When you first plant the grapevine, put the drippers right beside the vine. At the one year in the ground point, move the poly pipe and dripper 12 inches to the side of the vine. Add another dripper 12 inches away from the vine on the other side. That will allow the water to spread in around a 4 foot space. Most people seem to use 1 or 2 gal drippers, but it really depends upon how porous your soil is and how fast it absorbs the water. Hope this helps.
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