A year ago, I wrote about how warm weather in the winter could be hazardous to the buds of fruit crops.
At that time, I did not imagine the 2012 tree fruit crop would turn out as badly as it did.
Wine grapes came through better than tree fruits due to two things — grapes are much slower to break dormancy and grapes have a very special three-part bud. The slower start saved the grape crop in 2012; in years with significantly cold winter weather, it is the three-part bud that helps grapes survive.
What appears to be a single bud on a dormant grape cane is actually a very tightly compressed set of three individual buds. The three buds are called the primary, secondary and tertiary buds, and the combined set is called a node. Buds are initiated at the base of leaves on shoots during the previous growing season.
By the time shoots mature to canes in the fall, the buds contain tiny primordial clusters that are the potential fruit of the vine for the next growing season.
The primary bud is the largest and it is positioned between the other two. Its potential crop is the greatest, but it is typically less cold-hardy than the other buds. It is the first to break dormancy, and its growth actually inhibits the further development of the secondary and tertiary bud.
The secondary bud is slightly smaller in size and potential crop, but it is more cold-hardy than the primary bud; it usually breaks dormancy later than the primary bud. The tertiary bud is small and it typically does not have any fruiting potential. It breaks dormancy the latest, but it is the most cold-resistant of the buds.
With this three-bud system, grapes have a very functional form of biological crop insurance. If winter temperatures dip low enough to kill the primary buds, the secondary buds may make it through and still produce shoots and a crop. If the temperatures drop lower and take out the secondary buds, the vine can at least put out a set of healthy shoots and carry on the annual cycle of growth and bud initiation for the following year.
This also works if a spring frost kills the shoots from primary buds. Secondary buds will then begin growth to replace the injured primary shoots. Tertiary buds will take over if the secondary bud shoots are killed.
If things really go wrong and even the three-bud insurance plan fails, grapes have a couple of tricks left. Throughout the woody parts of the vine there are typically a number of hidden, dormant "latent" buds that usually do not grow out into shoots unless most of the normal buds and shoots have been killed.
Snow cover can also save vines from complete death in very cold weather. Unlike fruit trees, grapevines can be trained to produce numerous buds close to the ground. These buds are very well protected from extreme temperatures by the insulating effect of snow, providing almost certain survival of the basal portion of the vine.
Erwin "Duke" Elsner is a Michigan State University Extension small fruit educator.