164 WINES&VINES January 2017

GRAPEGROWING WINE EAST

Determining the Best Cold Hardiness MeasurementNearly two-dozen grape varieties tested for ability

to withstand below-freezing temperatures

By Imed Dami

Grapes contribute more than

$5 billion per year to the

economy of the eastern

United States, and the grape

and wine industries have

been expanding rapidly in

those regions. However, the sustainability of

profitable yet cold-sensitive cultivars is limited

by climatic constraints, primarily lethal freez-

ing temperatures that can drop well below

0° F. The severe economic losses that result

from freezing injury continue to be a major

setback for the continuous growth of the

grape and wine industries in this region.

In 2014, the polar vortex caused temperatures

to drop to critical levels for grape survival and

resulted in widespread damage and crop losses

across the Midwest and eastern United States.

Ohio experienced the worst damage and crop loss

in 20 years (valued at $12 million). This problem

was exacerbated with whole-vine loss of cold-

sensitive species V. vinifera, which have dominated

new plantings in the past 15 years in Ohio.

Therefore, the limitation imposed by the

sensitivity of the vinifera species has an im-

pact on the sustainable growth of the indus-

try, and it is critical for growers to know the

cold hardiness (CH) of the newly introduced

varieties, in addition to the standard viticul-

tural and enological characteristics. Sub-

freezing temperatures below -20° C (-4° F),

often damage V. vinifera, which is mostly

adapted to a Mediterranean-type climate.

Nonetheless, information about the CH of

the newly introduced vinifera varieties has

not been available.

Ohio’s cold-hardiness study The purpose of the study at the Ohio Agricultural

Research and Development Center in Wooster

was to determine the cold hardiness of 23 variet-

ies of recently introduced V. vinifera grapes so

that grapegrowers and nurseries would have a

better understanding of the ability of those variet-

ies to survive the harsh winter conditions in the

Midwest and northeastern United States. Re-

searchers also investigated whether there are

better ways to estimate the CH of vines.

Buds were collected from each variety bi-

weekly between September and March for three

dormant seasons (2011-14) and placed in a

freeze chamber. The freeze chamber was set up

indoors to conduct freezing tests that would

simulate cold damage in the vineyard. Cold

hardiness was determined by thermal analysis

and expressed by the lethal temperature that

kills 50% of the bud population, or LT50

.

We evaluated three methods to express cold

hardiness:

1) Standard method: computing mid-winter

hardiness based on a single and lowest

LT50

value;

2) Modified standard method: computing

mid-winter hardiness based on multiple

lowest LT50

values;

3) Annual freezing tolerance (AFT, or an-

nual cold hardiness): computing LT50

not only during mid-winter, but also

during fall acclimation and spring

deacclimation.

Cold hardiness of 23 varieties As expected, all 23 varieties behaved similarly

and followed the typical U-shaped pattern of

bud cold hardiness, which consists of three

stages: fall cold acclimation (September to

December), followed by maximum hardiness

in mid-winter (January to February) and deac-

climation in late winter and early spring.

However, there were differences of cold

hardiness among varieties. Gamay Noir, Rot-

berger, Pinotage and Regent were generally

KEY POINTS

The Ohio State University’s OARDC tested the cold hardiness of 23 varieties newly introduced in Mid-

west and northeastern vineyards using three methods of testing: the standard method of indexing

mid-winter bud cold hardiness (LT50); a modified standard method based on regression analysis; and

researchers developed a new cold hardiness index, the relative annual freezing tolerance (RAFT),

which compares annual freezing tolerances of these varieties with that of a benchmark variety.

These 23 varieties can be grouped into two distinct groups of most cold hardy (Gamay Noir, Chardon-

nay, Pinotage, Regent and Rotberger) and most cold sensitive (Barbera, Durif, Lagrein, Sangiovese,

Syrah, Tempranilllo and Teroldego). The remaining 11 varieties fall in the gray area, and their CH may

shift one way or another depending on other factors such as weather and cultural practices

The findings of this study provide research-based information for nurseries and on cold hardiness of

new varieties they sell. They will also guide growers making decisions on suitable varieties for sites

where winter damage is a concern.

A variety trial plot (shown here before the polar vortex of 2014) helped researchers understand

each variety’s cold tolerance.

Janaury 2017 WINES&VINES 165

WINE EAST GRAPEGROWING

among the most cold-hardy variet-

ies with the lowest LT50

values,

while Tempranillo, Lagrein, Bar-

bera and Durif were among the

most cold-sensitive varieties with

the highest LT50

values.

Also, this study showed that not

all varieties acclimated, de-accli-

mated or reached maximum hardi-

ness at the same time and same

level. “Cold Hardiness for Three

Seasons” shows three varieties to

illustrate this finding. Gamay Noir

was the most cold-hardy variety

(lowest LT50

in mid-winter) and

was the quickest (steepest slope)

to cold acclimate in the fall. Tem-

pranillo was the most cold sensitive

(highest LT50

in mid-winter) and

was the quickest to deacclimate in

late winter. Dolcetto acclimated the

slowest in the fall and reached

maximum cold hardiness later

than Gamay and Tempranillo.

Methods of assessing cold hardinessStandard method: Mid-winter bud

cold hardiness (LT50

) is the most

commonly used index to estimate

a given variety’s cold hardiness and

has been reported as such by re-

search institutions in several states

conducting this type of work in the

past 30 years. In Ohio, we found

that mid-winter cold hardiness var-

ied between LT50

= -18.3° C

(-0.9° F) and LT50

= -24.8° C (-12.6°

F). “Cold Hardiness Based on Stan-

dard Method” shows that Gamay

Noir was the most cold-hardy vari-

ety, whereas Tempranillo was the

most cold sensitive in this study.

We also found that the standard

method has a few drawbacks: a)

the lowest LT50

in mid-winter is

generally based on a single LT50

measurement; b) not all varieties

reached their maximum cold hardi-

ness on the same date; and c) wide

variability of LT50

within each cul-

tivar made it difficult to distinguish

cold hardiness between two variet-

ies if the difference between their

LT50

was less than 3° C (5.4° F).

During the polar vortex in

January and February 2014, the

research vineyard in Wooster ex-

perienced successive low tempera-

tures that dropped below the

MATERIALS AND METHODSVarieties tested: Vitis vinifera—Arneis, Barbera, Cabernet Franc, Cabernet Sau-vignon, Carmenère, Chardonnay, Dol-cetto, Durif, Gamay Noir, Kerner, Lagrein, Malbec, Malvasia Bianca, Merlot, Pino-tage, Rotberger, Sangiovese, Sauvignon Blanc, Siegerrebe, Syrah, Tempranillo, Teroldego; and Vitis sp.—Regent (a Ger-man cross of Diana and Chambourcin)

Rootstock: 101-14 Mgt. (V. riparia × V. rupestris)

Location: OSU Research Vineyard at OARDC in Wooster, Ohio (lat. 40°44’16” N; long. 81°54’12” W)

Elevation: 1,165 feet above sea level

USDA Plant Hardiness Zone: Zone 6a (-10° F to -5° F).

Spacing: 9 feet x 6 feet (row x vine)

Training: Bilateral low cordon (40-inch height) with vertical shoot position (VSP)

Pruning: Spur pruning to 30 buds per vine (two to three buds per spur)

COLD HARDINESS FOR THREE SEASONS

LT50

(°C

)

0

-5

-10

-15

-20

-25252 287 312 341 3 46 82

Day of Year

Cold hardiness changes of Gamay Noir, Dolcetto and Tempranillo in the

Wooster research vineyard by day of year for three seasons. Plot points

represent LT50 (lethal temperature that kills 50% population) collected for three

seasons. Note the U-shaped trend lines of cold hardiness in the three varieties.

Gamay Noir

Tempranillo

Dolcetto

166 WINES&VINES January 2017

GRAPEGROWING WINE EAST

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COLD HARDINESS BASED ON STANDARD METHOD

LT50

(°C

)

-14

-16

-18

-20

-22

-24

-26

-28

-30

Cultivar

Cold hardiness is calculated based on the lowest LT50 of each variety in mid-winter.

Gamay

Noir

Pinota

ge

Caber

net S

auvig

non

Rotber

ger

Carm

enèr

e

Dolcet

to

Arneis

Malv

asia

Syrah

Regen

tDur

if

Sieger

rebe

Sangio

vese

Tero

ldego

Sauvig

non

Blanc

Lagr

ein

Mer

lot

Barber

a

Tem

pranil

lo

ANNUAL FREEZING TOLERANCE FOR CALCULATING COLD HARDINESS

AF

T (°

C)

-13

-14

-15

-16

-17

-18

-19

-20

CultivarThe annual freezing tolerance (AFT) is based on computing LT50 throughout the dormant season.

Gamay

Noir

Pinota

ge

Rotber

ger

Chard

onna

y

Malv

asia

Bianca

Regen

t

Sieger

rebe

Carm

enèr

e

Caber

net S

auvig

non

Sauvig

non

Blanc

Caber

net F

ranc

Malb

ec

Arneis

Mer

lot

Kerne

r

Tero

ldego

Sangio

vese

Syrah

Lagr

ein

Tem

pranil

loDur

if

Barber

a

Dolcet

to

Janaury 2017 WINES&VINES 167

WINE EAST GRAPEGROWING

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critical temperature of -20° C (-4° F) five times

(-20.1° C [-4.2° F] to -24.5° C [-12.1° F]). All

23 varieties sustained trunk damage, with the

exception of Regent. The extent of whole-vine

damage was assessed in late summer 2014,

and vine death was recorded.

In general, variety vine mortality corre-

sponded to the computed LT50

. For example,

Tempranillo, with the highest LT50

(most cold

sensitive), sustained the highest vine mortal-

ity (100%), while Gamay Noir, with the lowest

LT50

(most cold hardy), sustained the least

vine mortality (17%). Therefore, the 2014

winter damage sustained in the vineyard co-

incidently confirmed our cold hardiness de-

termination in the laboratory.

Modified standard method: This method

based on regression analysis was explored to

alleviate the limitation of the standard method.

It proved superior to the standard method since

it distinguished the CH between two varieties

at 2° C (3.6° F). However, it was not possible

to obtain estimates for all varieties tested since

more LT50

observations were needed.

A new index–the AFT: The annual freezing

tolerance (AFT) or annual cold hardiness con-

sisted of averaging LT50

throughout the dor-

mant season. AFT has several advantages over

the standard method currently used by nurser-

ies, growers and researchers. First, AFT pro-

vides an estimate of CH across all stages of cold

acclimation (i.e., autumn acclimation, mid-

winter maximum hardiness and spring deac-

climation). As a result, the AFT had the least

LT50

variability, and differences between two

varieties were detected at the 1° C (~ 2° F)

level (see “Annual Freezing Tolerance for Cal-

culating Cold Hardiness”). Second, the stan-

dard method provides an LT50

of a given variety

determined on a specific date and location.

Hence, this absolute value provides an arbi-

trary measure of CH, which will vary with time

and space. In other words, one cannot extrapo-

late LT50

published values determined in one

location (e.g., in New York) to equate the ac-

tual LT50

in another region (e.g., in Missouri).

This may be common knowledge within the

research community, but it is frequently misin-

terpreted by vineyard owners and nurseries.

To overcome this issue, we propose to com-

pare the CH of varieties to a standard or bench-

mark variety grown in the same location. We

call the new index the relative annual freezing

tolerance, or RAFT. Merlot was selected since

it is a common variety grown in different re-

gions worldwide and has mid-range CH.

The figure above illustrates RAFT with nega-

tive numbers indicating more cold-hardy varieties

and positive numbers more cold-sensitive variet-

ies than Merlot. For example, Gamay has a RAFT

of -2.5° C, meaning Gamay is more hardy than

Merlot by ~2.5° C. However, Barbera has a RAFT

of ~1.5° C, meaning Barbera is less cold hardy

than Merlot. In other words, if a region is known

to have Merlot as a borderline variety for winter

survival, then a grower should not even think

about planting Barbera in the same region.

Finally, among the three methods, AFT pro-

duced the best prediction of whole-vine CH.

Imed Dami is professor of viticulture in the Department

of Horticulture and Crop Science, Ohio Agricultural

Research and Development Center, The Ohio State

University, in Wooster, Ohio.

The author thanks project participants Dr. Yi Zhang and

Shouxin Li and the USDA-NIFA (agreement No. 2010-

51181-21599), Ohio Grape Industries Program and OSU

Department of Horticulture and Crop Science for their

financial support.

RELATIVE ANNUAL FREEZING TOLERANCE

RA

FT

(˚C

)

3

2

1

0

-1

-2

-3

Cultivar

Annual cold hardiness is shown relative to that of Merlot, computed by subtracting the annual mean LT50 of

each variety from that of Merlot (baseline 0º C).

Gam

ay N

oir

Pin

otag

e

Rot

ber

ger

Cha

rdon

nay

Mal

vasi

Reg

ent

Sie

gerr

ebe

Car

men

ère

Cab

erne

t S

auvi

gnon

Sau

vign

on B

lanc

Cab

erne

t Fr

anc

Mal

bec

Arn

eis

Mer

lot

Ker

ner

Tero

ldeg

o

San

giov

ese

Syr

ah

Lagr

ein

Tem

pra

nillo

Dur

if

Bar

ber

a

Dol

cett

o

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