Winter Storage and Prep.

November
Outdoor:

This is a time to be prepared for the coming winter, once night temperatures consistently fall to around 25 F trees will need protection. Decide where and how you will protect your trees; mulch bed, cold
storage house, cold greenhouse, etc. Acquire any needed materials now so that you are not caught without needed supplies when it is time to store the trees. Keep trees outdoors on cool fall nights. Do not bring winter-hardy trees indoors. Adjust watering to the changing temperatures. The soil is likely to stay damp longer on cool and in particular cloudy, days. Fertilization with a very low nitrogen formula may continue or may be stopped at this time. Light pruning can be done but heavy pruning is not recommended. Keep an eye on the wiring and remove any that are becoming tight. Wiring may be done and is often easier on deciduous trees after the leaves have fallen. (Some advise that wire may transmit cold to secondary and tertiary branches and remove wire for the winter as a precaution, but this is still up for debate.) Remove the remaining foliage on deciduous trees and any weeds or debris on the soil before placing the trees into winter protection. Spraying with a general fungicide can be beneficial. Some people spray dormant oil on the trunk and branches of deciduous trees.

Indoor:

All tropical trees should be brought indoors when outdoor night temperatures are in the 50’s. It is best to have them in their winter locations 2 to 4 weeks before the furnace begins regular operation. Water as needed for your indoor conditions (light, temperature, humidity). Trees at windows, under fluorescent, or LED lights may be given fertilizer. Pruning and wiring may be done. Repotting is not advised at this time.

Bonsai Winter Storage for the IL/Wis. Border

If you get caught off guard by a quick freeze and do not have enough time to get your plants to their winter quarters or only need protection for a day or two, simply lower bonsai under their summer stands and if needed drape plastic or a tarp over the stand.

Conifers:

I store my conifers in two different locations. My plants that are in development are in my garden in a layer of mulch. They have a building blocking the wind from the north and I planted asparagus to act as a wind block from the west, I leave the stocks up until spring and then cut them down when I want to get the additional sunlight in the evening in the spring.

Junipers in a layer of mulch, make sure the area directly below the pot is level with no air pockets. This will help keep mice and moles from wintering under your pots. Trees are yellow because of the sunset!

Another way to over-winter plants is in the garden. I have a high water table in the spring so I made wood frames and then add dirt to the top of the pots. I will add more dirt after taking this picture.

My more valuable trees are located in a location that has buildings blocking wind from the West and North. These trees are on the bed of gravel I use for my summer Bonsai stand. The stand is nothing more than cinder blocks and 2*10’s. I leave the stack of cinder blocks to help block wind and move the 2×10’s to the front of the cinder blocks to block wind from this direction then place landscaping fabric or an old bed sheet, place my bonsai pots and fill in the gaps either with gravel or wood chips. The bed of gravel helps with drainage. 

(I live in an area that can get soggy in the spring or during a thaw and have had problems with junipers that were dug into the ground getting too much water.

I like to let my trees cool off in their new location and will add mulch or gravel the following weekend. Also, there is a maple in this batch that will be moved later.

Deciduous and less cold-tolerant trees

These trees are placed in a barn to get out of the wind. I place pink foam insulation down. Plants are then placed in plastic crates with 1” of rubber mulch on the bottom for insulation and drainage. I have also found that insects and mice do not seem to like the smell of the rubber. Once I have placed as many plants as possible in the crates without any touching the sides, I fill in the gaps with rubber mulch to the top of the tallest plant in the crate. Then I simply stack the crates next to each other. If things get very cold I have some old lambing pens that I enclose the collection in and cover with a plastic tarp. A 75-watt bulb will usually keep things around freezing on cold nights. I picked up a thermal cube from Fleet and Farm that turns on at 20 degrees to make sure things do not get too warm or waste electricity.

The crate on the left has about an inch of mulch and is just waiting for some plants, the crate on the right has a mix of rubber mulch from the prior year that I will use to fill in gaps around the pots.

Crate with 2nd-year cuttings getting ready for their first winter outside. Notice leaves and weeds have been removed

Winter storage strategies (Source Bonsai Tonight)

Identifying suitable winter storage options for your bonsai starts with an understanding of the local weather. Knowing what kind of weather to expect can help you determine how much shelter your trees will need.

Your best resource for this will be local enthusiasts who have experience overwintering bonsai in your area. Checking with your local bonsai club – or the nearest club with similar weather – can be a great starting point for determining what kind of shelter will work for your collection.

Before looking at effective storage strategies, here are some goals to keep in mind.

• Avoid wind. Wind is the most dangerous element in winter as cold air can dry out trees quickly. Trees that dry out in winter are more likely to lose branches or die than trees that are sheltered from the wind.
• Keep trees well-watered. Dormant trees don’t completely shut down in winter. As such, they will continue to need water. Trees in warmer and brighter spots may need watering up to once a week; in cooler spots, watering once a month might be appropriate.
• Reduce temperature fluctuations. For most temperate species, the goal is to maintain temperatures between 33 and 40 degrees F throughout winter. This is cold enough to meet dormancy requirements without exposing trees to harmful temperatures.
• Reduce freeze-thaw cycles. Trees that freeze and thaw frequently experience more stress than trees that consistently stay just above or just below freezing.
• Maintain dormancy for as long as possible. As we’ll cover below, spring care can be challenging in cold climates as late freezes can damage new growth. By keeping trees dormant until freezes become infrequent, you can help your trees grow successfully in spring.

With these criteria in mind, here are some notes about popular approaches to winter storage for bonsai.

• Place trees under benches. If your winters are relatively mild but temperatures can drop into the 20s (F), placing trees on the ground or under benches can provide additional warmth and shelter trees from heavy snow loads.
• Bury trees in the garden. By digging holes big enough for bonsai pots and backfilling them with dirt just over the lip of the pot, you can provide insulation for the roots while exposing the tops of trees to the elements. Shoveling snow around the trees up to the first branches can provide additional insulation.
• Place trees in covered or uncovered mulch beds. As an alternative to burying the pots in soil, you can create mulch beds and pack the spaces between pots with bark, mulch, leaves, or straw up to the first branch. For extra protection, place trees next to a fence or wall and cover them with plastic sheeting.
• Store trees in unheated garages, sheds, or enclosed porches or patios. Storing trees on tables, shelves, or directly on the floor can work, as can building insulated spaces within the garage. Space heaters plugged into a standalone thermostat can provide additional heat.
• Store trees in cold frames. Freestanding cold frames are essentially greenhouses that are kept cool. Simple cold frames can be built with wood and plastic sheeting. Lining greenhouses with foam insulation can provide additional protection. See Winters in Canada by Derek Delisi for how-to instructions for building a cold frame.
• Store trees in heated greenhouses. If your trees require storage within a narrow temperature range, heated greenhouses offer great flexibility. Warm greenhouses kept at 70 degrees F can be used for tropical species while greenhouses between 33 and 40 degrees F are perfect for temperate species that don’t like to freeze. Temperature-controlled exhaust vents can help maintain cool temps when the sun is out.

Bonsai under a layer of insulative snow (photo courtesy Joe in Kentucky)

The following tips can help you avoid common pitfalls of winter bonsai storage.

• For trees stored indoors, using fans to circulate the air can cut down on fungus problems.
• For trees stored outdoors, find a location next to a fence or wall to cut down on wind. Shady locations are best for keeping trees cool.
• In areas that receive excessive rain (the Pacific Northwest and the United Kingdom come to mind), trees may need protection to prevent root rot or general weakness in spring. Give these trees a break from the rain by placing them under an eve, keeping them in a greenhouse or cold frame, or placing a plastic sheet over the soil (and checking regularly to make sure the tree gets water when it needs it). Using soil mixes that retain less water can help when excessive moisture is a problem year round.
• Greenhouses and cold frames can heat up rapidly when the sun is out. When possible, set up greenhouses in a shady spot to reduce overheating. Strategies for keeping temperatures cool inside range from covering structures with shade cloth (mounted inside if large snow loads are expected), installing vents, lining the walls, floor, and ceiling with solid insulation, or using buckets of water as heat sinks.
• Heat pads or heat coils can help maintain root temperatures indoors, but note that trees kept on heat pads may require more frequent watering.
• Many species will benefit from light in winter. LEDs on timers are a popular choice for trees stored indoors as they produce little heat.
• Rats, mice, voles, deer, gophers, and rabbits have been known to munch on bonsai over winter. Wire screen can keep critters away from trees in the garden, and bait or traps (that aren’t accessible to pets) can be effective indoors.
• When using heaters to regulate temperatures indoors, consider standalone thermostats as they can offer greater flexibility and reliability than thermostats built into heaters.
• All but the toughest bonsai pots are susceptible to breaking over the course of a cold winter. To ensure your pots remain intact over winter, place trees planted in precious pots in climate-controlled spaces that are kept no colder than 33-40 degrees F.

Standalone cold frame (photo courtesy Derek in Nova Scotia)