by George Buehler 

Caring for your bonsai in the spring, summer or fall is a fairly easy undertaking (this is said tongue in cheek) since so much is written for these times of the year.  However, when it comes time for winter, not much is out there, although it seems like more has been written in the last few years.  If the tree is not cared for properly during the cold months, all the pruning, wiring and summer growth can be lost or damaged.  What is the best method to use for any particular tree?

The first place to start is to find out the particular species of tree you need to “winterize”.  Just because it is a pine doesn’t mean that it really can stay outdoors in the winter (i.e. Norfolk Island Pines wouldn’t make it outside in the type of weather we have in the Ohio Valley).  The USDA has a fairly extensive list of plants used in the landscape and their hardiness (hardiness zones).  However, I must stress that this is only an indication of what the tree will take when it comes to the cold.  This list will tell you whether the trees are:

Tropical – cannot take temperatures below about 50-60 oF for any period of time and really will suffer if they get to these temperatures.  They prefer much hotter temperatures, and, in general, once the temperature starts cycling toward the 55-60 oF overnight, they need protection.

Semi-tropical – can take temperatures to about 40 oF, but can’t be exposed to frost.  There are different types of semi-tropical plants; some can go to the lower limits (and really prefer it to ‘complete’ their yearly cycle), while others need protection when the temperature gets to the 55 oF range. 

Temperate – can take freezing temperatures and, in fact, needs to be exposed to a hard frost to complete their growth cycle.

As previously mentioned, the information you find in a botanical guide or hardiness zone map is only an indication when it comes to bonsai.  The information listed in these guides or maps pertain to plant material buried in the landscape.  If the landscape plant loses a few twigs due to freezing, you will hardly notice it – not so with our bonsai.  Of course, landscape plants have extensive roots, many are below the frost line and, therefore, these roots cannot freeze. The frost line is the depth to which the ground will freeze in the coldest winter in the area (generally considered about 18-25 inches in the Louisville area).  No matter how cold it gets, some roots will be in moist ground and be able to bring water to the various trees branches.  Again, this is not so with our bonsai in their small pots which are also several feet off the ground on growing benches.  Once the soil in the pot freezes, there’s no moisture available.  If the temperature stays below freezing for a few days and nights in a row, the trees could be irreparably damaged.  With the trees several feet off the ground, the plants are exposed to even more extreme ranges of temperatures and to the drying winds of winter.  Therefore, the temperature ranges above are only general guidelines.

Let’s look at what we should do now to protect our trees from the winter coming up.

Find out the Latin names of your trees and determine their general susceptibility using the USDA hardiness.

Look at other landscape plants near your benches.  Are they still blooming or sending out new growth?  You may have a different microclimate in the area.  However, be cautious.  Remember, the nighttime temperature can drop rapidly when the sun goes down, and plants on benches can be exposed to freezing temperatures rapidly.  Setting the trees directly on the ground may be indicated in this case.

Talk to other club members.  If they have had trees like yours, they can give you their experiences on “winterizing” their plants.

Start a log of your trees.  Keep track of where you have them currently located and note the low temperatures at night.  Also look at them in the early morning to see if they are showing any signs of leaf curl or suffering from the temperatures.  This sometimes is an indication that the plants are trying to protect themselves from the cold.  Don’t confuse the normal plant responses with trouble signs.  When the daylight starts getting shorter, the trees will respond accordingly by losing leaves or changing colors.  This log will help you next year to make better or timelier decisions.  Also, if there is a question as to whether the plants are suffering, move them to a protected location.  Better safe than sorry.

 Before we start discussing the methods of winterizing the different categories of trees, there are several things to do before any type of tree is put away for the winter.

Clean all dead leaves and other debris from the soil and branches.  Remove any cobwebs and examine the plant carefully for scale or other insects.  Use tweezers and cotton Q-tips dipped in alcohol to remove the scale.

If the trunks or branches have algae growing on them, remove it with a water spray and an old toothbrush.  Use of a very dilute amount of dish detergent will help remove the algae.  Don’t use a lot of force with the toothbrush.  You just want to remove the algae and any “critters” in the fissures of the bark.  Pay particular attention to trees with flaky or cracked bark.

You may want to use a dormant oil spray on your deciduous trees.  Mixing the oil with an insecticide at this time of year will also help eliminate scale and insects.  I have even read about using the dormant oil and insecticide on your evergreen trees at this time but haven’t tried it.

Examine the spaces beneath the nebari since these are  favorite hiding spots for some critters

Check the drainage holes of the pots and clean or open up if necessary.

Lush moss on top of your soil helps accentuate the bonsai.  However, in winter, it can be a haven for both insects and their larvae.  It is probably a good idea to remove the moss before you put the trees away.  You can peel it off the top of the soil and put it in a seed tray that has moist soil or sand in it.  Place the tray in a semi-protected area where it won’t be exposed to the hard winter rains but where it will get some rain.  You should put some screen wire over the top to keep out the marauding squirrels and birds.  When spring arrives, the moss can be taken out of the trays and placed back on your tree soil.

 TROPICALS

When the temperature starts cycling to the 60 oF range overnight, it is time to bring in your tropicals.  When I say bring them in, this may not be for the winter duration.  Generally in our area, we get a cold snap for a night or two.  If you have your tropicals outside for the summer, you will need to bring them in and then take them back out when the daytime temperature gets back to the 70-80 oF range.  This can be quite time consuming as well as frustrating.  My philosophy is to let the trees enjoy the outside as long as possible, so I take mine in and out for as long as the temperatures allow.

When it is time for permanent winter storage, where do you put them?  Several members put them close to a south-facing window where they will receive plenty of light during the day.  These windows are equipped with drapes or blinds that are closed at dark to keep in the heat.  You have to be careful if you use this method because with the sun shining in through the window, you can overheat your plant when we get one of the warm days we generally get during the winter.  You also have to watch the soil moisture level since it can dry out rather quickly in our dry homes.

One of our members, Gene Sewell, has grown tropicals indoors year -round using fluorescent lights.  Gene uses one Gro-lux and one cool white bulb and has three or four 4-foot fixtures above his plant trays, depending on the size of the tree.  Gene cautions that the cool white should have 3000 lumens in order to provide enough light. For those of us who take our tropicals outside in the hot summers, bringing them indoors when the weather cools and putting them under grow lights is an ideal way to keep them healthy.  Remember, the plants need light to function and even survive.  Use of incandescent lights is not recommended since they produce a lot of heat and can both dry out the soil quickly and burn the leaves if the light is too close.  If you move the bulb far enough away to not burn the leaves, the light benefit is greatly reduced.  Therefore, fluorescent lights are the only viable solution.  In general, the fluorescent should hang about 3 to 4 inches above the top of the plant (no more than 6”).  The tallest tree can almost touch the bulbs.  If you have a variety of different sizes, you can use a platform or box to raise the shorter ones.  You can use link chain to hang the fixtures so that you can raise or lower the fixtures as needed.  I have my lights mounted on a piece of plywood that has been painted with reflective paint.  The plywood is held in place by the link chain.  I use a timer to keep the lights on for about 12 hours a day in November through January.  In late January, I start increasing the time the lights are on about 15 minutes each month, so that by late April they are on for about 14 hours a day.  I do this to try to simulate the natural increase in daylight the plants would experience in the wild.

I use a simple metal shelving unit placed in the basement to put my tropicals on.  I have always used only two 4-foot fluorescents above each shelf. However, after seeing Gene’s setup, and talking to him about his experience, I am going to add an additional fluorescent fixture to give more light.  The temperature around the pots stays at about 70 o F.  I am considering getting a small electric heater with a blower that will keep the temperature closer to 70-75 oF,  since my basement stays at about 65 oF.  In addition to keeping the temperature slightly hotter, it will also move the air around the plants, which may help them grow during their winter hibernation.  This will dry out the soil quicker, and I will have to monitor the dryness more often.

I also put some of the plants on a humidity tray filled with small rocks and ‘try’ to keep water in the tray all the time.  I say “try”, since the trays dry out rather quickly.  If I use the small portable heater, the trays and soil will dry out even quicker.  I check the plants about every two or three days to see if they need watering.  If they do, I use water that I have obtained from the city supply but have left open for about a week next to the plant stands.  This lets the water warm up to the room temperature and also lets some of the chlorine evaporate from the water.  I also fertilize the plants with a low nitrogen fertilizer once a month during their winter storage.  I use Liquid Bloom full strength mixed with Pro-Tekt full strength (both are Dyna-Gro products and can be obtained from several mail order bonsai sites).  Any low nitrogen fertilizer should do the job.

 SEMI TROPICAL

 For my semi-tropicals, I wait until the temperature starts cycling around 45-50 oF overnight before I bring them into the house.  If there is a chance of frost, I put them inside and then, when the temperature warms back up, I take them back outside.  Once the temperature stays in the 45-50 range, I put them with my tropicals and treat them just like the tropicals.

There is one plant that I treat differently, and that is my Portulacaria.  In past years, I watered it just like the tropicals.  I determined (from my log) that the watering routine had to be monitored more carefully.  If I watered it the same as the tropicals, it soon started losing its leaves and suffered.  I kept forgetting that this was a desert plant that didn’t need large amounts of water.  Watering once a month is probably sufficient.  Once again, you have to know the species requirements and remember them or, as mentioned in the beginning of this article, keep a log and refer to it often.

CAUTION:  When the weather warms up and it is time to move your trees back outside, get them out as early as possible when the weather permits.  However, after a winter under fluorescent lights, you cannot put the trees directly out in the sun.  You will need to move them to a semi-shady area (remember the temperature in the shade is cooler than in the sun).  The basic idea is to gradually expose the trees to more and more sunlight.  This protects the tree from getting “sunburn”.  Agriculturists call the process hardening off and it is extremely important.  The process should take about two weeks if you are diligent and daily move them to more sun.  Another point to watch during the hardening off process is the moisture level in the soil.  This is especially important as the trees are in the full sun for several hours a day.  With the springtime “growth spurts” and the drying sun, plants can quickly dry out.  So check them regularly.

 TEMPERATE

There are probably more bonsai in this category than the other two groups.  It includes both deciduous and evergreen trees.  There appears to be several methods to winterize your temperate plants.  Some are more complex than others.  In general before you put your temperate plants away for the winter, they should go through at least two hard frosts, so that they are going dormant or are completely dormant.  Let them stay outside and unprotected until the temperatures start staying in the 32 oF range.  At this point, you need to begin your cold protection plan.

Winter protection of temperate trees falls into four categories: cold frames, unheated indoor protection, burying, and box storage.

 Cold Frames

There are several plans for cold frames.  Since our frost line in Louisville is 18-25 inches below the ground, the cold frame pit should be dug in a shady area (to prevent overheating from sunlight) about 4-8 inches below this level.  The reason is that below the frost line the earth is always above freezing temperatures.  Since the ground is always above freezing, it will actually heat the air above it; and since heat rises, the entire inside of the cold frame will be warmed.  Depending on several factors associated with the cold frame, the bonsai may freeze but it wont be as severe as if the trees were at ground level.

The best size of the cold frame is determined by how many trees you will need to protect.  Place all your trees that you want to protect on the ground close to each other – but not touching – about 2-4” apart.  Then measure this area.  Add about 4-6” to each side of the measured area, and this is the size of the hole you will need to dig.  You should line the inside of the pit you dug with something to keep the dirt wall from falling.  Wood will work but will rot in time (pressure treated lumber should not be used since the chemicals will eventually leach out and could damage your trees).  Solid fiberglass sheets (obtainable from lumber centers) are long lasting and provide additional insulation.  The fiberglass can be secured with metal rods or PVC piping driven into the ground.  The cold frame should be covered with a lid around the pit above the ground.  There are a number of plans for cold frames on the web.

The lid should be removable or easy to open and placed on an angle to the ground.  It can be solid wood or the old-fashioned storm window type.  This allows the frame to be opened either partially or fully, and also lets in some light if glass is used.  The drawback to the glass is that if the sun hits it, the pit will warm up quickly.  You should situate your cover so that it is facing away from the rays of the sun.  If you use wood, the trees will be in darkness.  If the trees are fully dormant, they don’t need the light (only some water) to make it through the winter.  This is only true if the trees are completely dormant.  Therefore, they should go through several “hard” frosts prior to putting them in the pit.  Evergreens also are OK with this method as long as the temperature is around freezing.

You will have to monitor the temperature in the frame to ensure that it doesn’t get too hot.  Moisture levels also have to be monitored.  You can purchase automatic openers that will raise the lid if the temperature gets too high.  I have also read that you can use the cold frame pit to put your trees in, pack them in well with cypress mulch, and have the solid wood cover several inches above the frame.  This keeps out the rain but allows the plants to remain cold and to get some air circulation

Cold frames are not easy to build but have been around for years, so they must work well.  An added benefit is that they can be used after the trees are taken out for germinating your spring flowers. 

I personally think a cold frame is a lot of work when there are other easier methods.  Also, cold frames are much safer to use in northern climates where the temperatures are lower and remain that way for longer periods of time.

 Burying

I have had discussions with several of our members on “winterizing” trees.  Both Lee Squires and Earl Cormney use a similar method.  Both take their trees to a protected corner of their back yard where they have placed a layer of mulch (cypress or hardwood).  They place their trees on top of the mulch with about a 2-4 inch distance between each pot.  They then pack more wet mulch around each pot and tree (up to the first branch).  They then put a plywood barrier around the outside of the area.  The height of the barrier is higher than the tallest tree in the area.  This protects the trees from the drying wind.  They then wet down all of the mulch, make sure it is pushed around each tree, and  leave it for the winter.  If we do not get any rain or snow, they will wet down the area when the temperature is above freezing.

 Box or Tub
Depending on the size of the tree and the number you have to protect, you can utilize what I call the “box method”.  It is similar to the bury method above.  I purchased several plastic storage containers large enough to protect the pot and lower part of the trees.  I drilled holes in the bottom to allow drainage,  put several inches of wet mulch in the bottom, placed the tree on top and packed more wet mulch around the tree.  The plastic container was large enough that I had several inches of mulch packed between the pot and the sides of the plastic box.  I used this method for junipers, pines and boxwoods, and it has worked well.  I place the trees in the plastic box next to the house where they are protected from the wind but get naturally watered.  The only time I would water them was if we didn’t get any rain or snow for a week or so. Then I would check them for dryness.
One other point on this method and probably could be used for the bury method, is that when I place the pot on the mulch, I would cover the soil and pot with several sheets of newspaper to make it easier to keep the mulch out of the soil.  After I put the newspaper on, I would wet the newspaper prior to putting on the final mulch.

 Unheated Indoors Protection

For the last 8 or so years, I have placed my deciduous trees (after they had lost all their leaves) in my unheated attached garage that has several windows in it.  The only evergreen that I place in the garage is my boxwood.  I had previously used the plastic tub method above to overwinter it, but I noticed at the end of several winters that it had leaf burn (probably due to the dry wind or severe cold snaps we had).  Anyway, I now put it in the garage with the deciduous trees.  They sit on an outside wall on benches the farthest point away from the house.

Several of our members have said this is not a good way to store them, but it has worked for me.  Bud break has not been a problem.  One year, it did get warm enough for the buds to start swelling, but this stopped when the temperature turned cold again, and it didn’t seem to be a problem in the spring.

I like this storage method, since I can take the trees inside to trim, style or repot without having to clean off any mulch.  I can also monitor the dryness with ease.  The only time I open the garage doors is to take the cars out.  On that point, I do not let the cars warm up inside the garage.  Once they are started, they are rapidly moved outside to finish warming up.

WINTER JOBS

Just because our bonsai are in their winter homes doesn’t mean that our bonsai jobs are done.  This is an ideal time to either sharpen our tools or get them sharpened by a professional service.  Display benches should be examined and repaired when the weather allows.  You may also need to build more stands or benches, and this is an ideal time to do it.  Look at your log and see which of your trees will need to be repotted.  Decide if you will use the same pot or whether you need to get a different one.  There is nothing worse than starting to repot a tree in the spring and suddenly realizing that you want to put it into a better pot.  Do you have enough soil and wire for the spring rush?  Last spring when you were in such a hurry to do all the springtime tasks, did you put some pots away unwashed?  Once the trees are hibernating, it is an ideal time to wash and sterilize your empty pots (washing them with a solution of one cup household bleach per gallon of water works well).  Also, if you use city water, you probably have a crust of salts on the top of your pots.  If the pot is empty, it can be scrubbed with household vinegar that will, with a little effort, remove the salts.  If the pot has a tree in it, you will have to scrub the pot with a stiff brush and perhaps pick off some of the scale with a knife.

 Hopefully, this article will help you get your trees through the winter months with no damage.  If you use a different method than listed above, send me a message and I will pass it on.