Chillin’ isn’t just for teenagers…plants need it too. Spelled a bit differently, though.


For plants, this is serious, un-boring business. Without chilling, the plant won’t be able to grow in the spring. Without hardiness, the plant may be damaged by cold weather. Let’s break this down a bit.

There are two things at work here. Hardiness. And Chilling. 

  • Hardiness: The ability of the plant to survive cold. Light frosts improve the ability of a plant to withstand deeper cold. Once plants begin growing in spring they gradually lose their cold hardiness.
  • Chilling: Time, in hours, needed in the 33-50 F range before the plant will grow in spring. Freezing has no effect on chilling, and time spent below freezing is not ‘logged’ in the plant’s countdown. Temperatures above 60 are detrimental to chilling. Unmet chilling requirements are what prevent a plant from growing too soon in the spring, and if chilling requirements are met and the plant is still not growing it’s because the weather is too cold. Once temperatures are in the 40’s trees generally begin growing.

So. What does this mean to those who grow temperate species as bonsai?

  • Some plants have a lot of chilling requirement or need a higher temperature in spring before growth is initiated, and burst out so late in spring you might wonder if they’d died, such as beech. Others have very little chilling requirement, like some southern blueberries. You may notice differences between coastal trees and high elevation or northern trees. Knowing these differences help us to anticipate and protect those that will begin growing earlier in spring.
  • You might notice some kinds of growth, like flowering, that may happen while the snow is still flying on camellia or ume (flowering apricot). These plants will still take a while to begin shoot growth, however, which is more tender to cold.
  • Understanding chilling also helps us to program trees that are in need of chilling for health, such as temperate trees we’re trying to keep in warmer climates. In those warm areas, we may need to place temperate bonsai in the shade, keep them cooler, and keep that up for as long as we can in the winter to help them meet their chilling requirements.
  • Although understanding whether our trees can or cannot survive freezing is very important, hardiness maps for plants are nearly useless to bonsai growers, who have root hardiness to consider. Roots are considerably more tender to cold.
  • Be very careful in the spring, when bonsai are growing and are beginning to lose their hardiness. Even very hardy kinds of plants like spruce, once growing, can be damaged by cold. And cold damage is irreversible, one of those things where one night of prevention is worth years of bonsai training. Isn’t there a saying like that?

Also take a look at the first in this set of articles on cold and bonsai: http://crataegus.com/2015/11/19/seasonal-care-for-cold-weather/

Every year I try to wrap a blog post around the snarly issue of protecting our trees from cold. This year we’ll try a new approach, and take it in a few bite sized chunks, in a couple of blog posts.

Firstly, we need to bear in mind that the top of a tree or shrub is much more cold hardy than the roots. When we read about the ‘cold hardiness’ of a plant in a garden book, that designation is assuming the darn thing is the ground, where it’s supposed to be. The designation takes no heed of silly bonsaiists who will put it up on a bench. Roots are much more tender than their tops!


A lot of people don’t know this, apparently even a few snow sculptures…

Nextly… on the ground is a good first place to put a bonsai in cold weather, generally when it is dropping to about 27 F / -2.8 C overnight. On the ground a pot might be as much as 7 F warmer than on a bench, three feet higher (according to our experiments here at Crataegus Bonsai). Which is a significantly warmer temperature, if you’re a root, and you’re used to being in the ground.

Once the thermometer drops lower, below 27 F / -2.8 C, many temperate trees and shrubs need more protection. You might need a greenhouse, poly tunnel, or coldframe, either for the whole winter or for the short severe cold snap that might last a few days. Many trees are OK outside on the ground down lower than this, such as some mountain pines and junipers, but beware the wind/cold combo…

Wind can be as damaging as cold, and both together are a real whiz-bang yikes thingy that can deposit a dead tree at the doorstep of spring. A frozen rootball with wind is seriously not good. The bonsai can desiccate, causing if not death often some branch dieback in the growing season. Keep your trees hydrated, and keep them frequently thawed out (even if they freeze occasionally, they shouldn’t remain that way).

You can work on many bonsai in the winter, including wiring and bending. If you’ve recently wired a tree, or done severe bending, however, they cannot be put out in wind and cold. Protect them.

Beware also of keeping temperate bonsai at too high a temperature overwinter, which may cause weakening the following year. Keeping bonsai over 50 F / 10 C creates some problems with chilling requirements and the ability to grow out in the spring nice and strong.

Next up on this miniseries…’Winter dormancy and chilling requirements’, which will get a bit nerdy…


(Please note: All temperature notations are approximations, and everything related to cold hardiness also depends on the species of tree, how late in the year it was growing, its health…all those things and others play into this discussion).

…or get smaller, to be more accurate.

When we style a juniper, very often the live vein changes in size. It’s normally an adjustment to foliage loss, or branch loss, both of which tend to happen during styling.

Most commonly, the sides of the vein will shrink, so that the vein will become narrower. Except on the very youngest of trees, a vein will never become wider. (This reluctance to cover a wound is why we don’t cut off a large branch cleanly to the trunk, as one might on a pine or maple, expecting it to callus and close up. A juniper won’t do that, so we make a jin).

Although it won’t widen, over time the live vein will grow more bulbous, growing out from the trunk. The top of this juniper shows an old live vein that has a rounded character to it. This is a very old live vein. The deadwood on either side died many years ago, probably decades ago in the mountains.

With this juniper we got lucky. The vein diminished to the point that it unexpectedly added complexity. It added a spiral and became more dramatic. On this Rocky Mountain Juniper, I noticed that the bark was getting loose in one area, and after investigating discovered that the live vein had retreated after the tree was styled years ago. There was a fair bit of dead bark that could be removed. I did that, and cleaned up the exposed deadwood with sandpaper. Now the tree looks quite a bit jazzier than it was…


A spiraling live vein on a Rocky Mountain Juniper. On investigation it was found that the live vein had retreated, and there was some dead bark to remove. The reddish areas are the dead areas—compare with the photo below, after removing the dead bark. (‘Dead bark’ is an odd set of words; all bark is dead, technically. What I meant was that there was no live cambium or phloem underneath the bark). 

DSC_0147 - Version 2

Cleaning the bark off reveals the true nature of the veining.


The whole tree, 28″ / 71 cm. This tree actually has two live veins going into the soil, which supply two parts of the tree. The cascading branch has one vein, and the upright trunk has another. Tree was collected by Randy Knight about 8 years ago. This Rocky has finer foliage than many, also greener, more like Itoigawa than most Rockies. There is so much variability in juniper foliage that it makes it quite exciting and full of personality from tree to tree.

This is a tangly question…there’s what’s best for the tree, and there’s what we might want to do because fall leaves are rather nice looking.

For Japanese Maple, the fall color is often the most eye-catching thing in the yard. We want to see it for a while. But, if we wait until the leaves dry and fall off, we won’t be able to do any fall canopy pruning. The reason is that the tree will bleed for a long time if we cut after the leaves have fallen off naturally. For most Japanese Maples the window of time after leaf fall that you can safely prune is less than a week, much less than the 10 days often reported.

There’s two solutions to this…take the leaves off when at the height of color or just after, and do a careful fall shoot shortening immediately, or wait until spring to do this pruning. The first option shortens the time you get to see your tree in full color, the second will offer a possibly rougher looking tree during the winter silhouette time. Maybe the solution is ask yourself if you’re a color person or a shape person…either way, it’s a decision partly based on the tree’s needs, partly based on bonsai technique, and partly on personal preference. (And if you like to keep a tidy yard, pulling leaves off will greatly reduce picking up leaves off the ground).

One final point. If leaves are taken off while they are still green the tree loses resources. In the fall the chlorophyll is broken down to salvage amino acids and nitrogen, which is then brought back into the tree before leaf abscission. This helps strengthen the tree though the winter. (Anyone still convinced of the 0-10-10 fertilizer regimen in the fall for temperate trees should give this a long think…)


Taking leaves off a Japanese Maple in October. My mother was visiting and wanted to do something in the bonsai yard, so I suggested this pleasant task.


The first tree in this photo essay is a whimsical juniper that used to be a needle juniper. Although my client enjoyed the needle juniper, it wasn’t doing very well where he lived and was getting weaker. I gave him a few options, and he decided we’d ‘change the clothes’ of the tree, so to speak, and make it happier. Essentially, we made it into something we could do bonsai work with, and not just eek along and ‘keep it going’, which isn’t really in the bonsai textbook of desired results.

Three years ago I grafted itoigawa scions on it. It was roughly styled about a year and a half ago, the whole tree created from the original four small veneer/cleft grafts. I have mixed feelings about itoigawa, to be honest, but for very small trees or those with some delicacy about them it does seem appropriate.

Itoigawa, if you’re going to go that route, is a very strongly growing plant (one of my issues with it). Some varieties of itoigawa are so strong that the branches can very rapidly get overly thick, and will soon look rather muscular and out of character with the foliage. So controlling the energy and growth on this type of juniper is particularly important.

Also, at the end of this photo essay, I include a different grafted tree, with a very different feeling…it’s a ponderosa pine that we grafted black pine onto. Not that ponderosa pine isn’t an easy tree to grow, it’s just that my client doesn’t like ponderosa very much…so that too was grafted. Different preferences for different people. Enjoy the photos!


Itoigawa scions veneer grafted onto needle juniper. March, 2012.


Itoigawa grafts growing strongly off the top, needle juniper below. March 2014.


Tree is completely itoigawa juniper now, all the needle juniper foliage has been cut off. Four grafts were used.


After a bit of cleanup, before wiring. October 2015.


Styled tree, October 2015. Three years from grafting.  38″ / 96 cm high.


Details of the branching and foliage pads in the next few photos-

IMG_1998 IMG_1999 IMG_2007 IMG_2018


Our second grafting project was a black pine grafted onto ponderosa. To get a sense of the mass and scale of this tree, the red rectangle on the box is the tab of a handtruck. That’s a big two person box.


After all the ponderosa foliage had been cut off, only black pine remains.


We’d not paid much attention to this tree for a while, and it had grown wildly for several years. Embarrassingly, I needed a jack to bend one of the massive grafted limbs, which, had I been awake, would have needed only a modest wire only a couple years earlier. Sigh. What I get for being an idiot, more work. In any event I’ll likely do a post about this one when there’s something worth photographing. One or two years I think.


Blogger Oscar Jonker of Holland came to my garden during the Artisans Cup and made this video interview. We walked around the yard and talked about a handful of the trees. He spent quite a bit of time with this, and I was delighted with the product. And, to many of my students who have chastised me for being invisible on the web, at least in video format, here’s a step in the direction of visibility… A big thank you Oscar and Bonsai Empire for creating this! Enjoy-

Many of you are now familiar with the prize winners of The Artisans Cup. This week it was found that a fourth tree was a winner as well…scoring the same 50 points as the Third Place winner. Along with Amy Blanton’s Rocky Mountain Juniper, the Japanese White Pine of Konnor Jenson will also be awarded third place.

An accompanying award certificate, plaque, and $3,000 purse will be awarded to both Mr. Jenson and Ms. Blanton. Congratulations to them both!

Konnor Pine Artisans Cup

Sharing Third Place at the Artisans Cup is Konnor Jenson’s Japanese White Pine


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