Once we have suddenly dipped back into winter with below freezing temperatures, frost damage occurs. So what effect will this have on our trees and shrubs? The real question should be how much freezing will kill a plant. Freeze damage to plant tissue can be detrimental to plants. A light frost in spring typically doesn't cause major damage, with exception to very tender plants, but a hard frost (where air and ground freeze) freezes water in plant cells, causing dehydration and damage to cell walls. Cold injury is more likely to occur as the sun comes up. As a result of these damaged cell walls, the plant defrosts too quickly, killing leaves and stems.
Luckily woody plants are perennial, long lived plants that have evolved and adapted to such issues over tens of thousands of years. Woody plants are terrific at storing reserve energy in their woody twigs. That energy is stored in the form of starch (water insoluble carbohydrates) in ray cells and near buds. This insurance policy that the tree or shrub has developed is the main reason they can re-sprout new leaves and twigs (or water sprouts) when we prune too much off the plant, or an insect like gypsy moth eats all the leaves in the spring, or a late freeze damages the first flush of leaves.
A late freeze is certainly not good for our trees and shrubs that broke bud, because they expended energy starting to create new leaves that will not give them a return on the investment - making food for the plant, but they will not die! A healthy tree or shrub will have enough reserve, stored energy to create a second flush of leaves. It might take it some time, but don't be quick to give up on it and cut the plant down. The new growth killed by the freeze may remain brown and stay on the plant for a while before the new leaves emerge. Just give the plant time.
The bad news, however, is that flower buds are much more tender, and prone to damage from cold temperatures. If your flowering trees or shrubs (such as hydrangea) have begun to break dormancy, your blooms are gone. If you are trying to grow fruit on those trees in your yard, you might not be seeing much fruit set this year.
Best suggestion at this point, since we cannot control the weather, is to keep the plant healthy this season. Make sure it is protected from any leaf destroying insects (caterpillars) or diseases (anthracnose or fireblight), watered during hot dry periods this summer, mulched properly to reduce competition with grasses and weeds, and pruned properly removing only deadwood and leaving as much leafy growth as possible.
For more information on this and other gardening topics, visit the Penn State Extension website or contact a local office or Master Gardener for advice.