Cold temperatures can affect the growth of grape vines and buds at different stages of growth. Grape bud development stages are as follows:

Spring Growth Stages in Grapes
	Scale Crack (SC), is the first visible indication of growth. A small crack occurs between the hard outermost bud scales as the bud begins to swell.
	During First Swell (S-1), the bud has swollen out of the hard outer bud scale and is globular, light brown(doe-colored), and fuzzy. No green or pink is visible as of yet.
	When Full Swell (S-2) occurs, the bud has elongated (about 1.5 to 2 times as long as wide) one or more bulges of leaf tissue are visible and appear green or pink. The bud remains closed around the growing point.
	During the fourth step, Bud Burst (B), the leaves have separated at the tip, usually exposing the growing point. No leaf is, as yet, at a right angle to the growing stem.
	Expanded Shoot (ES) is the final step and occurs when the stem is 4-6 cm in length with 1-3 small leaves at right angles to the stem.

LT50 is a term used to signify the temperature estimated to cause 50% kill.
The following table gives the LT50 for Concord Buds at Different Growth Stages:

*The duration of conditions favoring a black frost is crucial. A black frost is when the dew point is less than the low temperature reached. The protection here is ALL via supercooling of the bud  and the length of exposure  influences critical temperature. For Michigan, the more conservative White Frost (HOAR FROST) conditions are more appropriate. A white frost is when the dewpoint is reached and frost or dew forms.   As water vapor condenses out of the air it gives up haet as it changes from a gas to a solid.  This heat warms the bud.

REFERENCE:
G.S. Howell,
Department of Horticulture
Michigan State University

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