Week 11 (Aug. 26th - 30th)

We spent a few days touching up the nursery this week so that Peter can more easily evaluate the progress of the young vines. The nursery floor was covered with a mat of purslane, a weedy succulent, but this was not necessarily a bad thing. Due it its density, the purslane helped retained soil moisture and out competed other more difficult-to-control weeds from spreading. We tilled the aisles, hoed around the the plants, and then tilled the aisles again. Afterwards we had to carefully inspect the drip tape for leaks since our hoes were pretty sharp. 

New shoots, whether they on a spur or young vines like this, are hardening off now that the days are getting shorter. In doing this the vine is sealing itself off to the environment that surrounds it while forming a periderm that will be able to withstand the winter. Since vascular flow is gravity assisted, shoots that are pointing upwards are hardening off much faster than their downward facing counterparts. Due to this, a VSP training system is one of the most effective for cold hardiness.

Some varieties, such as many vinifera, are poor at depositing and maintaining appropriate levels of callose in their vascular system during the winter months. Therefore, water may begin to flow into the trunk during warm days and then freeze once the temperature drops back below freezing at night. This causes trunks to split, such as in the photo above, and the plant to die. Growers need to carefully judge how much fruit they allow each vine to bear during the growing season, which will determine how stressed on resources the vine is going into the process of hardening off. Also, if possible, they need to allow the vine time to stabilize and prepare itself for winter after harvest and before the first frost. It's a tricky to be able to judge what to do when in order to maximize winter hardiness in cold-climate grapes.  

Black rot is still affecting some grapes. At first I thought this was merely sunburn, but then I realized that sunburn is more widespread over exposed clusters, while this discoloration on select berries. Temperatures over 95F inhibit evapotranspiration, which helps cool the plant, and intense sun lyses enzymes in the pulp of the berry, which is effectively a small greenhouse. Below is a photo of what sunburn looks like.

In order to prevent sunburn one needs to be selective in their leaf-thinning. Without the leaves to shade the clusters, the berries heat up too much. This causes them to raisin and fall off.