Overwinter Plan

[turnyface]

I have a small collection of pre-bonsai I’m trying preserve over a Minnesota winter (Zone 5b). Initially I was planning on moving everything to my unheated, attached garage once temps dropped below 28F, but now I’m wondering if it’s a better idea to keep them outside and heavily mulched in for the winter.

My house is surrounded by river stones, so whatever contraption I use will need to be on top of the rocks (is this identical in terms of ground warmth to bare, grass covered ground?).

One option is to use cinder blocks to create a space around the trees, set the trees on the rocks, and heavily mulch them in like these photos:




Another option is to build a box like this and heavily mulch them in like the previous photos:



This is the image of the space I have on the north side of my house:



I have about ten young Itoigawa in 4” pots and 8 young JBP in 2” pots. I also have five Japanese maples and two azaleas (the azaleas will be garaged for the winter).

 

[turnyface]

The trees.

 

[Kodama]

I agree with keeping them outside and mulched in. I do that as well but you might consider adding a wind barrier such as a wall of clear poly greenhouse plastic as winter winds can kill. I keep deciduous in unheated garage but still have to shuffle if temps bounce around and is a little risky in breaking dormancy too early.

 

[Paradox]

The cement blocks and mulch should work.
The glass top box could cause some problems with sun heating the inside too much.
Would act like a greenhouse

 

[turnyface]

Do the trees have to be touching the rocks on the ground? What if I built a box insulated with rigid pink foam and set the trees in there with a lot of mulch?

I’d keep the lid open except for the week or two in January when temps dip below zero.

 

[Jrmcmich]

You will want the soil (rocks) to help keep the pots warm as the ground temp is warmer than the air temp.

 

[turnyface]

Would a wood frame without a bottom work as well? Just thinking about having to move the cinderblocks down and to the back (it's a walkout). Could also add a wood lid that I could close on the coldest of days.

 

[TrevorLarsen]

I found I valuable my first winter to leave some of the trees I didn’t care much about out exposed all winter just to see what the limits of survivability are for my yard.

 

[Jrmcmich]

Would a wood frame without a bottom work as well? Just thinking about having to move the cinderblocks down and to the back (it's a walkout). Could also add a wood lid that I could close on the coldest of days.

That’s what I made Last year. Simple wood frame with open bottom wrapped in double plastic for wind protection and had a top that had clear plastic that I could close when temps were low

 

[Rambles]

If you don't mind appearances and live close to a decent sized town, you can often pick up free pallets. Four pallets attached at the corners and wrapped in some kind of plastic sheeting makes a solid, easy winter box that can do double duty as a summer composting bin

 

[turnyface]

Finally got my winter protection setup (less trees and mulch).

I’m now starting to panic and wonder if the ground and mulch is enough to protect the trees inside this frame? Particularly worried about the Japanese Black Pines, Itoigawa Shimpakus, and Japanese maples.

I’m in zone 5a and it could get very cold for a week or two in January.

Should I rethink my plan and put my trees in an attached, unheated garage instead of mulching them in (was going to mulch up and over the pots)?

 

[turnyface]

Solution!

De-icing cables on the ground, pots on top, that will turn on if the temp of the pots drops below 20F. I read in Bonsai Heresy the root kill temp of JBP is 10F.

 

[dbonsaiw]

Granted I'm in 7b, but I like my trees staying outside to the extent possible. Better air and humidity, not to mention that watering is way simpler outside of the garage. Mulched in and protected from wind, your trees should be fine (not 100% on the azaleas as I have no luck with them). IMHO, de-icing cables and the like are overkill. Having the pots on the ground and mulched in provides a decent amount of protection for the roots. In fact, I find that the roots must be growing during winter as I place the boxes on the ground in November and the roots have grown into the ground by April. The snow will also add a layer of insulation. I no longer worry about the roots over winter and am more concerned about branches drying out.

 

[Cable]

Heh, those first two pics are mine. I haven't overwintered with that setup in years! It worked really well for the trees I had at the time but the year after that I started adding burlap as a wind barrier and now I have a full-on high tunnel hoop house.

I haven't overwintered any trees inside my garage in years but I don't recommend it when you have space to do it outdoors like you do. You're on the right path.

 

[MSU JBoots]

I’m in a similar zone in west Michigan and I’ve always kept my trees outside including a couple JBP. I’ve lost one tree to winter and it was a tiny grafted cork bark JBP. This is my set up.

I bury the pots slight underground leaving the top couple inches exposed then cover with mulch. I also put a little mulch in the bottom of each hole to help with drainage. I use the tarp and plywood as a wind break. I put the pine and most winter hardy trees on the outside and the least hardy on the inner most area. By the time we usually get really cold there’s a nice blanket of snow to insulate the roots. I’ll be doing this again in the next couple weeks usually the beginning of November.

 

[leatherback]

No real recommendations. Just noting that seeing this thread really brings home home pleasant my winters really are, with only the occasional dip in real cold territory, and mostly in the 20-ies as night temps when freezing. Temperature swing winters here.

 

[stewarjm192]

Your plans will all probably work, but the stone ground is not helping to insulate as some are saying

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[penumbra]

I am in zone 6 but it really has been zone 7 for a few years. Still, I am prepared for zone 6. I know a lot of people do the garage shuffle, but honestly, I wouldn't do it if I even had a garage. I have over 1,000 plants outside, bonsai and prebonsai. Some go in coldframes but most are in mulch. I have a few coldframes that are covered with 2 layers of frost blanket instead of plastic or glass. I just got a large coldframe with 8mm insulated poly carbonate panels with an auto opening lid to let the heat out, It is oriented to not receive any sunlight, I also have a few low hoop house that are covered in winter with heavy duty frost blankets. Some of my larger very cold hardy plants sit on the ground and some stay on the benches.
I love the winter because I have a lot more time to work on other projects, and it is the only practical time to travel when you have so many plants. I am getting rid of a lot of my tropicals to give me a bit more winter freedom. Still have over 200, down from 400.

 

[turnyface]

Thanks for the input!

I ended up removing all the rock and exposed the ground layer as I was worried the rocks wouldn’t give me a true ground temperature. I also decided to add some de-icing cables hooked up to a probe thermometer that will turn on if the soil temp of my trees drops below 20F. I also have two ancillary thermometers to monitor the soil temp of trees on the edge of the frame to manually turn on the cables if they get too cold.

Hoping to not use the de-icing cables, but it’ll be nice to have it just in case.

Now I need to add some molemax and liquid fence spray before I let the trees rest, mulched in for the winter.

 

[turnyface]

Well, not so good news. My HOA sent me a letter telling me to remove my cold frame or I'll be fined. I'm not going to get into the details of why my cold frame is any different than storage boxes for patio furniture outside because it makes me irritated.

So now I need to rethink what the heck I'm going to do.

1. Store them in the garage and do the shuffle. My garage is unheated, but attached with a good insulated door. I have never tracked temps in the garage, but I do think it'll be 40-50 in the garage all winter unless I keep it cracked open at all times.
2. Store them on my deck in a large plastic bin/large cooler/foam cooler, mulched above the rim, and on top of heating coils. I'm thinking of using a method I found on Bonsai Empire:
   • Lay down 2" of foam
   • Lay down plywood on top
   • Set bin on top of plywood
   • Lay down thin layer of mulch in the bin
   • Add the heating coils
   • Set trees on top of the coils
   • Mulch trees above the rim
   • This would also offer me the flexibility to move them into the garage if it gets way too cold (I'll be monitoring soil temps with digital thermometers), but definitely something I want to avoid.

This selective enforcement of HOA rules is crap. Why should I be under more scrutiny just because I live nextdoor to the HOA president? Blah...I digress.