Newcomers to bonsai are often surprised to learn that most bonsai are outdoor plants. Usually it isn’t long before the question comes up: what do you do with them in the winter?!
Winter care is a subject for another time but there are multiple silver linings for the bonsai hobby in winter. First, you can get some valuable “down-time” to rest, recharge, and then brush up on your bonsai knowledge.
When it comes to deciduous trees, you can more easily work on them without leaves on the branches. Without foliage (an important distinction because there are deciduous needle conifers) you can more easily see the branch structure and work on the tree without the hindrance of the foliage. You do still need to be careful you don’t damage buds on the branches but certainly wiring is a lot easier.
As an example, consider this Chinese elm.
This was taken fairly early in the season yet the foliage still obscures most of the branch structure.
But when dormant you can much more easily see the branch structure and be able to make better decisions regarding where to prune or wire.
Not only that, but there are even bonsai shows in winter (not this year, though, thank you COVID…) where deciduous trees can be enjoyed for their “nude” form. In winter silhouette shows you can truly appreciate the ramification of the branches while they’re not hidden by the foliage.
When bonsai are displayed in this state at a winter silhouette show it really reveals the work that went into the creation of the tree. Scars, poor branching, graft unions, and artist mistakes can be easily hidden by foliage. With bare branches, every inch of the tree’s structure can be seen, admired, and/or critiqued. That can be nerve-wracking for the artist but quite beautiful for the show attendees. It is often said that beginners grow trees for their summer form but seasoned artists grow for the winter form. The pictures accompanying this article really show the beauty of these winter nudes.
There is an interesting concern often raised about winter shows that goes back to the fact that bonsai are primarily outdoor plants: is it harmful to the tree to be indoors and warm in winter?
Usually not! Early winter shows are safer than late winter shows in that the tree hasn’t been dormant for long enough to be easily woken up. The hormones that keep a tree asleep are broken down by light and heat so as long as they’re not inside in the warm temperatures for more than a couple of days it is no more harmful to a tree than a few warm, sunny days in the middle of winter. This isn’t to say you should tempt fate by regularly bringing dormant trees indoors to enjoy them in the winter but occasionally displaying a tree in a winter silhouette show is generally a safe endeavor.
As you can see, it isn’t all gloom and doom for a bonsai hobbyist in winter. Take advantage of the opportunity to refine your collection’s branching, you and your trees can get some rest, and maybe next winter you can take in a Silhouette Show!
In the next few days most of the northern United States and Canada are expected to experience unseasonably cold weather. Here in the Cleveland, Ohio area, for example, it is predicted to drop to 27° tonight and 19° tomorrow night when the averages for those days are 38° and 37°.
Why should that matter for bonsai trees that are hardy to lower temperatures and do we need to provide extra protection to our trees?
I would advise that extra protection be given to trees, even if hardy to those temperatures, because there is a chance the trees have not yet acquired their full winter hardiness this early in the season.
Part of cold hardiness comes from the concentration of sugars in plant cells. This concentration increases starting in early September and reaches maximum in late November to early December. Then it holds steady until early April and begins decreasing as the plants wake up and begin to utilize that sugar for spring growth. Since plants reach their maximum concentration in late November that is when they have their maximum tolerance to cold temperatures.
The fact that it is early November means that it would be prudent to provide additional protection against the cold temperatures but care should be taken to avoid keeping the plants too warm which could delay cold hardiness.
Some good options for protection for the next couple of days include an unheated garage or shed, a cold house or cold frame, or setting on the ground and covering with mulch.
Your trees might not be ready yet for winter storage but would benefit from a little protection until temperatures return to normal in a few days.
One topic that comes up frequently, particularly as winter approaches, is how to safely overwinter bonsai trees. There is no one answer but there are several important guidelines to keep in mind.
First is the hardiness of the tree. The hardiness zone refers to the annual extreme minimum temperature of a region. It is used as an indicator of what plants can survive winters in that region. For example, northeast Ohio is zone 6a and generally doesn’t exceed -10°F. It is important to remember that this is for a plant growing in the ground. A plant in a pot does not have any insulation for its deeper growing roots so a good rule of thumb is that you lose a zone when the plant is in a pot unless additional measures are taken. Therefore a plant that is hardy to zone 6 is really only hardy to zone 7 when it is in a pot and unprotected.
When we talk about winter protection for a plant there are really two things we’re looking at: temperature and wind damage.
For plants that are hardy beyond your zone, (for example a zone 3 plant living in zone 6) you likely will not need any temperature protection. Even in those cases, though, it is advisable to give some protection to the roots to ensure the best chance of survival. So how do you do that?
One method is to simply put the plant into the ground. Dig a hole and either remote the plant from the pot or put it into the hole pot and all (assuming your pot is capable of withstanding the cold temperature). Some bonsai practitioners will remove their tree from its ceramic pot and put it into a plastic pot for overwintering in the ground. Then, cover the rootball as you would when planting a tree in your yard. Be careful to not plant it lower in the ground than the current surface level of the tree to reduce the chance of water damage. For easier removal in the spring, some choose to backfill the hole with woodchips, mulch, or sand.
Another similar method is to place the plant on the ground and “heel-in” with mulch piled up to the lip of the pot. Again, sand or woodchips works here. Some people use straw or hay but there is a danger of weed seeds getting into the bonsai soil if you do that. Hay and straw can also be attractive to mice which can then chew on the roots or trunks and kill the tree.
Speaking of mice, it is generally advisable to wait until everything is well frozen before putting away your trees for the winter. Some people in the USA use Thanksgiving as the target weekend. Doing so increases the chance that vermin will have already located their winter quarters rather than making a home in your trees. To further decrease the odds of rodent damage you can also spread around some mouse bait (taking care that your dog doesn’t get into it if you have one!).
Some people choose to use a structure such as a greenhouse, shed or unheated garage. That can work but there is a danger of temperature fluctuations. Generally, you want to buffer the plant against rapid changes in temperature. If you get a warm winter day or a stretch of warmish days in the early spring you don’t want your trees to warm up too much as they could come out of dormancy, lose their ability to handle extreme cold, and then die when it cools back down. If you have no other option you may end up doing the “two-step” in the spring and taking your plants outside during the day and bringing them back inside every night.
Wintering inside a heated structure can be deadly for plants that require dormancy. If a plant needs dormancy and doesn’t get it then it will usually weaken and die within three years. So, bringing them into your house, a heated garage, or a warm greenhouse will usually kill it. This does not apply if it is a tropical plant (which doesn’t require dormancy and will die if it gets too cold) or if you’re merely applying minimal heat to keep the temperature consistently around freezing.
The other factor to consider is wind. This is usually the most dangerous aspect of a bonsai tree’s winter survival.
As wind moves across a plant it causes moisture in the plant to evaporate. This is the primary reason why deciduous trees lose their leaves in winter. However, there can also be moisture loss through the bark and buds. Even trees with thick, waxy leaves or needles (which are designed to minimize moisture loss) will often benefit from wind protection.
As we noted earlier, trees growing in the ground year round have roots below the frost level in the soil so never freeze. If the tree loses moisture, the unfrozen roots are able to replace it. Bonsai trees, even if put into a hole in the ground, are not below the frost level so once the soil freezes the roots are not able to replace the lost moisture so the tree can desiccate and die.
There are many ways of offering wind protection. Most people place their trees up against a building. This is often on the north side of the building to lessen the chance of the sun warming the plant too much in the winter or early spring even though many prevalent winds come from the north. Keeping in mind that you need to balance temperature protection with wind protection it is often advisable to supplement being up against the building with leaning or erecting a cold-frame (clear plastic or glass), snow fence, or some other structure to further block or slow the wind.
Wind protection is one of the reasons people often want to use an unheated garage or shed but we already discussed those dangers. Underground structures, such as root cellars, can lessen the chance of temperature swings. However, if using such a structure be advised that evergreens continue to photosynthesize in the winter so will not do best in a completely dark location.
While temperature and wind are the two greatest dangers their are other things to keep in mind when overwintering bonsai trees. Take care to ensure they don’t dry out (you might need to water them on warm days). Beware of varmint damage from mice, voles, rabbits, and deer. If stored in a location with little air movement then mold could be an issue. You also need to keep in mind your particular microclimate as influenced by factors particular to your yard such as buildings, landscaping, etc.
While winter can be a dangerous time for bonsai trees, with a little preparation there is a good chance your trees will be fine!