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!