Winterizing Raised Perennial Flowerbeds

In My Humble Opinion
1

I’m an old farm boy, give me something to stick in the ground and I can generally make it grow.

My wife was having some back trouble, so last year I made 10 raised beds, all 4’ tall, ranging from 4’ x 4’ to 4’ x 8’. I then dug up and moved all my perennial flowering plants to the raised beds. Everything went great, didn’t lose a single plant.

Now the bad part. I only live here part of the year because it’s just too cold. Guaranteed - 20F for a few weeks and a few days will be -30F. Wind chills hitting -50+F are not unusual. Average about 40" of snow per year but have had over 100" twice since we bought the place. Other than a stray one every couple of years I never lost any plants.

After the first winter in the raised beds, I lost about 20 plants over all the beds. Again, these were all established plants that I’ve had for years.

“Aha”, I think to myself, “I should google to see if I should cover my beds since they are above ground.” Of course, Google gives me many different answers. Use straw, don’t use straw, use only mulched straw. Use grass, don’t use grass, use only mulched grass. And on and on for everything you can imagine.

So, I’m here for some real world advice. To mulch or not to mulch, and with what?

In case it matters, all the beds are homemade compost only. The plants are generally used to attract, bees, butterflies and hummingbirds. So, things like Butterfly Bush, Russian Sage, Milkweed, Bee Balm, Coneflowers etc.

2

The rule of thumb for successfully overwintering in containers (and essentially you’ve built large containers), is to plant things that are two zones below your own. So in USDA zone 5, you could successfully overwinter plants that are zone 3.

Of course, nothing is set in stone, and each winter is different. Mulching thickly (2”-3”) would certainly help. Doesn’t matter with what: straw, shredded bark mulch, shredded leaves. Even better, you could mulch and cover the entire bed with a permeable frost cloth.

If you’re feeling really ambitious, you could also wrap the exterior in burlap or frost cloth.

3

In that kind of climate, it sounds like a given that soil in the raised beds will freeze to below root depth. A bigger problem may be frost heaving, where plants are at least partially heaved out of the ground as a result of cycles of freezing and thawing. Raised beds warm up faster than surrounding soil and so are prone to this phenomenon when mild spells alternate with very cold temps.

I’d mulch fairly heavily with something like straw or straw mixed with shredded leaves. The goal is to keep soil in the beds at a fairly even temperature, not to try to keep the plants warm.

4

Thanks for the info. Googling frost cloth gives me a wide variety of prices. Do I lean towards the high price or will something in the middle work as well? For the mulch, in the spring do you remove it or just leave it to compost into the soil?

No thawing periods here. Once it starts snowing in November, that snow will stick around until spring. If it makes any difference I should mention that the snow covers all the beds completely. The ones near the driveway and sidewalk usually have 4 or 5 feet of snow covering them because the cleared snow is shoveled on top of them. The others will vary depending on how much the wind is blowing during the snowfall.

I’ll ask you also, what would you do with the mulch in the spring? Mix it in with the soil or remove it? I’m not big on the looks of mulch in beds during the growing season but moving 10 beds of 4" inches of mulch to my composting site every year doesn’t sound like much fun. Maybe I need to start selling compost to my neighbors. Thanks for the help.