Question: Why is it that plants can be killed by temperatures one time and not damaged at all by the very same temperature another time?
Answer: There are several variables. One is called “hardening,” where a plant is gradually conditioned to lower and lower temperatures for several weeks before a really hard freeze. Using oleanders as an example, they can normally withstand temperatures to 15 degrees if they have received prior conditioning at, say 30, 25, and 20 degrees over several weeks. However, if the first freeze hits 20 degrees, those same plants may be frozen to the ground or killed entirely. How much wind accompanies the cold also enters the picture, as will the number of hours that the minimum temperature is maintained. You’ll see more damage in a citrus orchard, for example, if it’s below freezing for 15 hours than you will if the same temperature is held for only 3 hours. You’ll see more damage if you have applied a high-nitrogen fertilizer just a few weeks prior to the cold, and you’ll see much more damage if the plants are in water stress when the low temperatures hit.