GRAPE FROST TOLERANCE AND
FROST CONTROL METHODS

Springtime in the foothills of the Colorado Rocky Mountains. This is a perilous time for the Rist Canyon Vineyards. In most grape growing regions, buds from the vines push forth in late March or early April. In our Colorado grape growing area, vines typically try to break bud in late April. But the early buds are usually killed by frosts. These frosts may occur as late as the 25th of May. This means that our grapevines don't get their start until the beginning to mid-May.

Late frosts affect the developing grape flowers of the primary buds and diminish the yield of fruit dramatically. The frosts also shorten the growing season considerably, making the proper ripening of fruit difficult. This makes the Rist Canyon Vineyards one of the shortest grape growing regions in the country. It is precisely this marginal growing condition that makes Rist Canyon Vineyards the perfect Colorado grape growing area to conduct a grape varietal trial and perform grape research.

Grape varieties not only have to endure the cold winters but also must be able to produce a crop during a very short growing season from mid-May to late-September. On the average, this is 135-140 frost-free days. Fortunately for us, the degree-growing days (dgd) averages about 2000 dgd base 50F. The high heat units of this area are what allow us to grow grapes at this altitude, but not without some manipulation.

Fluctuating late winter and spring temperatures are seen as major restrictions to grape growing in this area. High daytime temperatures followed by frost during the night may cause more damage to grapes than the coldest days of winter. Frost tolerance is a must for any variety to be able to properly set and mature grapes under these Colorado grape growing conditions. Spring frost control is also a must. A combination of the right variety and frost control can guarantee a favorable outcome in the form of a good yield of grapes with high quality.

What Is Frost?

Frost, in the general sense, refers to the deposit of ice crystals on the ground and other exposed surfaces when the air temperature falls below 32F. The actual temperature at which ice forms will vary with humidity, causing either "white frost" or "black frost". White frost occurs when the air is moist. Dew will form on the ground before the temperature turns to 32F. When the freezing temperature is reached, a coating of whitish crystals forms, thus giving the name "white frost". Black frost forms when the dew point is below 32F and frost develops without the familiar whitish frost crystals.

There are two types of frost episodes. These types are known as radiation and advection frost. Radiation frost happens when there is little or no wind, clear skies, and low humidity. Heat is lost by radiation when there is no barrier (such as clouds) to prevent this heat loss. Temperatures will fall faster in this dry air than in moist air. This usually happens in the early morning hours. Radiation frost occurs from the ground upward, affecting the lower portions of a plant more than higher parts. Advection frost occurs when a cold air mass moves into an area displacing warm air. Winds often mix warmer air over the cold air, pushing the cold toward the ground. Advection frosts happen earlier in the night and while the freezing process is the same as with radiation frosts, the effects are usually seen on the upper parts of the plants.

Site Selection

Site selection is an important initial step in controlling frost in the vineyard. A sloped site with good air drainage is necessary in controlling spring and fall freezes. Cold air drains downward, close to the ground, and vines situated where cold air collects (frost pockets) are certain to be caught by Jack Frost. Planting tolerant varieties is also essential.

The Ideal Rist Canyon Grape Variety

The ideal grape variety for this area would be one that buds out late, grows at a rapid rate, and ripens very early in the season. This ideal grape variety would be able to avoid the late spring frosts by this late budding. Rapid growth would allow the variety to obtain maximum photosynthetic area quickly. Grape maturation in mid-August to early September with a high brix and fairly low acid content would make for useable fruit and would allow several weeks for carbohydrate accumulation, periderm hardening before the first killing frosts in the fall, and acclimation for maximum winter hardiness. Another characteristic of the ideal variety would be to bear a crop from the secondary buds in the event of frost damage. Of course this variety doesn't exist (yet).

Grape Frost Susceptibility-Varietal Differences

Each grape variety varies with its budding date. Some like Foch, Aurora, and Frontenac bud out very early. Such varieties are very susceptible to late spring frosts. But even these bud out at different rates. Foch swells early but has a slow growth rate allowing it to survive many of the early frosts. Fontenac and Aurora have a more rapid growth rate. Thus shoots are elongated at an earlier date making them more susceptible to death with late frosts. Buds and growing shoots are differential in their response to frosts. Buds are at their hardiest in mid-winter after proper hardening off and acclimation to winter temperatures. Varieties vary in what temperatures they can endure. Vitis riparia has been known to be able to withstand cold temperatures down to -49F in parts of its range while Vitis vinifera varieties show damage when the temperature begins to dip below 5F. American and hybrid varieties range in-between these in there tolerance for cold temperatures.

After mid-winter, vines de-acclimate quickly with warm temperatures. This is when fluctuating warm and cold periods can cause major damage to the grapevine. A January or February "thaw" followed by extremely cold weather can be one of the major reasons for bud or vine death. Research has shown that vines vary in their response to re-acclimation after a warm period. Generally, a vine takes 3-5 days to re-gain tolerance to cold temperatures after a warm episode. Often the cold temperatures come on suddenly and this isn't a quick enough response to avoid death to buds or other vine parts. Even with a re-acclimation response, the vine never again reaches the resistance to cold that it had in mid-winter. A perfect winter would be one where the temperature gradually gets colder through the fall and then stays cold until spring arrives. At the Rist Canyon Vineyards this never happens. Fluctuating temperatures are the norm.

Springtime in the vineyard can be summarized in growth stages. The minimum temperature at which the bud/shoot can survive varies with each stage it goes through. The following table outlines the critical temperatures for grapes in the spring:

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

From the above table you can see that as growth proceeds from bud swell to shoot elongation the minumum temperature which produces damage to the developing shoot increases. This means that a temperature of 25F, as occured IN 2002 at the Rist Canyon Vineyards during bud swell of many varieties, can have a devastating effect on the potential crop for that year.

The grape bud is a compound bud. There are actually three buds within - Primary, Secondary, and Tertiary. It is the primary bud which bears fruit. The primary bud is also the most sensitive to frost and cold damage. The secondary and tertiary buds are able to withstand colder temperatures than the primary bud. In some varieties, the primary bud can be killed and a small crop will form from the secondary bud. But most varieties available only produce flowers from the primary bud. The chart above shows killing temperatures for the primary bud only.

Cultural Manipulations for Frost Control

There are several things that can be done to control or limit frost damage in the vineyard. Good site selection is, of course, the first priority. Wind machines, sprinklers, and heaters are the tradition means to limit frost damage. These can be expensive. The Rist Canyon Vineyards is on a limited budget and these traditional means are not within our reach at this point in time. We have been experimenting with other methods of control. Frost protectants were experimented with in the past year. We have experimented with three commercially available frost protectants. All of these products claim to protect growing plants to 25F, well below the frost point of 32F. They all are organic products which protect by either antitranspirant properties or by bactericidal properties. The antitranspirants protect the plants by forming a barrier that keeps in water and heat, thus preventing freezing. The antibacterials kill ice nucleating bacteria, thus preventing ice formation. Our findings on their useage in the vineyard - Don't Bother, they Don't Work! In fact, we found that some of the products can actually lead to more of a frost problem. This is especially seen in the spring when developing shoots are much more tender than leaves in the fall. Even in the fall, none that we tested would protect the vines below 30F.

Research at Colorado State University by Imid Dami and Dr. Cecil Stushnof has lead to a means of spring frost control. The idea that these scientists came up with is to chemically delay bud break until the weather is frost-free.The solution is sodium alginate. Alginate is a naturally occurring kelp extract. It is painted or sprayed onto the vine spurs and canes after mixing it with 1M sucrose solution. This is then followed by a cross-linking spray of calcium chloride. The calcium chloride hardens the alginate/sucrose mixture on the vine so it can't be washed off. Alginate applied this way delays bud break 2-3 weeks allowing the vine to pass through the spring frost period. Maximum protection is achieved when combined with delayed pruning (another traditional frost control method). This means of frost control has been used commercially in New Zealand vineyards with good success. Experiments with the alginate method will be used during the spring 2003 growing season. We feel that this may be the key to success for early budding varieties. (See the photo at the top of this page to see the results).