How many grape vines would i need for a pergola that’s 18ft by 13ft?
The vines I'm looking into produce black grapes. They are already established vines and are to be removed for transplant.
Normally 8 - 12 feet spacing on a trellis. But assuming that you intend for the vines to grow up the posts and spread overhead on top of the pergola, I would say 6 plants, one on each post. That's if there is a post in the middle of the 18 foot span as well as corner posts. If there are only 4 posts overall, then one on each post will be okay, but won't create as dense a cover as fast as it would with 6 plants. I wouldn't put two on a post, but you wanted closer spacing, you could provide other vertical support with a trellis or wire.
If they are table grapes, prepare the soil well, with a deep and wide planting hole and rich composted and mineralized soil, it will provide high nutrient density, support more fullness of growth to cover the pergola and better quality fruit.