Advice on fertilizing and irrigating conventional vineyards is often easy to come by, but the rules change in a bonsai vineyard where you plant grape vines in pots and trim the roots every year. You must supply water and fertilizer to keep the vines healthy and to get a good crop. The key question is how much.
Grapes are famous for growing in poor soil, and it’s easy to over fertilize. The excess vigor can encourage vegetative growth at the expense of fruit and make the vine hard to manage. Over watering can be a problem too, especially late in the season when the vines are ripening fruit. Excess water will find its way into the grapes, leaving the grower with great looking plump fruit that is unsuitable for making wine. This is why growers often withhold water late in the season.
Vines in a conventional vineyard can send their roots deep into the ground in search of nutrients and water, so they’re almost never cut off the way a potted vine can be. My approach is to always water with a dilute solution (about a tenth of concentration recommended for outdoor plants) of complete fertilizer, like Miracle-Gro, then water according to where we are in the season and what the vine needs. When the vine is growing and demanding water, it gets more fertilizer. Late in the season after most growth is finished and the vine is preparing for dormancy, it gets much less. This simplifies the problem by reducing it to one question, “how much water does the vine need?”
Unfortunately, I can’t give a one-size-fits-all answer like this many gallons (or liters) per week. I usually start with a tentative schedule, like watering (until the well drained soil is thoroughly wet and just starting to drip through the drainage holes) once a week, and watching for signs of water stress to see if I need to supplement. Are the leaves and tendrils strong or limp? Does the soil dry out very quickly? These signs would push me to provide more water. If it’s early in the season, I have a bias toward more water. Later in the season I might tolerate some water stress to avoid over watering. The ripening fruit gives clues too. Firm bulging grapes look terrific, but don’t make great wine and indicate too much water. If the fruit starts to shrivel, like raisins, then you’re at the other extreme of too little water. If you know how to listen, your vines will tell you.