suzanne

This depends on whether the vine was planted from a graft on rootstock or
self rooted. If it was a grafted vine it depends where the suckers are
located. If it was a grafted vine make sure the sucker(s) is above the the
graft union otherwise you will be growing the root stock instead of what
you want.

Generally every year it is a good idea to allow a couple suckers to grow to
shoots in case you need to replace the trunk or cordon because of winter
kill or disease. If they are not needed, you can prune them off the
following year or prune back to one or two buds.

If you presently have a couple shoots near the head of the vine (where the
trunk goes horizontal to form the cordons), you could cut off your old
cordons and use the shoots for your new cordon. This would in effect be
cane pruning and the shoots would produce fruit. There is a LOT of energy
in the old trunk and roots. You will be surprised at how fast the vine
jumps back to vigor.