Caring for Crepe in Colder Climes
A. Crepe Myrtles are supposed to be 'woody perennials'; like roses and rhododendrons, their above-ground growth persists over winter.
But even though their range seems to be extending Northward by the year, crepe myrtles are not 'normally' hardy North of UDSA Zone 7; and Muncie is either a warm USDA Zone 5 or a chilly Zone 6, which is why your plants died back to the ground four of the last five winters.
So, what to do?
First, ixnay on the black plastic. Although many people think that black plastic is like gardening duct tape, it's really only good for one thing I can think of, which is to cover garden beds where heat loving summer crops like tropical melons are growing in colder-than-ideal climes (hey; like Muncie!). You are correct that dormant plants can't process sunlight in the winter, but that's not the issue. Covering any plant in plastic, whether clear or black, could easily kill the plant by cooking it on sunny winter days. But you have the right basic idea.
It may be too late this year (I'm writing this in mid-December in Northeast Pennsylvania, and we have already had nights in the teens), but a windbreak could well make the difference in an in-between winter. But again, no plastic! Take a page from the cold weather fig growers handbook and drive stakes into the ground about a foot away from the plants and then wrap burlap around the stakes, NOT around the plants themselves.
(I suspect you want to use black plastic {quote} 'because you have it'. Those are often the last words a plant hears: "Why'd you use THAT?" "Eh; because I had it." You Bet Your Garden Rule #3: "Just because you have a lot of something doesn't mean you should use it in the garden. If fact, it generally means the opposite.") [See wood ash, fresh manure, sawdust, etc.]
Now, why are we wrapping the burlap around stakes instead of the plants themselves? Because burlap gets wet. Wet burlap alone can weigh enough to severely damage a plant. Then, when it freezes hard, severe damage can escalate to death. By allowing a small distance between burlap and plant, you cut the desiccating winds of winter, a seemingly small gesture that can greatly increase plant survival. Some dedicated fig growers in chilly regions (it is no coincidence that one popular fig variety is named "Chicago") will frame out wooden enclosures around their figs and then wrap the lumber in burlap. Opinions vary on the roof, but I would leave it uncovered of burlap, although a few extra boards up there would preserve more heat inside.
(If all this seems like too much trouble, don't grow semi-tropical or Mediterranean plants anywhere you get lake effect snow.)
Again, the fig comparison. Virtually all figs will regrow from their roots if winter kills the above-ground growth; but that doesn't give the plant enough time to grow figs. Protecting the above-ground growth, even imperfectly, means the fig will start out with a good amount of biomass and have adequate time to set and ripen fruit. A little trim in the Spring will help stimulate new growth faster.
Same with a crepe Myrtle. Their flowers appear on new growth, and those flowers are some of the last to appear on our landscape plants, so a little winter protection might be all you need. A nice mulch of shredded leaves around the base of the plant wouldn't hurt either.
Two final notes.
One: There are several cultivars of crepe Myrtle that have been bred to endure USDA Zone 6 winters, and these should be the first choice of people who want to grow these magnificent late-season beauties in chilly regions. Plant them in a protected location (at the highest spot on your property and/or near a wall or two) and you further improve your odds.
And finally: Pruning in a Myrtle-friendly climate: Leave the plant alone after the end of the season. When new growth appears the following year, trim off the amount that the plant grew the season before. That keeps the plant at a manageable size, and the flowers close to the eyes. Do NOT cut the poor thing back to the ground; that's 'crepe Murder.'
