Minnesota Wintering Tips and Tricks from MBS members

Let there be light! One word – give them lights. Don’t keep them in the dark. Keep azalea, boxwood, holly, and shimpaku junipers under lights. I think they do better with light as they are not kept cold enough to be completely dormant… I’d put all trees with foliage under lights if I could afford it.

Fungus among us:
• When we built our new cold room, it was so airtight that it kept a humidity of about 90-100% all winter… the first winter we had a lot of fungus and killed a few trees. We installed an exhaust fan plugged into a dehumidistat, and set the humidity max at about 85%, and that solved the problem. Sucked up more electricity, but saved the trees
• Don’t let pot bottoms touch standing water in a tray. A shallow puddle is okay for humidity, but a too-humid room will cause fungus growth on soil surfaces. This is your clue to leave less standing water
• Make sure winter storage has air movement. This will help to eliminate fungus build-up.
• Treat all plants with a systemic fungicide and pesticide about a week before they are put away.

Bugging out:
• I spray all my tropical trees (except succulants) with dormant oil before I bring them in. It helps to control any scale, and makes them a more intense green color as an extra benefit!
• Spray deciduous trees with lime sulfur to kill bugs and keep down fungus.
• I put a couple of dog flea collars in my cold frame to take care of any pests that came in with the trees.
• Not only remove moss from your trees but ALL dead & fallen leaves. It’s a perfect place for mold & unwanted critters to hide. Shop-vacs are great for sucking up hard to pickup leaves.

• This may sound weird, but one of my biggest problems in wintering my trees is squirrels, possibly just one squirrel who gives the others a bad name. I am reasonably certain that the squirrel who got into my greenhouse last week is the same one that I trapped two years ago and moved to the University campus. He was back last year. He had dismembered the clay/muck rim of a slab planting and scattered it all over the greenhouse just for snits and giggles. He chews his way through the Styrofoam insulation which forms the soft joints with the house. I have had to replace all of these. He won’t eat the Chips Ahoy that my mother used to use to get rid of chipmunks. I love squirrels and admire their intelligence and persistence, but this one is pushing it. When I lived in Florida, I trapped a squirrel that was clawing his way through my bromeliads. I painted a green stripe down his back before relocating him 15 miles away on the other side of town and a big lake. He was back in my yard two months later.
• I keep Raid mouse blocks scattered around floor areas (and on the top of the cinder block in my basement) to keep the field mice out. When temps are moderate and the door is open during the day, I keep a section of bench across the door when it is open to keep the bunnies out. Mice and bunnies chew the bark off the trees. Maples are their favorites. I inspect the ground contacts each year to make sure there are no burrows.
• A soup can on its side with D-Con or bar bait in it will you trees protect from small critters.

• I only grow tropicals and there are 2 important issues. Watering and root temps. I water about once a week in the winter, Portulacarias once a month. If the root temps get too low rot will set in. This is mostly due to watering practices and where the trees are kept. Also I do very little or no pruning during the winter months, growth is weak and it is difficult to have enough light to keep the internode spacing tight. Better to prune right before moving back outside, also winter pruning may cause branch dieback
• Rotate your trees in winter store too. You would be surprised how much area you can miss watering on the back & corners when your trees are packed into a tight or small area.
• When watering your trees, make sure the temperature of the water is the same as the area your tree is living in for the winter. If it’s too hot or ice cold from the tap it will shock your tree.
• The size and shape of a pot affects watering needs in every season, and plant species matter too. Tall pots drain faster than flat ones. Cacti and jack pines can go a lot longer without water than fruit trees

COLD ROOM TIPS for trees kept at 32-40 degrees F. Overwatering kills the most trees. Remember that plants are dormant now and using much less water than during the growing season. If the soil is a free-draining, loose gravel mix, a thorough weekly watering should be fine at this temperature. If the soil is garden/potting soil, the danger is overwatering. Check weekly with a finger, and water each plant accordingly. Plants in mud will need water less often.

Heating and Insulation
• Most of the heat in my greenhouse is supplied by the blower from my Jenn Aire stove, which blows in house air. I learned the hard way that this has to be evenly distributed: when I expanded the greenhouse a number of years ago, this put one section of bench in the direct blast of the vent, and the repeated freeze/thaw killed off a number of my pines. Now I attach a section of flex vent each winter, snake it across the floor, and punch big holes in it. It also helps keep the temperature more uniform. I tape over the blower switch on the stove to prevent well-meaning wives from turning it off.
• I have been putting panels of thin Styrofoam in the roof to keep it from radiating too much heat at night, and it helps keep it from overheating in the summer. I find that the plants near the roof are especially vulnerable to heat/sun damage as spring draws near

A Few More Words of Wisdom:
• In your bonsai journey, you WILL lose trees, but its part of the learning process. Experience is still the best teacher. Know each plant’s temperature tolerance. Some will do better in a cool bedroom over winter. If you don’t know, check online information or ask an experienced club member.
• Keep trees out until completely dormant, I leave mine out until it threatens to snow, or we get to Thanksgiving.
• Don’t wire until early spring and buds begin to swell.

8 comments on “Minnesota Wintering Tips and Tricks from MBS members

  1. i’m a new member have had juniper bonsai’s for years never kept one over winter tell now.’ i missed selecting pot and repotting basic class,but will be at april basic class. should i wait to repot my juniper at april class or do it before class, was going to use training pot .i ‘m thinking of training it cascade style as it is growing in that direction. any advice will be appreciated. see you at next meeting.

  2. Hi Troy,

    You can still repot at the April class. Please bring all the supplies needed to repot your tree. See you then!


  3. I just bought a juniper at the auction. The notecard said it’s hardy. Should I bury the pot outside and cover with leaves or something else?

  4. You can bury the pot (assuming it is a frost proof pot) and cover the base with some mulch or leaves. Keep it away from the wind and sun too. Placing chicken wire around it will keep it safe from rabbits and moth balls around the base will stop rodents from girdling the trunk. Make sure it is watered and keep some snow on the base to ensure if gets water throughout the winter. Good luck!

  5. Thank you! So, I shouldn’t let it get covered by snow, just keep some at the base? It’s only four inches tall.

  6. Keeping snow at the base is ideal, but I don’t think it would hurt if it got covered in snow. Keep it protected from the wind too.

  7. I have several eastern white pines & 3 coastal redwoods that I need information on how to get them through the winter. I live in Bemidji MN. Cold is an issue. I bought a small portable green house to use to winter the trees in. Suggestions? I do have a big garage I can winter the trees

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