What a spring frost means for local vineyards

This article was featured in the April edition of the Pinnacle Ridge Winery e-newsletter.

Spring has sprung - early! A prolonged stretch of unusually warm weather in March has sent a message to many plants that it is time to grow. This early warm weather launches the growing season earlier than normal which means (everything else being equal) we should have an earlier harvest than normal. Years with an early harvest are generally better years as the fruit has more time to hang and get fully ripe in warmer weather. It tends to be especially helpful in getting the late red varieties fully ripe. However, there is one caveat that can not be overlooked - the danger that Mr. Jack Frost will visit while tender green shoots are exposed.

In the last 20 years of growing grapes, we have observed killing frosts as late as May 20. It is not normal to have frost that late but it does happen. The first week or two in May would generally be regarded as the last frost date. So what happens to the vines in this situation?  Like most things it is not as simple as it seems.

Typical spring frosts in our area occur on very clear, cloudless nights. The absence of clouds allows the warmer air to move up into the atmosphere while the colder (more dense) air sinks to low spots. Naturally any low spots in a vineyard are prone to freezing before higher spots. So within a vineyard site there will be  areas that are colder than others.

The other variable is the level of exposure of the new shoots on the vine. Closed buds on a vine can withstand cold weather down to zero degrees or lower. As the root systems awaken in the spring and push sap into the vine. the cold hardiness of the buds diminishes. As the bud swells and starts to open it becomes more and more susceptible to frost/freeze injury. A swollen bud can withstand temperatures down to around 25 degrees whereas new shoots with several leaves showing will suffer injury at 30 degrees.

Different varieties push their shoots at different times and not all shoots push at exactly the same time. Our earliest varieties are our Pinot Noir and Chardonnay. The shoots on these varieties push weeks earlier than Cabernet Sauvignon or Chambourcin. So during a frost/freeze event the level of damage depends on location in the vineyard and on the progress of shoot development on the different varieties. Frost injury usually ends up hitting one or two varieties harder than others and mainly in lower, colder locations in the vineyard.

Once a shoot has been injured the vines will push secondary or tertiary shoots. On most varieties of vines these shoots will not carry much fruit (if any) but they do allow the vine to produce leaves which allows photosynthesis to occur and the plant to survive. So frost doesn't generally kill plants but does reduce the
yield of grapes. Our buds in the Chardonnay have started to swell and we will most likely have bud break in the first days of April. This is exactly what happened in 2010.

So keep your fingers crossed that we don't have any of those clear, cold nights for the rest of April!