In January we had a blast of cold weather. The coldest it got down to was -26, cold enough to kill the primary buds on Vinifera. There could be a good side to that cold weather that invaded Prince Edward County, (Remember its always better to look at the glass as half full that way you never go thirsty.) it has made pruning decisions simple on all Vinifera. Everything above the dirt is done.
-23 Celsius is the killing temperature for all Vinifera. That means that here in the County all buds that were exposed have been severely damaged if not killed. If the buds produce anything from the secondaries, the chances are it will not produce any fruit. The fruit for this year was produced last year and is held in the dormant bud that is formed at the base of every leaf along the growing shoot. Inside this little bump along the cane is the leaves, shoot, and the fruit cluster for this coming growing season. All of this is held in the primary bud. Damage to the primaries can start at -15 Celsius.
Although the dormant bud looks like a simple structure, it is actually a compound bud consisting of three growing points, sometimes referred to as the primary,secondary, and tertiary buds within one bud. The distinction between secondary and tertiary buds is sometimes difficult to make when observing the bud visually and is often of little importance, so it is common to refer to both of the smaller buds as secondary buds. These three buds are packaged together within a group of external protective bud scales within the compound bud.
The dormant bud is the focal point during dormant pruning, since it contains clusterprimordia (the fruit-producing potential for the next season). It is called dormant to reflect the fact that it does not normally grow out in the same season in which it develops. The dormant bud initiates the year prior to its growth as a shoot. During that prior season, it undergoes considerable development. The three growing points of the compound bud each produce a rudimentary shoot that ultimately will contain the same basic components that comprise the current season’s fully grown shoot: leaves, tendrils, and in some cases flower clusters. The primary bud develops first; therefore it is the largest and most fully developed by the time the bud goes dormant. If it is produced under the right growing conditions, and it will contain a flower cluster primordia before the end of the growing season. The flower cluster primordia thus represent the fruiting potential of the bud in the following season. Reflecting the sequence of development, the secondary and tertiary buds are progressively smaller and less developed. They generally will be less fruitful (have fewer and smaller clusters) than the primary bud. Bud fruitfulness (potential to produce fruit) is a function of the varietal and vineyard management practices. Dormant buds that develop under unfavorable conditions (shade of a dense canopy, poor nutrition, disease etc.) produce fewer flower cluster primordia for the following season.
In most cases, only the primary bud grows, producing the primary shoot in the following season. The secondary bud can be thought of as a “backup system” for the vine; normally, it grows only when the primary bud or young shoot has been damaged, oftentimes from freezing County cold or late frosts in spring. Such as what happened April 2012. Tertiary buds provide additional backup if both the primary and secondary buds are damaged, but they usually have no flower clusters and therefore no fruit. If only the primary shoot grows, the secondary and tertiary buds remain alive, but dormant at the base of the shoot.
The protection of the primary buds is the reason that we here in beautiful Prince Edward County, “tie down” and bury canes every fall on our Vinifera. So that is the reason when we get a cold snap of -26 Celsius, our pruning decision is easy. The fruit this year is going to come from that cane that we buried last fall.