Cold returns for a few days

The cold has returned to Wisconsin.   Last night we got down to about 26F.    Some buds may have already seen some damage.  At least foch and Lacrescent had already approached full swell.


Below is a nice table that shows critical temperatures for bud survival.  The table below was originally on the Michigan State University Extension web site.    That link is now dead.   (I will gladly link to another source if someone has it.)



CRITICAL TEMPERATURES FOR GRAPES

Growth Stage	Description	No Damage	10% Kill	50% Kill	90% Kill
Dormant	Bud not swollen.		Varies with conditions.
First Swell	Early swell, bud brown, no color.		13	7	-3
Full Swell	Late Swell, swollen bud looks pink.		26	21	10
Bud Burst	Leaves at tip of bud separate.	30	28	25	16
First Leaf	One leaf out at 90° to the stem.	30	28.5	27	21
Second Leaf	Two leaves out, 90° to the stem.	30	29	28	22
Third Leaf	Three leaves out, 90° to the stem.		28		26
Fourth Leaf	Four leaves out at 90° to the stem.		28		27

(The above table and photos originally from Michigan State University Extension) 

I believe the 10% and 90% kill numbers originally came from Rutgers and refers to Wine grapes.   The 50% kill numbers come from Washington State University and refer to concord grapes.  I do not know the accuracy of the numbers with respect to cold hardy vines such as French Hybrids, University of Minnesota cultivars, or Swenson cultivars.   It is interesting none the less.