Ali Drew, of Christchurch, asks :-
When picking grapes from a garden grapevine I noticed several bunches had a few ripe grapes and many undeveloped small green ones. Why didn't they all develop?
Glen Creasy, a viticulturist at Lincoln University, responded.
There could be several reasons for this, but given the weather we've had this past growing season, I'm putting my money on it being caused by cool and wet weather.
Depending on where you are in New Zealand, grapes flower in late November through to late December. When the flowers open (a time called anthesis, or for grapes in particular, capfall) there needs to be dry and warm weather in order for the pollen to be transferred to the flower and the pollen tube to grow down into the flower were fertilisation of the embryo can take place and form a seed.
If the weather is wet, the pollen can be washed off the flower, and if it is cold the pollen tube doesn't grow so well, leading to unfertilised embryos and seedless berries, which are smaller than those that have at least one seed. As well, cold weather can damage the embryos before the pollen tube can reach them, also resulting in seedless berries.
Usually, the unfertilised flowers fall off quite soon after the cap has fallen off (up to 10 days usually), but sometimes there is some growth of the flower's ovary leading to a small berry (because it has no seed). This can become a small but coloured and ripe berry (often, they are more sweet than the seeded berries), but sometimes this doesn't happen and the berries stay small and green. Usually these are very small, say, a few millimetres across.
If you saw bigger green berries, then it's likely this was also due to the poor weather around flowering. Cool temperatures stretch out the length of time that the flowers open on a grape cluster. In warm climates, all the flowers on a cluster can open and set fruit in a week or even less. In cooler climates, this can go on for up to a month! If this happens, you can have berries on a cluster that are one month less developed than the berry next to them, leading to smaller and still-green berries alongside ripe ones. This latter fruit set issue is of major importance to the grape industry in New Zealand, as the unripe berries contribute green aromas and flavours to the wines, and are virtually impossible not to pick alongside the ripe ones. So we all wish for nice weather in the month or so leading up to Christmas.
Grapevines are not alone in having issues with poor weather during flowering - other fruit crops (e.g. apples and kiwifruit.) are also susceptible, because they rely on pollen tubes fertilising the embryos as well.