And there are a few things you can do to help Mother Nature
The soil is too wet
A soil that holds excessive moisture over the winter will rot plant crowns. A winter that has a great deal of freeze-thaw cycles will create wet soil with a layer of ice over top and this is certain death for many plants.
While I used to keep my mulch over top of all my plants in the winter, I now pull it back from all the crowns at least 8-inches. This space allows the crowns to breathe and lose excessive moisture (as long as there isn’t a layer of ice on the soil). The plants also come up faster in the spring with this clear area around the crowns.
If you have problems with overwintering perennials or if you want to push the gardening zone on perennials, the single most important thing you can do is increase the drainage in the garden or grow those tender perennials in well-drained areas.
It’s too cold out there
Excessive cold will kill tender perennials planted in colder areas.
Butterfly bush is an excellent example. Many garden centers sell it as a hardy perennial but in USDA zone 4, it is marginally hardy.
Buy plants hardy in your zone
Plants in containers on a deck or in a garage
I remember some research from “back in the day” saying plant roots died at 5F or -15C. So if you’re trying to overwinter a plant in a garage or unheated porch, that’s the temperature you need to avoid. Note small containers also have water issues with the soil drying out plus the soil freezing and this is a double dose of death.
Many beginning gardeners don’t understand that the lifespan of a perennial isn’t “forever”. Most perennials live 3–5 years before they go to the great compost heap in the sky. The longer-lived ones (peonies, daylilies, hosta, astilbe) can easily reach 15–20 years but most others are shorter-lived.
In my perennial garden, I plan on losing 15% every year just “because”–with no reason other than “it’s dead”.
Who knows but everybody loses this one
In the nursery trade, we know that winter will kill a different plant every year. Some year, the unique pattern of weather might wipe out Shasta daisies. The next it might be bleeding heart. This won’t happen in every garden but there will be a general regional plant loss that will mystify every garden expert and cause a run on that plant in the garden shops. Plants that are otherwise bone-hardy will suddenly die with no clear reason. But it sure annoys the heck out of gardeners.
What can gardeners do?
Mulch helps even out the swings in temperature and it also helps hold the snow over top of the plants. You’ll lose more plants in a year with a fluctuating spring (from high to low and up again) than you will in a year where everything stays frozen until it is time to grow. While keeping my plants frozen a little longer with a thick mulch may mean the early spring bloomers are a few days late blooming, I know they are alive.
One of the greatest “mulches” for tender perennials is to put old Christmas tree branches over top of the very tender plants. This holds the snow in around the plant and keeps them happy until spring. Remove the branches when the snow melts.
How to think about your perennials in the spring
Do not give up on a plant until you know the plant. For example, every year I’ll get letters asking why this plant or that plant is dead. And I write back suggesting they wait another few weeks because the plant is a notoriously late starter (Coreopsis verticillata) or just sulking a little (Hibiscus).
When I moved, put my Hibiscus in a holding garden in October 05 and then to its garden location in April 06. It finally showed up in early July with two shoots, grew six-feet tall in 6 weeks and started blooming the end of August. Go figure. Even I had given up on this one but it surprised me.
If you’re poking around, you can check to see if the root is soft and mushy (dead) under the mulch or whether the root and eyes (small growing points) are hard (alive). If a lavender or woody plant, you can gently scrape the bark with your thumbnail and if it is bright green under the bark, it is alive. If brown, that branch is dead. Enough dead branches and the plant is dead.
Often it’s something the gardener does…
This always assumes that you put your plant in the right light and soil areas. If you try to grow a plant out of its preferred location, you stress it. If you stress it enough, it will winter-kill. So clay soils that hold moisture will rot out roots of plants that want drainage. Plants that want full sunlight and are planted in part sun will go into the winter weakened. I note you may get away with this for a few years but eventually, the stress of the growing condition will show itself over the winter.