Please help me to understand how I can protect my bonsai's roots this winter. I received a Japanese juniper bonsai for a gift, and today the temperature dropped below -10 C. I understand that my tree needs to stay outside as often as possible, but also read this variety needs to be "protected" under these temperatures. From the overwintering section of this site I read my concern is to protect the roots, possibly with a pot cover.

What would be a suitable cover for my tree's pot? A towel? A sweater?? Is this something I could improvise using common materials, or something special I should begin shopping for?

Thanks in advance!

Hey there.
Can you tell us where you are from. treatment of your tree this time of year varies depending on what zone you are in.

Yes, I live in Rochester, New York. Apparently we're in
hardiness zone 5b-6a.

I've found a suggestion that I may want to bring my bonsai inside, if that was how the retailer (Wegmans) was keeping it. bonsai4me: "Trees that are purchased during the winter should be kept in similar conditions to those that they had been kept in by the retailer, even if this was indoors. To put a tree outside in the middle of winter when it has previously been grown indoors and is still active would kill it."

Here's my concern with your tree. Junipers are outdoor trees normally. They can survive inside for a while, but will never thrive.
If this tree was purchased from a grower that had this tree outside, it was dormant, and should have stayed outside until spring. Bringing it inside now is pretty detrimental to it's health.
If the tree were purchased from a grower that kept it inside, it wasn't dormant, and putting it outside this time of year probably shocked the tree and its dying or dead.
Trees need to be kept outside all the time starting in late summer to get ready for the winter SLOWLY.
My advise to you is this...
keep the tree inside, away from any heat source or drying drafts or breezes until late spring. When danger of frost has passed, put it outside in a spot that gets morning sun...and leave it. check it daily to see if it needs watering. Next winter, move it to a location outside where it won't get any wind. wind in the winter will dry the needles out and they will brown.
I Have a juniper. its happily sleeping outside.

Indeed. Never jump a tree fromwarm indoors to winter outdoors. What you could do, if you have a space like this, use an unheated room where temperature does not drop below freesing. A garage attached to the house, or shed against the house wall typically will do. This will allow the plant to go dormant. This is also the spot I would put plants that are not completely hardy for your zone. Nice and cold (maybe 4C) yet not completely frozen. Only very few plants need long frost to thrive.

Make sure you do not let the tree dry out completely. It may look dead in winter, but it is still slightly active. Here especially: Read up on the species: An european olive may stand a few degrees of frost, but only if it is pretty much completely dry, whereas Oleander will die if kept completely dry in winter.

Thank you for your responses. I've brought my tree inside to my room. Before you object, please understand that I'm a resident at the Rochester Zen Center, and we tend to use minimal heat during the winter, especially in the dorm where I live. I've visited several rooms in our buildings to check the temperature, and concluded if I wanted to put my bonsai in a colder room it would be very dark with minimal ventilation (storage closets). So I hope this current location will be satisfactory for now.

My sister's confirmed she bought my tree at Wegmans, so I'm pretty confident it would not be dormant. I wonder if it would be possible to confirm that by examining the tree?

Don't put the tree in a dark room. evergreens need sunlight, even in winter.