Question: I have a four-year-old Hamburgh grapevine and now I have some bunches of grapes but they aren't very big. What can I do to improve the crop and get bigger grapes and more of them? Should I be feeding the vine, and thinning the bunches? Any help you can offer would be appreciated.
Answer: I am assuming that your ‘Hamburg’ is ‘Black Hamburg' and is growing in a glasshouse – it does need the extra warmth to do well. The easiest way to train the vine is to grow it as a single cordon. The branches that grow from this stem and produce fruit are laterals and are pruned each winter back to two buds.
In summer, you can pinch out the growing tips of flowering side branches to two leaves beyond the cluster of flowers. Thinning consists of allowing only one flower cluster to develop per side branch. This will concentrate the vines’ resources into grape production. In February, the vine would benefit from an application of a 5-8cm (2-3in) layer of well-rotted manure and general purpose fertiliser such as Growmore or fish, blood and bone. Regular watering will also help support the development of fruit.
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