By January most of them will be able to withstand temperatures 30 degrees below zero. Some could tolerate even colder temperatures. However, these same plants would be killed now if the temperature suddenly dropped that low. That's because something happens inside plants that allows them to prepare for winter. But first, plants must stop growth.
Growth slows for most plants as the days shorten and grow colder.
Scientists have found that short days and cold temperatures trigger the development of cold hardiness. But it isn't known whether the plant mechanism that slows growth also starts cold hardiness development or whether they just happen to occur simultaneously. You should avoid practices that stimulate late summer growth of trees and shrubs. For example, nitrogen fertilization in August or early September may encourage a late flush of stems that can't turn off their growth before frosts. Roots take up the nutrients and store them in the roots and stems, stimulating vigorous growth next spring.