I have my Thyme Bonsia as well as a Tree of heaven bonsai. I dont have access to a greenhouse or cold frame. I was wondering if sinking the bonsai into dirt, pot and all, and mulching it in with straw would keep them protected?

My Tree of Heaven is placed on a rock so I'm not really sure how to care for that one during the winter because of its exposed roots.

Thyme is a very hardy plant (hardiness 4a to 8b). You can leave the plant on it's normal spot during most of the fall/winter. Just protect the pot when the temperature drops below -7/-10ºC. You can do this by packing some straw around the base.

You must keep in mind a few things though. First of all: Low temperatures alone are not that big of a problem. The cells in plants are able to withstand very low temperatures. However, Low temperatures in combination with wet/moist soils is the real killer. When the water in the pot freezes, it expands crushing all the roots. So Make sure the soil of your trees is relatively dry. Don't water your trees when it freezes of before it starts to freeze and when it rains a lot in your area, it's probably best to place some kind of roof over the trees.

Second: When it freezes for more than a few days the danger of drying out starts lurking. What can you do about this? First, put the plant on a darker place where there is no wind. Wind and sunlight causes the tree to evaporate faster. second, water the tree as soon as a freezing period is over. When there is a lot of snow, you could also decide to pack the pot in a thick layer of snow. Snow insulates even better than straw and it also gives water to the tree as soon as it melts.

If you consider the above, putting the tree in the shed during harshest periods of the winter is probably the best thing to do. Plants don't need much sunlight during the winter while they're dormant so they can even be held a few days/weeks in the shed too without any consequences.
Putting the tree in the ground is only a good idea if you can find a place in your garden where it's relatively warm, and you can protect it from the rain, wind and sunlight.

All of my bonsai plants are 100% zone hardy. However, I really dont want to move my plants into a garage or shed or move them around during the winter.

I think my problem is that my pots are terrecotta. They split and shatter very easily. I think I'll have to put my bonsai strait into the ground. At least this year anyways.

Do traditional bonsai pots have greater tolerance to cold and ice thereby making them less likely to crack??

I have really nice loamy garden soil so I think that just gently sinking the whole plant to the soil level and mulching in a little with straw should save them from shock and death.

If anyone has some suggestions for what I'm to do please let me know.

Gnarlwood wrote:

Do traditional bonsai pots have greater tolerance to cold and ice thereby making them less likely to crack??

Absolutely! Real bonsai pots are made from a much better quality clay than terracotta. Especially when you go for the somewhat more expensive pots like Tokoname. Also take care to place deciduous trees in a glazed pot and conifers in a non-glazed one. This is better for the tree as well as the pot.

Gnarlwood wrote:

I have really nice loamy garden soil so I think that just gently sinking the whole plant to the soil level and mulching in a little with straw should save them from shock and death.

This gives my a very good clue on why your pots tend to break and your trees are less hardy. The soil you are using is just too compact and holds too much water. This is fine for garden plants which are generally less valuable but totally not suitable for bonsai. Because this soil is so compact and holds so much water, there is no room for the water to expand when it starts to freeze so eventually the roots get crushed and the pot breaks. For this reason we generally use a more grainy and porous mixture of akadama and kiryu for our bonsai or kanuma for azaleas. Sometimes cat litter can be used as a cheaper alternative, but it also requires more cleaning work before it can be used as bonsai soil.

I really advise you to spend the effort of transplanting your bonsai into a mixture which is better suitable for bonsai. For conifers you could already transplant them into a mixture of 1/2 akadama and 1/2 kiryu now and give them a better chance of surviving the winter. For deciduous trees, I would wait until spring to transplant them into a mixture of 2/3 akadama and 1/3 kiryu. You must protect the deciduous trees in a shed and you can leave them there al winter. Deciduous trees don't need any sun-light while they're dormant (No leaves).

Thanks for the advice. I'm strapped for cash at the moment while I'm going for my horticulteral degree but in the spring I plan on potting them properly. My Thyme will be fine. The other trees are being field planted so I think they could wait until next year for their potting.