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By: Imed Dami, Diane Kinney, and Megan Soehnlen, HCS-OSU
Last week, we experienced one of the coldest temperatures since 2019 and minimum temperatures ranged between 1F and -12F across the state. On January 22nd, temperatures dropped to -4.4 F (for few minutes only) and -9.2F at the OARDC in Wooster and the AARS in Kingsville, respectively. The good news is that these temperatures occurred during the maximum cold hardiness of grapevines. To address growers’ concern about the impact of this cold snap on grapevines, our group conducted freezing tests in the lab to assess cold hardiness status of several grape varieties. We found that grapevines, in general, had achieved “good” cold hardiness despite a mild December. In fact, cold hardiness expressed as LT50, or temperature that kills 50% of primary buds, ranged between -4F (most cold tender vinifera) and -16F (most cold hardy hybrids). Based on this information, grapevines most likely sustained minimum to no bud injury following the freezing event on January 22. To verify that claim, and on January 24, our group collected canes and visually assessed bud injury of the most cold tender varieties grown at the OSU research vineyard in Wooster. Vitis vinifera varieties Arneis and Verdejo sustained 8% and 10% primary bud injury, respectively, with no injury of secondary buds.
We may have dodged the bullet from the January 22 event. However, we are not out of the woods yet as we still have four more weeks of potential damaging cold events. The status of bud injury reported here is only valid up to January 24th. In other words, % bud injury could remain the same as of January 24th or increases if damaging temperatures occur again. In both situations, it is important this year to assess bud damage prior to pruning.
Information on assessing winter damage and pruning adjustment are from previously published articles in OGEN and excerpts are listed below. Most information is from the book titled “Winter Injury to grapevines and Protection Methods” which I strongly recommend (online order: msue.anr.msu.edu/resources/winter_injury_to_grapevines_and_methods_of_protection_e2930 ).
- Prune cold hardy varieties that break buds late then those that break buds early and finally most tender varieties last.
- Collect enough canes to yield 100 “representative” nodes per variety. By representative I mean evaluate nodes that you would otherwise retain as spurs or canes when pruning.
- Place canes indoor to thaw for 48-72 hours.
- Using a sharp razor blade, cut across the bud tip at a third or half of its height.
- Visually assess if the primary bud (largest size) is alive (green color) or dead (brown). You may also evaluate the status of secondary buds if many primary buds are dead.
- We have added photos and links to You-Tube videos to assist with assessing bud winter damage (see below).
- A data sheet could be used to record and compute bud mortality as a percent.
- Conduct bud damage assessment for each variety separately and sometimes for each block of same variety separately (for example one block of chardonnay on top of the hill will likely have different bud damage than a block of same variety at the bottom of the hill).
- If primary bud damage = 0 to 14%, then no adjustment of pruning is needed.
- If primary bud damage = 15 to 34%, then leave about 35% extra buds. For example, if you prune to leave 30 buds/vine, and bud damage = 20% then leave an extra 35% or 40 buds/vine.
- If primary bud damage = 35 to 50%, then double the number of buds retained.
- If primary bud damage >50%, then it is best to minimally prune vines by hedging.
- Generally, basal buds (buds on the basal positions of the cane) are more cold hardy than distal buds. Thus, it is best to increase the number of spurs per vine than buds per spur when adjusting bud number per vine.
- Note that hybrids with fruitful secondary and base buds will produce a normal crop even with relatively high % primary bud injury. Examples include, DeChaunac, Seyval, and Vidal.
Video links for assessing bud injury: