Why should we cover rose plants in the winter?

Question

We live in Minnesota, where it gets quite cold in the winter and the ground freezes well below the depth of the roses.

We have a row of rose bushes which we inherited from the previous owners of our home. For the first two winters we have followed the instructions given us by the previous owner: prune back to about 18", mound up dirt on each plant, encircle with chicken wire, fill chicken wire with straw (or with leaves and top with straw). We have not lost a bush yet.

My questions are: Why does this work, and what parts are necessary?

It's not preventing them from freezing, since the ground freezes down pretty deep. And it's not protecting them from wind chill, since they are roses and not warm-blooded animals who would perceive wind chill. (If the air temperature is -5, and the wind chill is -20, anything exposed to the air will tend toward -5 and not get any colder.)

The portion of the plant that is beneath the ground or the mounded up dirt will certainly be below freezing, and well below freezing as the winter goes on. It will, however, be marginally warmer than the air temperature. If the ground is frozen down to about 3 ft, and the average air temperature is, say, 15 degrees, then the ground between the surface (15 degrees) and 36" down (32 degrees) will be on a continuum between those two temperatures.

So does mounding up the dirt help to keep the plants slightly warmer than they would be without it (e.g. 20 degrees instead of 15)? If that is the case, I don't think the straw on top would contribute anything to this, or at least would be no better than snow piled on top.

It is to reduce the temperature swings? If so, are any temperature swings hard on the plants or only crossing the freeze/thaw line?

Why does covering the roses work, and what parts are necessary?

Answer 1

In the UK, where I live, none of this is necessary, so I've just been researching this subject. It seems that what you are doing, all parts of the procedure, are essential if you want your roses to survive undamaged through such hard winter weather. Primarily, mounding up is to protect the graft union, because if that gets damaged, the graft will fail; the loose litter protection added on top will help in protecting the buried stems from extra damage. Even if air temperatures are only -5 degrees, a wind chill factor of -20 degrees would actually dessicate, whiten and damage stems which are not adequately protected - plant stems can be just as vulnerable to wind chill and prone to damage in severe temperatures as any living thing would be were it not insulated in some fashion.

In fact, the method you've been using seems to be somewhat easier than the primary, trenching method recommended by the Minnesota Rose Society, and as it works (and follows the old and wise advice 'if it ain't broke, don't fix it') I wouldn't be changing any part of it if you want to keep your roses healthy. Their information is here: Winterizing Roses (Minnesota Rose Society). Similar advice is given by the University of Illinois, describing the same methods.

UPDATE: Further to your comments, I'd add that what you're doing when you protect your roses in this way is necessary because of where you live. The grafted part of the rose plant isn't something that will survive because of your cold winters, so if you want to grow plants like varieties of roses which would not naturally thrive on their own year round in your part of the world, then taking the steps you describe to protect them is, I'm afraid, unavoidable.

Answer 2

The rule for plants' vulnerable parts, from most vulnerable to least are; Deciduous leaves then roots then stems of woody perennials. It is the reason most deciduous plants living with real winters survive because they seasonally drop their leaves before winter. There are a few smart conifers such as Larch that shed their needles. This expands their territory to be able to survive where others can not. Not everybody or every plant can live in the tropics. The roots are very vulnerable and for that reason, plants in pots in freezing weather have a low chance to survive. The roots are exposed to the cold. Roots beneath the garden soil are protected even if the soil freezes (the water in the soil) because a large body of soil can regulate the temperatures and keep them fairly even,stable. Freeze damage happens when plant material thaws too quickly. The extra mulch protects that graft union and the roots. The grafted plant below the surface is usually a hardier variety of rose with a less hardy one above. Stems can survive and like J.Musser said just wait until spring to do any pruning or clean up of plant material. What does freeze above ground adds to the protection of the roots and vulnerable grafts. Snow cover is..a very good thing. Snow insulates. So if you leave the stems, old leaves they help form air pockets under the snow. Decomposition never stops, slows yes, but the decomposition process in the soil and mulch adds heat to the equation. If you are having solid soil freezing temperatures with no snow, that might kill a few plants, roots and all. Mulching your beds in the fall will ensure almost all plants will thrive. Covering the bottoms of your roses is really all one needs to do. Wrapping your plants with christmas lights and burlap would really be a bit over the top but will increase chances of success. Spring, learn how to prune roses correctly with bypass pruners, not anvil. Clean pruners with alcohol and pull that mound of mulch/soil away from the bottom of your roses for the season. In fall, cover them up again. If any of your stems die down to the graft, killing the upper plant, you'll get growth from the part of the graft below of the hardier rose stock. I've lived in dastardly cold places and only fragile, older roses have died. I now live in the most dastardly cold place for plants except for Antarctica right now, grins. I actually have blue berry shrubs that have made it 4 years! A little tough with pollinators and flower timing but they are still alive. No grafts but still...plants are far hardier than we think. Just remember to pull the mulch and soil away from the base of your roses in the spring, rake the pile off to a pile nearby that you can use again. Don't leave thick piles of mulch on the rose bed as that will inhibit water and air getting to the roots during their growing season. Great question!