Should I Thin Out These Tiny Grapes Now Or Later ?

Question

It’s an old grapevine that’s been neglected for some time. In a greenhouse, in Scotland.

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Answer 1

I wouldn't, necessarily, thin the fruit. If it is fruiting poorly, then it probably needs a good hard pruning. This should be done in winter, or during dormancy. You may want to test the soil, as well. If it doesn't fruit as well as you want it to, then pruning may not be enough. The soil may be lacking in phosphorus and potassium. Likely, though, pruning should help the issue.

https://www.gardeningknowhow.com/edible/fruits/grapes/prune-grape-vine.htm

https://www.gardeningknowhow.com/edible/fruits/grapes/growing-grapevines.htm