My grandpa owned 7 40+year old grapevines. in his backyard before he passed away. my cousin was recently granted the ability to transplant them onto different property. he cut the root system as close to the plant as he could. do you know of a method to keeping the remaining root system alive? and maybe it continuing to grow back into grapevines? It is clear that the land has been able to support 7 grapevines with a probably extravagant root system for at least 40 years.
Jefferson County Wisconsin viticulture
It is actually easier to propagate new vines rather than try to transplant old vines. And you are more likely to be successful. You can take cuttings of the current season's growth this fall/early winter when the canes are completely dormant. Store them in a refrigerator crisper wrapped in moist paper towel or potting soil. Don't let them dry out. Next spring you can stick the cuttings in a garden plot, flower pot, etc. Be sure to keep the "polarity" current. End of the shoot is the top. Base of the shoot goes into the soil. They root easily. There are many resources available on rooting cuttings. I suggest you take many cuttings (50 or so) and give the extra vines away to friends and family. You should have 75% or more success.
If you decide to transplant the old vines, I suggest you cut the trunk off about a foot or two above the soil line, dig as much of the root system as you can and transplant to the new spot as soon as possible. If not immediately, heal the roots into a trench and cover with mulch, soil, etc. I would do this after the vines go dormant this fall. Next spring you should get new shoot growth from below ground and off the old trunks. But there will not be much of a root system to maintain the new shoots. Be sure to keep well watered and weeded. Train the shoots upward on the trellis to establish new vines. Don't expect 100% success as grapes don't have many roots close to the trunk so it is not possible to get much of a root system.