“Why watch the temperature drop? I know that it is cold!”
The temperature, high or low, hot or cold, is one of the governing aspects of plant development. Consider that temperature, light, moisture and fertility interact with each other to determine if plant growth can take place and if so, how rapidly development can take place. Different degrees of temperature will determine if seed germination can happen and when and how rapidly it proceeds.
To explain: Seed of many perennials, shrubs and vines must be exposed to a given period of chilling (moist stratification) before the seed can germinate. Some seed like holly must be exposed to cold, moist conditions followed by warm and again cold before seedlings show above ground (two full years from sowing to shoot growth).
In a general way, gardeners understand that cold temperatures slow chemical reactions, and warmth speeds chemical activity. Understand that all plant development is based on chemical reactions. If seeds like beans are sown in cold soil (below 50 degrees F), in the spring the seed often rots before germination can happen.
Some plants, like Christmas cactus, require an exposure to chilling temperatures before they are able to flower. Indeed, temperature-induced dormancy is so powerful that even if plants have a restricted day length and dormancy that is satisfied, flowering cannot take place until the chill requirement is satisfied.
Trees and shrubs each has a level and range of cool temperatures needed before trees can flower. Called cold units, the number of days exposed to cold governs where apple trees and lilacs, among others, can flower and fruit.
Gardeners think of temperature as hot or cold. Plants experience temperatures on a more complex level.