Winter Heater Protection for Japanese Maples

Winter is here and it's trying to kill your Japanese maples! Well, not really, but if you live in colder climates where temperatures can fall below -10 degrees Celsius then you need to take extra steps to protect your containerized trees especially. Trees in the ground, properly mulched have the ability to be able to withstand temperatures in excess of -20 degrees Celsius, depending on the variety/cultivar. Japonicums and Shirasawanums are particulary hardy and it's not unusual to see many of them survive beyond -20C without difficulty. Many Palmatums can also withstand these extremes as well, but regardless of species, potted trees are not this winter hardy. 

Although it doesn't seem like it, the earth is warmer than the air above it, which is exactly why trees in containers face colder temperatures than trees with their roots in the ground. I want you to keep -10C in your mind as the magic number for container trees. We generally don't want to expose our containerized trees' roots to anything below this without protection. So how can we protect them?

The most basic ways to help protect your trees include, bringing trees into unheated or lightly heated spaces. The trees need to remain in dormancy so temperatures generally need to remain in low single digits on the Celsius scale or low minus single digits. For example, -9C to 9C. The space could be a greenhouse, a garage, or a shed for example. The trees don't need light because the leaves, if any left on them, are not photosynthesizing. Less effective options include, bringing containers up close to your house/apartment in a fairly sheltered location. If you look at your house when there's snow on the ground you will notice that there's little or no snow directly next to the outside walls of your house. It's warmer there! It also doesn't hurt to wrap row/crop cover (some call it reemay) or burlap around the trees. If the outside temperatures are dipping to -20C, don't expect that moving your trees up against the house is going to provide the enough protection. However, when temperatures are less extreme, it's certainly warmer next to the house and you're better off placing the trees there than out in the open. 

You can also bury your tree's rootball in the snow. Basically make a snowman out of your tree. Snow is a great insulator and I have had many trees in containers survive harsh winter temperatures by having their rootball being buried. It can be labour intensive and it's not attractive, but if it's all you have then it's better than doing nothing. In the picture below, I have some rootstock trees which will be used for grafting. Last winter they were buried out in the open without protection other than being covered in snow. This year, they are under a shelter but unheated so I buried them in snow again. It's not ideal, but it can work.

Far more ideal and what I highly recommend is an electric greenhouse heater. One such heater is the Bio Green "Palma", and the optional external Bio Green thermostat. I've been using it for a few seasons and find it very useful. This heater is 1500W so it's basically like a large baseboard heater in your home. The one I use is 120V but 240V heaters are also available from Bio Green. A 120V heater is going to be more expensive than a 240V heater to operate however, a 120V heater can be plugged into your standard wall outlet.

The heater's ability to reach or maintain your preferred temperature is going to depend on several factors including, the building materials of your structure, whether it's insulated or not, and the size of the space, outside temperatures, and so forth. The heater itself has two dials. One selector to select either fan(summer) or heat/fan(winter) and one selector to choose the intensity. If you choose to use the optional external thermostat you dramatically increase the functionality because the digital display tells you exactly what the temperature is, and you can create a set point temperature of when you want the heater to come on. The thermostat does have a wide range from the minus side to the plus side and you can choose a Farenheit or Celsius scale. What I really like about this heater is that I can set the thermostat to minus numbers. This is especially useful when you aren't looking for warmth but rather a less colder space than the very cold space outside your shelter. And with Japanese maples, we have trees that are dormant so they don't need to be warm, they just need to be protected from extreme winter temperatures, if they are containerized.

Generally, in a smaller enclosure such as a little shed about 10'x10', you will be able to maintain a wide variety of temperatures. This gives the Bio Green "Palma" heater the ability to be used in many situations beyond even cold protection. You can use it to kick start vegetable seedling, or use as a fan for summer ventilation, and more. I have used the heater in situations where I required temperatures in the high teens (for grafting purposes) in an enclosure in February with outside temperatures well below freezing. And more recently, I am using it to take the sting off the very cold outside temperatures.

Recently the outside, overnight temperatures were around -20/-21C and so I had the thermostat set to -8C. In the large space where I was using the heater (approx. 26'x18') it came close to maintaining the set point but it struggled to reach the -8C set point. When the outside temperatures were a little colder than the set point, the heater was effective. The fact is, the space is too big and the insulation value of the plastic greenhouse is low, especially at night with no solar radiation (sunshine!). In this situation, I used a portable propane heater to compensate. The handy little unit called, Portable Buddy, by Mr. Heater.

Using a propane heater is not my preferred method. I'd rather not burn the fossil fuels. But from a business perspective, since I don't have a more permanent, sophisticated set up, and I have hundreds of trees to protect against extreme cold, I need to do what I can. I know what you're thinking, "Wait a minute, you were pumping up the tires about how great the Bio Green heater is but you're saying it can't keep up?". 

Everything has its limitations and sometimes we try to stretch those. And in my case, I'm stretching the limitations. For example, here at Simply Japanese Maples the main greenhouse structure while structurally solid, it's not optimal from an insulation perspective. It's fairly well sealed from air leakage but it's covered in a single layer of greenhouse plastic, versus two layers, which have fans which blow air between layers providing additional insulation value. Furthermore, we have no permanently installed heater systems such as a larger commercial nursery would have. We are doing what we can as we grow the business. Getting back to our situation, we only use one Bio Green heater, for a space that is much too large especially as the temperature dips lower and lower. With no change in the greenhouse limitations, I am fully confident that two of these Bio Green heaters would have accomplished the task without the use of a propane heater. For you, you likely would be using this heater in a significantly smaller space than 26'x18' so the Bio Green would be an excellent choice.

Now -20C is around the coldest temperature we get in our area, and it's infrequent. Typically, winter temperatures will range from just above freezing to about -12C. This means that for the most part I don't even need to utilize the heater until it gets closer to -10C. And when it does, it's not that hard for this single heater to keep the greenhouse temperature around -7 or -8C when the outside temperature is perhaps -10 or -12C. The lower it gets the more difficult maintaining our thermostat set point will be, without help. During the daytime in these extreme cold periods it's typically sunny and solar radiation heats the greenhouse up to about 10-12 degrees warmer than the outside temperatures. So, it might be -14C outside but -2C inside. Again, many factors affect what the temperature difference might be for you. At night however, in unheated greenhouses the temperature quickly equalizes and there's virtually no difference between outside and inside. 

By the way, the Portable Buddy when teamed up with a 20 lb propane tank can last several days. Again, many factors affect the duration. In our situation, in our large greenhouse, the propane is only on at night, on low setting, and we get many nights of use from it. The downside is that you can't regulate the temperature as well as the electric heater/thermostat combo of the Bio Green set up. The Portable Buddy has only a low and a high setting. On the flip side, it's a lifesaver for us since we only have one of the Bio Green heaters. And, it can be used on camping or hunting trips or other outdoor pursuits. I used to use a Mr. Heater "Portable Buddy" inside an ice fishing tent when outside temperatures were close to -30C. It would be toasty warm in that tent. Apparently, these heaters are safe to use indoors thanks to their carbon monoxide detection system which will turn off the heater if a potentially dangerous situation occurs.

For the record, we are not affiliated with or sponsored by the creators of the Bio Green "Palma" and the Mr. Heater "Portable Buddy". I just wanted to give you two great products that I've used that can be used to protect your potted trees against extreme cold temperatures. There are numerous ways and products out there to use to protect your trees. Remember, -10C is the "magic number". Below this temperature you are risking not only damage to the roots of your trees but also possibly killing your trees. If your trees are in containers they aren't hardy enough to survive well beyond these temperatures. If you want to see the real magic of watching your trees leaves unfold in springtime, then protect your Japanese maples from the extremes of winter cold. And if you live in coastal locations in the PNW, well, you are one lucky duck!  None of this really matter to you.

Stay warm everyone.


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