I have a new black grapevine (name not known) growing in a large pot inside an unheated greenhouse. I made sure that the variety did not require greenhouse heating, but it appears to be loosing some of its leaves and some are turning yellow. I've checked for diseases and it appears clear. Is it normal for grapevines to drop their leaves in winter?
Yes, it is normal for grapes to lose their leaves in the winter. It is entering dormancy and will come back in the spring.
I have just transplanted grapevines. Are there any fruits or veggies that can be planted at the base of the grapes?
There are no beneficial plants to grow with grapes but you should not grow beans, cole crops, chives, radish and peas with it.
I have an older grapevine plant (as in several years). We need to move it to a new location, too close to someone else's property. How do we go about moving the plant?
Make sure you dig up as much of the root system and surrounding soil as possible and transplant it to a suitable location. The following articles should be of some help to you: https://www.gardeningknowhow.com/edible/fruits/grapes/growing-grapevines.htm
Is it true that you can plant grapes from cutting its branches and planting them back?
Grapes can be started from dormant, new growth (hardwood cuttings) as they are just beginning to bud. Cuttings are usually made in late fall or early winter. Place the cuttings in cold storage (32-35 degrees F) or bury them in a well-drained trench, covering with soil and mulch until spring. Once the soil can be worked in spring, remove cuttings and plant them in desired location.
Can I plant Concord grape plant in a large flowerbox above ground?
You can grow any plant in a container as long as you can give it enough water and nutrients. A grape vine can do fine in a large container for many years. You may find that as it gets established, it needs to be watered 1-2 times a day in the summer, but if you can provide that, it will be fine.
We want to plant grapevine for the first time. Is it necessary to plant two grapevines to produce fruit? When reading the directions, we saw we needed 8 feet between and we are limited in space and wondered if one plant would live and give us some fruit.
You can certainly plant just one grapevine, especially if space is limited. This can be easily trained along a fence, trellis, or other suitable support.
I have two and three year old grape vines. I would like to start new vines from existing vines, if possible. Can this be done?
Yes, it can. This article has a section on propagation that should help you:
http://aces.nmsu.edu/ces/yard/2006/102806.html