When and How to Prune Your Frost-Damaged Plants
When a frost leaves your plants wilted and discolored, your first instinct may be to prune them immediately. Those ugly, damaged leaves must go. Pruning your plants too soon after frost damage can, however, further damage or even kill them. The wilted foliage will actually protect your plants during future frost and freeze events.
How the damaged leaves protect the plant beneath
When frost damages a plant, it impacts the most exposed portions first and worst. Foliage at least partially shields the inner structure of the plant from the frost, thereby lessening the damage. When you see wilted brown leaves, you may think the branch they're on is also dead. In fact, that may not be the case. If you move the dead leaves aside, you may see undamaged growth beneath. Sometimes it may be necessary to look all the way down to the plant's base before you see evidence of life.
The dead and dying leaves impacted by the frost form an outer layer of protection around the portions of the plant that survived. Depending on the plant, that dead layer may remain in place throughout the winter and continue to provide protection to the rest of the plant until the frost threat is over. Removing it will expose the still-living parts of the plant to future frost events. Additionally, removing dead foliage stimulates new growth. Future frost events will kill the tender new growth and possibly the entire plant.
Don't even think about pruning until after your last frost date
You may not want to, but you should leave those ugly dead leaves in place through the winter. Wait to prune until the threat of frost in your area has passed and evidence of life within the plants is visible.
How will you know when the frost threat has passed? Numerous websites post the predicted last frost dates for your area. Perhaps the best known of these is the Old Farmers Almanac site at almanac.com/gardening/frostdates. You can read more about average last frost date predictions and see the date for your area by simply entering your city and state or ZIP code.
To be safe, wait to begin pruning until two or more weeks after the last frost date for your area has passed. Waiting will allow you to more easily identify the still-living parts of your plants and new growth -- the parts you shouldn't prune away.
Depending on the severity of the winter and the type of plant, new growth may not become evident until a couple of months after the last frost date has passed, and even then, it may only be visible at the plant's base. For this reason, if you find no evidence of life even a few weeks after the last frost, you shouldn't give up on the plant and remove it, nor should you prune it. Check it again in a month or so for new growth.
If you just can't wait...
For those who can't bear the thought of looking at ugly dead foliage for weeks or even months after the last frost date, you can lower the risk that early pruning will damage or kill your plants if you first carefully examine the plants' branches. Once the last frost date has passed, look for color differences that may indicate which branches or parts thereof are still living and which are not. Living areas should be greener.
You can also check branches for signs of life by bending them. If they snap, they're dead. If they bend, they most likely have some life left in them. Whichever method you use, the living parts you identify should be left in place.
Ways to prune plants damaged by frost
Once you've differentiated between the living and dead parts of your plants, it's time to prune. Depending on the damage, you'll either need to perform what is known as "renewal pruning" or just cut away the dead outer growth.
If the only growth evident is near the plant's base, renewal pruning is the best choice. You'll need loppers, and perhaps a saw, to cut away everything but the base where the growth is visible. This could mean leaving only a few inches of the plant exposed above ground. The roots are the least susceptible to cold and frost, so plants pruned using the renewal method usually begin to show new growth around the base soon after pruning.
If the frost damage is limited to outer branches and leaves, simply use loppers or hand pruners to cut those away and expose the unaffected areas.
Whichever method you use, pruning will not only remove that ugly dead layer but will also stimulate new growth. If you're patient and careful, you should see positive results soon.