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.
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.
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.
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.
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.
- The temperature under the box stayed warmer in February than in January, with a low of -6 deg C.
- 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.
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.
The following is the numerical data for this experiment.
- “Preparing Roses for Winter” by Iowa State University; http://www.extension.iastate.edu/article/yard-and-garden-preparing-roses-winter
- Photo of commercial Styrofoam rose cone: Rona Inc