Whether for making your own wine or simply for enjoying your own home-grown fruit, growing grapes is a suitable challenge for any ambitious gardener in a suitable climate. As with any other plants, an understanding of the normal life cycle of grape plants is essential to growing them successfully. Knowing when grapes are due to ripen, for instance, can help you monitor the overall health of your grape plants.
As a look at the highest wine-producing regions of the world will quickly indicate, grapes prefer lots of sunlight and a warm climate in order to grow to full ripeness. Grapes only develop their flavours fully when allowed to grow under ideal conditions. If you plant your grapes in a soil that is not well-drained, for example, standing water in the soil will cause the grapes before they have fully developed their flavour. Premature grapes will also result from planting grapes in too cold or windy a climate, so ensuring that you have planted your grape plants in the most ideal location possible is the first step in harvesting fully developed grapes.
It is difficult to estimate a single month of the year that all grapes ripen since ripening times depend heavily on specific grape varietals. Moreover, ripeness tends to be more a matter of personal taste than an objective scientific metric; in the word of Texas A&M's Edward Hellman on the school's website, "ripeness is in the eye of the beholder." From a purely scientific standpoint, a grape achieves full ripeness when its size increases and flesh softens; at this time, the grape's sugar content increases and its acidity softens. This is a purely scientific view of ripeness, but remember that more than anything, ripeness is a subjective matter of personal taste.
Specific Varietal Considerations
All grapes ripen fully in the spring, but the exact window of time when ideal ripeness is achieved depends entirely on the varietal of grape being grown. Ripening times for each varietal varies from very early spring to very late spring. Note, however, that ripening times are also affected by weather, soil and other natural conditions
Hellman's report on estimating grape ripeness recommends both taste sampling by the grower and the use of advanced devices like a hand-held refractometer to measure the grape's sugar content. These can be useful for large, commercial vineyards but may be overkill for small tracts of home-grown grapes. The best method of measuring ripeness is to simply taste the grapes on your vines. If the grapes are highly acidic, lacking in sugar content or the flesh is not soft enough, try a later harvest time next growing season. Conversely, if the flesh is too soft and the grape is too sweet and not acidic enough for your tastes, harvest the grapes earlier next growing season. Eventually you will settle on the perfect harvest time for the specific grapes you are growing.