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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?

Sue Saddest Farewell TGO GL
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MRD
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  • Welcome MRD! This is a great first question, full of the details we love at this site! Do you have a picture or two you can post? Even though the question is about a general concept, seeing your specific situation would help us and future visitors. The [ask] page in our [help] explains why pictures are important. Also, they attract people to the question, so you can get better answers. I see you've already heard from one of our many experts! More answers might also come along. Please stay with us and have fun! – Sue Saddest Farewell TGO GL Dec 15 '16 at 20:31

2 Answers2

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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.

Bamboo
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  • Thank you for your thorough and gracious answer. The fact that we've lost no roses argues strongly for continuing what we've done. However, I am not persuaded that protecting the branches from the wind matters. It does not matter with regard to temperature. Is it that the wind would dry the branches out? In the spring, we cut them back to about 6", and either new growth comes from those or there are new shoots that come up, so those branches drying out does not seem concerning to me. The entire plant is frozen thoroughly, so there would not be the movement of moisture through the plant. – MRD Dec 15 '16 at 01:50
  • @MRD Wind chill hits plants hard. Even in my area (USDA zone 6b), roses on hills and in wind ways lose a lot more growth to winter burn. than roses in valleys and by walls. In my area, fig trees have to be covered the way you described, roses don't need anything (except perhaps mounding if it's grafted). They get pruned in spring here, so winter killed areas can be removed with the regular pruning. – J. Musser Dec 15 '16 at 03:22
  • Thank you, J Musser. I suppose there is something to the loss of moisture due to wind. What else could it be? It's not a matter of wind chill, for the temperature of the plants is the same as the air temperature, wind or no wind. – MRD Dec 15 '16 at 04:06
  • @MRD: temperatures area complex topic. I agree that will chill has not much effect (it is a an effect on human skin), and plants (compared to humans/animals) cares much more about temperatures then heat loss/gain. The cover has much to do with **micro-climate**. Just covering has an huge effect on ground, because of blocking radiation of ground. I would say that during night radiation has a larger effect then the air temperature (but locally, near ground level, the air temperature is heavy effected by radiation). – Giacomo Catenazzi Dec 15 '16 at 07:49
  • @MRD: The temperature of the plants is NOT the same as the wind chill temperature, because they're buried and covered, which shelters them, though they will be pretty cold because of the ground and air temperatures.. This is about optimum care of your roses rather than just hoping they struggle through with some damage. – Bamboo Dec 15 '16 at 12:42
  • Thank you Giacomo. I believe you are right about the micro-climate and the radiation. That still leaves the question of whether hovering around 15 degrees is better than 10 or 5. I've never read anything that describes at what point the plants undergo damage due to temperature. – MRD Dec 15 '16 at 13:16
  • Bamboo -- thank you for chiming in. The temperature of the plants -- or rocks or anything else -- is not the same as the wind chill because nothing is. The plants will never be colder than the air temp. The covering shelters them, yes, but the ground will still be well below freezing. And I am not opposed to optimum care. I simply wish to understand optimum care, and I sense that various sources just repeat what others say without an understanding of why certain methods are effective, especially when there are references to protecting plants from wind chill. – MRD Dec 15 '16 at 13:21
  • @MRD - it is not true that the ground will never be colder than the air temp - even in the UK, in,, say, February or March, the air temperature can be 10 degrees higher than the ground temperature following a long and very cold winter. Soil freezes if temperatures remain below zero C day and night for a week - the depth to which it freezes depends how extreme and how long the cold temperatures persist for. Even we get occasional winters where crops have to be drilled out of the ground. Air temperature and wind chill factor are not the same thing though. – Bamboo Dec 15 '16 at 13:34
  • Of course as the air temp lowers and raises, the temp of the ground will trail behind. Whether harsh winter or mild, the ground will be colder than the air in the spring. Regarding wind chill -- wind chill does not make anything colder than the air temp. That's the point I was making. The ground will tend toward the temp of the air, and while the presence of wind will accelerate the change in ground temp, the plants will not ever become colder than ambient air temp. The ground cover is not protecting plants from wind chill, and are keeping the plants only marginally warmer than the air temp. – MRD Dec 15 '16 at 13:45
  • @MRD Yep, but according to the rose society, wind chill does dessicate and whiten stems. Up to you what you do, but I have to say, I don't like roses much anyway, and if I had to faff about in such a way every Fall, I wouldn't bother to grow them at all, so I admire everyone's dedication over there! – Bamboo Dec 15 '16 at 13:49
  • Thanks to all who have shared their observations and experience. I'm grateful for the insight of others. It seems that the loss of moisture due to wind may be one factor in how well roses survive the winter. I accept that. I still wonder how temperature affect the plants. Where I am, zone 4a, the plants will certainly be well below freezing. I have not read anything about whether it matters whether the plant is 15F or 5F or what. – MRD Dec 15 '16 at 13:51
  • @MRD - Primarily its the risk of damage to the graft - damage that, and you may end up with the rootstock being the only thing left growing. Leave a couple not covered up so you can see what happens, as an experiment. – Bamboo Dec 15 '16 at 13:56
  • What damages the graft? Temperature? If so, what temperature? – MRD Dec 15 '16 at 13:59
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/50138/discussion-between-bamboo-and-mrd). – Bamboo Dec 15 '16 at 14:00
  • 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. 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 – stormy Dec 15 '16 at 23:26
  • Roots beneath the 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 stable. Freeze damage happens when plant material thaws too quickly. The extra mulch protects that graft union and the roots. The graft 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 gets frozen adds to the protection of the roots and vulnerable grafts that are still viable. Snow cover is.. – stormy Dec 15 '16 at 23:33
  • ...a very good thing. Snow insulates. So if you leave the stems, old leaves they help form air pockets under the snow. 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 all you need. Burlap over the stems is a luxury that does add more 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! – stormy Dec 15 '16 at 23:37
  • @MRD and all, I've [asked a question about it](http://gardening.stackexchange.com/questions/30232/does-wind-chill-affect-plants), just to see people's organized thoughts. – J. Musser Dec 16 '16 at 00:42
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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!

stormy
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