Answer: Grapevines will usually accept as much water as they can get so unless the ground is not draining well or if you are watering everyday, I don't think overwatering is the cause of the browning leaves. However, just to be on the safe side, water your grapevine deeply once or twice a week during the growing season. Hot weather can burn some of the leaves, especially hot winds, so if your weather has been on the warm side, the browning leaves are probably due to weather. Another thing to keep in mind is that young plants are more suscetible to weather related stress because they have small root systems. Since it's near the end of the growing season for your grapevine, I would tend to just pick up the fallen leaves and water deeply once a week. As the roots establish themselves they'll have more reserves and will be able to support the plant during hot summer weather. I think you'll see lots of healthy new growth next spring.
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