Bob H outside his cold frame  -  Image by: Glenda J

While our Southern friends are still roaming around in their shorts and sandals we have to be concerned about which trees get stored where. I guess that’s the price we pay for not having fire ants, killer bees, or alligators.

For the purposes of winter storage we can classify trees as tropical, temperate, and hardy. Let’s take a look at each –

Tropical. I think of our last Caribbean cruise on the SS TooManyPeople. We saw lush islands covered with crepe myrtles, jaboticabas, ficus, and other tropical plants. Now there’s the ideal storage; move to the Bahamas. Lacking that, let’s first define “tropical”. The simplest way is to see what Bachman’s sells for indoor house plants. If it’s sold to live inside it’s probably tropical, or, if it comes from Florida it won’t like Minnesota, either. The best bet is to look up the hardiness zone on Google.

Tropicals in Minnesota have their own set of problems. The humidity is too low, light levels are too low, winter is too long, and, of course, the temperature is too low. Trees are pretty adaptive so we don’t have to totally duplicate their native environment. In the Fall we start moving them in when the nighttime temperature routinely hit 45F. In the Spring we put them out when the daytime temperatures routinely hits 65F (might have to trot them in at night). Let’s assume storage at a level between thriving and surviving. At a minimum, your indoor tropicals will be happy where you’re happy. You need to provide enough light for photosynthesis, enough humidity to prevent desiccation, and enough fertilizer to keep them healthy without promoting excessive growth.

Light can be natural or artificial. Natural light comes from a bright window. We happen to have a garden window on the south side of our living room. We’ve stored trees successfully there. The basic problem is duration. Around Christmas we only get 8-1/2 hours of sunlight and the intensity is pretty meager. Somehow our trees did make it through. The better solution is using shop lights. Standard cool white bulbs work fine. The problem is that you need lots of them. In order to provide enough intensity, lights need to be within a few inches of the foliage. We built a rack from pvc pipe that allowed us to hang lights on the front and back sides of our favorite trees. Regardless of location we’d rotate our trees once a month to even out their exposure.

Water and fertilizer. Naturally, you’ll want to keep your trees properly hydrated. This becomes a matter of selective watering. Not all trees use water at the same rate. Simply water as needed, just like outside. One difference is misting. It is very beneficial to squirt the foliage ever day or two. That helps control spider mites and keep the leaves clean, too. About once a month we’d soak the trees in the bathtub to help rinse out mineral buildup. We drew the line at showering with a tree in each hand. You don’t want to fertilize too much, just enough to keep the tree healthy. We’d do it at half strength every couple of weeks.

The other issue is pest control. No matter what you do, your tree will have bugs. We spray prior to storing them and then use a systemic insecticide either as a drench or granular. Even so, by mid-winter durable critters manifest themselves. At that point we use a spray orchid insecticide formulated for indoor use. Remember, anything you spray in the house stays in the house. Then again, maybe your significant other could benefit from a little insecticide.

Hardy. The other extreme. Hardy trees are those commonly used for landscaping, those that can live outside. These are the simplest trees to deal with. All you are going to do is spray for critters and set them on the ground where they are protected from sun, wind, and passing dogs with full bladders.

Both temperate and hardy trees need a dormancy every year. If you’ll remember your fifth grade science class, a tree stores its' energy (and sap) in its’ roots for the winter. Bear that in mind, we’ll come back to that. Remember the Halloween blizzard when we got 30” of snow on October 31? I felt very superior because my trees were put away and many of my friends had theirs on the benches, inaccessible. Not being fond of cold, windy, weather I tend to deal with my trees around mid-October. Don’t forget, though, you need to water the trees until everything is frozen.

Simply locate a protected spot, set the trees on the ground and mulch with something like oak leaves or cypress mulch. The idea is to use the earth as a heat sink. Heat will rise from the earth. We just want to insulate the pot so the heat moderates the pot temperature. Consider this, the soil seldom freezes below 36” and as soon as there is snow, the soil surface temperature is insulated from the air temperature. Just cover the pot up to the first branch with mulch and you’re good to go. Let’s get back to our science class. The reason we want a protected location, by that I mean shady, is that by February the sun has enough energy to achieve solar gain. Even on cold days sunlight can heat up objects, note your car’s interior when you get into it. The sun can desiccate your tree and the frozen roots are unable to replenish lost moisture, wind can do the same. Thick barked trees are less likely to suffer, but, trees like maples can be damaged. Thus, the protected location. We’ve dealt with sun and wind, how about critters? I wasn’t being facetious about dogs with full bladders. Animal urine, nesting bunnies, and gnawing rodents are an issue. We’d often put chicken wire around our stored trees along with a box of D-Con in a weatherproof container. That seemed to solve the problem. Once, a prominent member of our club lost much of his collection to mice gnawing bark.

In the Spring we recover our trees starting about mid-April. Whenever the forecast called for sub-freezing we just set the trees on the ground to moderate the pot temperature. The issue here is that new foliar growth is very susceptible to frost damage. When new growth appears, and has not hardened off, you may have to put the trees in the garage overnight.

Temperate. These trees are those that are left. I think of trees that survive in Chicago. All you have to do is keep these trees from falling below 10F. Once a tree is dormant it doesn’t need light or much water. As a gauge, I wait until leaves are shed to put the trees in storage. There are several options. I once bought junipers in mid-winter from a nursery that kept them in an unheated basement. The object here is to keep them either frozen or keep them from freezing. It’s the bouncing back and forth over the freezing point that causes the damage. An attached garage works fine, after all you keep your Coke out there and it doesn’t freeze. An enclosed porch on the north side works. Window wells and cold frames are great, too. Even a single thickness of poly film will gain one hardiness zone.

Trees go dormant around 40F. The closer you can get to that temperature the better off you are. We are fortunate enough to have a dedicated facility that we heat to 40F. If you have the facilities, even your hardy trees will appreciate indoor dormant storage.

If your trees are not frozen they will need occasional water. Just don’t let them sit soggy. Root rot is an issue and so is fungus. Keep an eye peeled for fungus, mold, and mildew. Remove any lingering fruit or flowers prior to storage. These are very susceptible to botrytis.

One of the problems with temperate trees is that they leaf out long before they can go outside. The low light conditions lead to large leaves and long internodes. The foliage is very tender and will tend to burn as soon as it’s exposed to sun or wind. We usually try to put them out in late April or early May. We put them in the shade when the forecast is for cloudy and rainy for two or three days. That seems to minimize leaf burn, but, does nothing for the oversized growth. Eventually we throw in the towel and totally defoliate the tree. We get a new crop of leaves within a couple of weeks. By waiting for a little while before defoliating, the tree has a chance to recover from the wintering experience.

Now you know everything I know. Go forth and store your trees. Just don’t call me if they don’t make it.


(For more wintering strategies check out the Wintering Strategies image gallery) .