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Styrofoam Cones – Do They Keep Roses Warm in Winter?

As winter approaches, we worry about our poor plants making it through the cold. All manor of wrapping devices are used to protect them and Styrofoam cones, also called rose cones, have become popular in recent years. Do these contraptions work? If they do, how much warmer do they keep your plants?

There is a lot of talk on the internet about Styrofoam cones. Many extension offices recommend them, and suppliers certainly tell you they work, but I was unable to find a single source of information that actually provided data to support the idea that Styrofoam cones keep plants warmer in winter.

So I decided to do some testing of my own.

Temperature changes with and without a Styrofoam Cone, by GardenMyths.com

Temperature changes with and without a Styrofoam Cone, by GardenMyths.com

Styrofoam Cones – How Do They Work?

Covering a plant, like a rose, with a Styrofoam cone will do two things. It will reduce evaporation due to reduced winds and it may increase the temperature around the plant.

The Styrofoam is thick enough to prevent wind from going through it so less wind will hit the plant. A reduction of wind, will reduce evaporation. The effect will be higher in evergreen plants than in deciduous plants since deciduous plants do not have leaves or stomata in winter – the main source of water loss.

What about the effect on temperature?

To better understand why a Styrofoam cone might increase the heat around a plant, have a look at my previous post How to Protect Plants from Frost. The only heat source is coming from the ground. Since Styrofoam is a good insulator, it should trap heat inside the cone. Let’s find out if that is really true.

Experiment Design

Last winter, I set up the following experiment.

Instead of a Styrofoam cone I used a Styrofoam fish box as pictured below. These boxes are used to ship fish around the country and can be picked up at restaurants and grocery stores for free. The thickness of the Styrofoam in these boxes is a bit thicker than some commercial rose cones, so my container might be a bit more efficient at trapping heat than a commercial product.

Styrofoam rose cone

Commercial Styrofoam rose cone

Most references that recommend using a Styrofoam cone to protect plants suggest putting some small holes in the cone. I am not sure if that is a requirement, but I did not do that in this experiment. The presence of holes might affect the amount of stored heat.

The Styrofoam box was set on the ground and a stone was placed on top to keep winds from moving it. The ground in this case was one of my garden beds. The soil is covered with about one inch of old wood chips. What this means is that there is not a perfect seal between the Styrofoam and soil/chips because wood chips have spaces between them.

A lab grade thermometer, accurate to 0.5 deg C was placed under the Styrofoam box, and left there. On selected days, between 10:00 am and 2:00 pm, I went out, lifted the box, and quickly read the temperature. This provided the temperature under the box. I then waited about 5 minutes for the thermometer to equilibrate to the air temperature and took the air temperature reading, making sure the sun was not shinning on the thermometer. The thermometer was then put back under the box until the next reading date.

Styrofoam cones - do they keep plants warm, by Robert Pavlis

Styrofoam fish box – do they keep roses warm, by Robert Pavlis

Results

The 2014-15 winter was a bit unusual. We got a bit of snow – maybe 5 cm (2″) in early January, but it did not stay around. The rest of January was cold, but mostly snow free. There was no real accumulation of snow until the first of February, when we received 10 cm of snow. New snow fall during the rest of February ensured that we had at least 10 cm of snow on the ground at any point, and most days it was more. The amount of snow was never enough to cover the Styrofoam box.

By early March the snow was melting and the experiment was stopped on March 13.

The above chart shows the results.

In January the temperature under the box and in the air were almost the same. On very cold days the temperature under the box was slightly warmer by about 2.5 degrees. On warmer days both temperatures were about the same.

In February two observations were made.

  1. The temperature under the box stayed warmer in February than in January, with a low of -6 deg C.
  2. The difference between inside and outside the box was greater in February (delta=7 deg) than in January (delta = 1 deg).

Styrofoam Cones – Do They Keep Roses Warm in Winter?

The answer is yes and no.

On two testing days, the temperature under the box was actually colder than the air temperature. The differences were small and can be explained by the fact that the Styrofoam box is an insulator. Changes in temperature will happen more quickly outside the box than inside the box. If the air suddenly warms up (either a warming trend or a sunny day) the air warms faster outside the box then inside the box.

On most days the temperature was warmer under the Styrofoam box than outside the box. Clearly the Styrofoam box does keep plants warmer, most of the time.

Why is January so different from February?

The months have nothing to do with it. February had the lowest air temperatures but January had the lowest temperatures under the box. The Styrofoam box worked better in February than in January. Why?

The difference between January and February, was the snow cover which did two things.

The snow on the ground made an air seal between the ground and the Styrofoam box. In January there was little or no snow and the seal between the box and the ground was not very good. Wind could easily blow under the box and remove any heat being held in by the box. Even if there was no wind, heat can more easily escape through gaps around the bottom edge of the box. It is similar to leaving your front door open a crack in winter – makes it hard to heat the house.

In February, a heavier snow fall made a good seal between the box and the ground. After each measurement, I was careful to make sure the snow was back in place to maintain the seal for the next reading. With the snow in place, the wind had no way to reach the inside of the box and cracks along the bottom edge of the box were sealed.

A second factor was also at play. With no snow on the ground, the ground around the box would cool off to the same temperature as the air, or very close to it. With no snow, the surface of the soil around the box was cold, making it harder for the box to keep the soil under the box warm. Once snow was on the ground the soil inside the box and outside the box were both warmer than the air, making it easier for the box to maintain it’s warm temperature.

These explanations may not be entirely correct, but they do follow basic thermodynamic principals. It is clear that the snow cover caused the difference.

Conclusions

Styrofoam cones do keep the air under them warmer in winter, provided that there is a good seal between the soil and the cone. In areas that do not have reliable snow fall, it would be a good idea to use soil around the cone to make the seal.

Styrofoam cones will not prevent the plant inside the cone from reaching freezing temperatures. In zone 5 (location of this experiment), a low temperature of at least -6 deg C should be expected.

Will Sun Warm The Cones?

The Iowa State University, in an article called “Preparing Roses for Winter” (ref 1), claims that “air temperatures inside the cones may get quite warm on sunny, mild winter days without ventilation holes. ” Other sites also make this claim.

Is it true?

The experiment in this post did not look at this point. However, on the warmest day in January, and on the warmest day in Feburary/March, the temperature under the box, was cooler than the air temperature. This suggests that the box actually keeps plants cooler on warm days.

This does make sense. If you take a Styrofoam cooler full of beer to the beech, the inside of the cooler stays colder even in the sun. Styrofoam is an insulator, which means it keeps the warmth of the sun out of the box.

The ground can still be frozen on a sunny warm winter day – warmth is relative – 0 degrees is warmer than -6 degrees. The Styrofoam cone will help keep the ground frozen, and keep the plant cooler, on a sunny day.

On a warm sunny day, ventilation holes will tend to warm up the inside of the box since it will let warm air enter. Do you poke holes in the lid off your cooler to keep the beer colder? Only when you’ve had too many!

This is not proof that the statement by the Iowa State University is wrong, but I think basic physics suggests it is wrong.

Adding the holes may still be a good idea to allow ventilation of humidity in very early or very late winter – I don’t know.

Raw Data

The following is the numerical data for this experiment.

Styrofoam cones, temperature data

Styrofoam cones, temperature data

References:

  1.  “Preparing Roses for Winter” by Iowa State University; http://www.extension.iastate.edu/article/yard-and-garden-preparing-roses-winter
  2. Photo of commercial Styrofoam rose cone: Rona Inc
Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

15 Responses to 'Styrofoam Cones – Do They Keep Roses Warm in Winter?'

  1. Paulette hackett says:

    I first tried rose cones 25 years ago.

    I did not poke the small holes & found that on warmer days mold covered the roses & eventually killed them..a lot of mulch & a surround with chicken wire filled with straw works just as well.Paulette, Gillette wy

  2. Marilynn says:

    I live northwest of Toronto, in a zone 4 environment. I’ve been looking everywhere for the styrofoam rose cones and can’t find any. I have about 15 old ones that I use every winter… I need a couple more to cover some new HTs and mini roses.
    Does anyone know where to find them in the greater toronto area?

  3. Gene Froc says:

    I live in Zone 3a of Saskatchewan and grow approximately 250 roses, about 1/3 of which are tender roses (hybrid tea and floribunda varieties), and about the same number of peonies, about 5 % of which are tree peonies. I have researched and tried many methods for winter protection over the past 30 years. In my area, the secret lies not in keeping the plants warmer during the winter, but rather keeping them protected against extreme freeze-thaw cycles in late fall and early spring when temperatures cycle between freezing at night and thawing during the daytime. The most critical period is during the early spring when the plants start to come out of dormancy and the buds start to swell. Do your best to keep your plants cold/frozen during this time or at least reduce the spread between maximum and minimum temperatures. The only practical way to do this is to provide insulation. I will not go into detail on how I do this, but this is the principle and it is supported by plant research conducted elsewhere at some universities. Note that it is practically impossible in my area to provide enough protection for those very tender plants that cannot at least tolerate some freezing. I have (at times) considered using a heat source (e.g. thermostat controlled light bulb placed inside a Styrofoam box), but my wife said that she would send me to a mental institution if I went to that extreme. So, I have resigned myself to the fact that I can’t grow some of the most tender plants 🙂

    • I agree with what you say. Maintaining a steady temperature may be more important than keeping a warmer temperature. I am not sure how well this has actually been tested. Having said that there is usually a lower limit of temperature that kills the plant.

      • Gene says:

        Yes, absolutely there is a lower limit for some plants. For example, I have found some hybrid tea roses that cannot withstand temperatures colder than minus 20 degrees C +/- even with all kinds of protection, whereas, I have some that can tolerate minus 35 to 40 (also with protection). I have also found that some Austin roses, that are supposedly very tender and cannot be grown in my area, are just as cold resistant as the Canadian bred hardy roses.

  4. growerjenn says:

    I sure appreciate reading your thoughtful and fact-based articles. I do not have roses on my land — mainly because I can’t eat them; and also because in my southern Montana zone 4 yard, it is too much work. However, I do have experience with a job I had for a woman with no such hesitation — she just paid me to do it. I coned 50 or so hybrid tea roses every year in November. I first pruned them quite severely so they would fit under the cones, some also needed to be tied. Then the cone went on with staples and a 5 or 6 inch deep and wide pile of soil around it. Into the cone went loose compost about halfway up the cone. No holes were in the cones. We generally have 2 or 3 weeks per winter when the temperature is consistently 20 below zero F. When I took the cones off in the spring, often there was ice still in the compost, and occasionally some damping damage on the roses, but they always survived. But it was very costly and very time consuming work. The roses were beautiful, though, and made the lady very happy.

  5. garcan says:

    Your interesting experiment illustrated that the moderating effectiveness of such cover could be sufficient for plant addicts to keep a few less hardy favourite plants (for their zone) i.e. with minimized extreme temperature dip (zone pushing?).
    Your data for no snow cover may be just showing the effect of a leaky insulated house. This can, perhaps, be mitigated by some form of mulching like wood chips to seal along the bottom edge?
    There are a few ‘I WANT IT’ roses I wouldn’t mind trying this cover to keep them alive if I can get hold them at all.
    It may be interesting to extend this experiment to compare the effectiveness of various winter protection tents available is the stores. I know of one gardener in Edmonton who uses such tents to keep a dwarf Japanese maple and a couple of Rhodos very lively for a few years now.

    • I agree that you can probably push the hardiness zone by one with the cones, provided you get a good seal around the bottom.

      I am fairly sure the effectiveness of any product will depend on the insulating properties of the material. Thin Styrofoam will give less protection. Other materials that are thinner and more heat conducting will provide even less protection. This is all fairly basic physics.

  6. Hi Robert
    I too am a Master Gardener and enjoy your contributions to the email list. My own theory about roses and winter is this: roses are either tough enough to survive winter and thrive the following year, or they aren’t. I do hill them up with compost in the late fall, but mostly for amending the soil as I don’t fertilize them either.
    The ones who survive are gorgeous, and the ones who don’t, I treat as annuals and they get shovel-pruned.
    I have over 85 roses in a small garden- with more planned – and the idea of covering them all with white boxes does not appeal to me…I am giggling at the thought of how it would look…like some extraterrestrial landscape : )
    Thanks for your research….
    Cauleen
    http://www.lush-gardens.com

    • I think 85 Styrofoam rose cones would look quite nice!

      I garden the same way. If it does not survive – I can go buy more plants. Why grow things that need to be covered?

      I can grow just about everything, but for some reason roses and I just don’t get along. I’ve never done well with them, but I have a few good ones, and I am building up a collection to try again.

  7. Deborah says:

    Also, the temperature variance under the box was less compared with outside temperature. Wouldn’t this also be a good thing?

    • I agree. The data does not show this very well, but the temperature under the box was modulated. We had some days where the outside temperature changed very fast. Under the Styrofoam box temperatures changed very slowly. As far as I know that is good for plants.

  8. rogerbrook says:

    Some of your figures, especially when cold is severe, are quite impressive in respect of the insulating properties of styrofoam, Robert
    I wonder how much the elevated temperatures under the insulation would be valid when the cold is prolonged – as opposed to temperatures dipping overnight.
    My impression is that my unheated greenhouse holds its temperature reasonably well overnight, but when cold persists for a long time and their is no sunshine it is nearly as cold as outside. The 2010 Winter over here went down to minus 20 degrees centigrade for several weeks and the temperatures inside my greenhouse were just as cold as outside
    When I look again at your figures they seem to answer my question – that the effect of the foam does maintain differences over longer periods ?

    • It is a good question. I don’t have the data to prove this, but I think that in a constant low outside temperature, the temperature under the Styrofoam would reach a constant temperature in about 12 hours. After that it should remain constant – provided there is snow cover. The reason for this is that the amount of heat coming from the soil is constant. The loss of heat through the box is constant for a given outside temperature. So the temperature inside the box should remain constant – provided there is snow cover.

      A greenhouse is different in that the volume of air above the ground is much larger, and it is not as good an insulator. The heat loss will be greater than the heat produced by the soil. If you cover your greenhouse with snow – it would work better 🙂

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