I have a 40 year old grape vine in a conservatory that I have been maintaining for many years. It is rooted externally and i don’t feed it or water it. In recent years crops of fruit have been quite small compared to prior years. I wonder if a new vegetable patch behind the conservatory may have taken some of its root area. There are concrete paths all around the conservatory and earth pits inside where it is planted. Should I feed or water it?
It could also be affected adversely by changes of water availability and water table, such as drying out or flooding.
Or its roots could have been attacked by vine weevil larvae.
You should feed it in late winter with some well-rotted compost and a shake of fertilizer. If the growth of leaves and shoots has been reduced, use a general fertilizer or a fruit fertilizer. If it is just lack of fruit and plenty of foliage, use sulphate of potash. About 50 grams per square metre of root area, probably over 8 square metres or so