We recently went through the coldest weather that I can remember for quite some time. What made it so rough on our trees was the duration of low temperatures. We sent out a quick note to remind everyone of the impending doom, but I thought we should go over our winter strategies a little more thoroughly.
Living in the Pacific Northwest is really great for growing bonsai. And it’s hard to find a climate much better than Portland itself. We get sunnier summers and believe it or not, drier winters than Seattle, and we don’t have the scorching sun of California. Our semi-marine weather allows azaleas and maples grow like weeds, while providing a buffer against hot summers and bitter winters. But not always. We can get lulled into a false sense of security only to be awakened by arctic air in the teens. Such was the case recently.
I haven’t found anyone who could identify tree damage due to cold with any reliability. There can certainly be some branch die back, and trees may just up and die. But it is really hard to tell if it was from cold. There are so many other factors that cause damage. But the best defense against cold is a healthy tree, just as it would be for any pest or disease.
And what is cold? That might be a matter of opinion, and when will a tree die or be damaged will vary by species, age, health, how it is potted up, and even where it is positioned in the garden. I personally don’t even think about the temperature unless it is going to dip below 25 or so. The first rule of order is to get the trees out of wind. Trees in a pot can be desiccated quite quickly by biting winds.
You can look up the cold hardiness of species online, and then throw it out the window. Or at least add a good 10-20 degrees. Trees in the ground enjoy the warmth of the earth, while our trees on the bench are all on their own. Ground soil rarely freezes more than an inch or two down, whereas the bonsai pot will easily freeze all the way through. A freezing rootball does not prescribe death, so don’t be misled. There are many trees that enjoy a good chill. But I can’t give you anything to predict a certain outcome. Just like you can’t predict a heart attack, but you can work on all the factors that would encourage such an event.
The thing to remember about low temperatures is that timing is everything. 20 degrees in November might kill some trees, but is pretty doable in January. Why? The trees drop into dormancy slowly. They don’t just flip a switch. You’ll notice some trees dropping their leaves early, and some hardly at all. Those are some clues to pay attention to. While the maples and elms are losing their leaves, many alpine conifers are still cranking away. Their waxy cuticle helps protect the needles so that they can function in more adverse conditions. It also allows them to function over several years. So the conifers are going into dormancy later than the deciduous trees, in general. Note how beeches and oaks tend to hold onto their leaves throughout the winter. The trees use this mechanism to protect next year’s buds, and thus these species seem to be among the most winter hardy, and are almost always the last to leaf out in spring.
I have quinces that almost never go completely dormant unless it gets really, really cold. By the time January rolls around, most trees are snugly set for the rest of the cold months and are able to handle cold temperatures much better. Let’s not forget that a cold snap can be just as dangerous in spring. My first exposure to BSOP itself was at a PNBCA convention in Beaverton in 1990 or so. The centerpieces for the banquet were all bonsai that had died the previous spring in a freakish cold snap in the teens during March, after many trees had started to leaf out. My rule of thumb is to always be prepared for cold weather by Thanksgiving and stay vigilant through March.
Early on, I had an alcove on the side of my house where sat an unused hot tub. This made a convenient location to cram all of my trees into for the winter. I simply walled it off with some plastic to keep the wind out. (You can tell that at this point, I was unmarried) As time went along, my collection got bigger in number, and the trees much larger. Winter with the hot tub was no longer an option. So I then left all of my trees out on the benches and prayed for moderate temperatures. When low temps came along, I huddled everything together under an open shed and wrapped up the whole thing to shoulder height in shrink wrap – what they use on commercial palettes. Silly, but it must have worked. Now, in our new place, I have nowhere to hide. And to my surprise, I can no longer move all of my trees on the spur of the moment. They’ve gotten bigger while my muscles have gotten smaller. So they stay out and I put them on the ground, if possible. The medium and smaller trees I move under the generous eaves of our house, where there is very little wind. A small respite, but essential, none the less.
Now, to strategies. If I know that cold weather is coming, I like to water my trees well, knowing that the wind will dry them out. This is especially important for trees undercover that do not receive our copious rain. Then be sure to let them drain well. If you are using good, granular bonsai soil, this is no problem. I then water them again after the cold has past. Since we wire our trees into the pot, some folks worry that they pots will break. This simply is not so if you are using regular bonsai soil. I have had only one pot break in over 25 years, and it was a nice one. The particular pine had not been repotted for 6-7 years, so the roots were packed very tight. And the pot had an incurve lip, so the roots couldn’t go anywhere. Add a little freezing water and boom, instant shards. I just taped the pot together until spring, since it was wired to the tree and there was no chance of it drying out.
Next, get the trees out of the wind. Snow is great, as it insulates the trees, but nature never seems to get things in the right order. A cold frame of some kind, recessed in the ground is the best. I wish I had that kind of space. Along the backside of a house is great. Longtime member Anne Spencer used to unpot her trees and place them in bark dust in a covered shed. She then had to repot everything the following spring. It worked for her, but I don’t recommend it.
My method is to leave everything out on the bench until I see a big event coming. Then I bundle up and spend an hour or two moving things to safer quarters. Plastic trays really help with the smaller trees and give just a little protection. Okay, call me crazy, but when I have had to leave a large tree on the bench, I sometimes wrap the pot in an old blanket.
Just nesting the trees into the ground will help a good 5-7 degrees, thanks to quantitative research by Mike Hagedorn. It takes some advantage of the earth’s own heat. And there you have it. It might seem a bit late, but we have to make it all the way through March, so there is plenty of cold yet to go.
This month marks the beginning of a new feature in the Tips articles. I am calling it Hard Won Truths. These will be just a sentence or two and will feature little tidbits of knowledge and advice that were either earned by costly mistakes, or because it took my dense brain too many words from my instructors before it sunk in. These are the hints that will really make a difference, starting now. If you don’t read anything else, read these.