Ask Mr. Smarty Plants

To quote from one of our favorite books, Botany for Gardeners, Revised Edition, by Brian Capon:

"In winter, when the leaves of deciduous species have fallen, water movement comes to a standstill in the plant. If the remaining water freezes in its cells, its expansion ruptures the delicate cell membranes-a condition from which there is no recovery."

Very generally speaking, plants die without water in their systems. In a quick frost, some of the top blooms or leaves might get brown or fall off, but if the water in the stems is not frozen hard, the plant itself will survive. A longer frost, a real freeze, can freeze that stem moisture to the point that the plant dies from dehydration, as well as the damage from expansion of remaining water, as cited above. Many plants, especially what you might refer to as "flowers", are annuals. They really are only programmed to live one season, bloom, make seeds after blooming, spread the seeds, and then their purpose in life, which is to make other plants just like them, is finished. If they're still getting water and sunlight, they may linger on for the warm months of fall, perhaps even put out a few unenthusiastic blooms, but it doesn't take much cold to convince them it's time to fold the tent. Their seeds will sleep, insulated by the earth, until warmth, water and sunlight in the spring cause them to sprout and rise again.

Other plants, usually referred to as perennials, which can include flowering plants, shrubs and trees, have deep root systems, again kept warmer by the earth. In its dormancy, and often without leaves, the plant is able to draw nutrients from the root material. The plant may drop leaves and blossoms, twigs die on the end and the whole plant can look like it's dead but, again, warmth and moisture will call them back to life after several months of dormancy.

More specifically, lots of grasses have still more protection, in that they already are growing low to the ground, where there is a thin layer of air warmed by the reflection of the sun on the earth. Decomposing organic material in the soil causes warmth to be generated year-round, which protects roots from freezing. There are parts of the world, of course, where even the soil freezes, and the plants that survive from year to year there have even more sturdy survival tactics. In parts of the world where it gets that cold, even grasses often have to be replanted every year. However, in the more temperate parts of the world, many grasses have rhizomes, underground root systems, really tubers, that can go as much as 18" deep in the dirt, ride out the cold, and pop up, usually where you don't want them, at the first sign of warm air.