Hi, welcome to NGA!
Mulch rings:
>> which provides better protection, rubber or coco fiber?
Maybe it is only me, but the very last thing I would put on my soil is a rubber rug made from recycled tires. Quite aside from the aesthetic effect of re-manufactured "industrial waste", I would expect things to leach out of recycled rubber that no plant ever evolved to handle. And the only time I saw rubber mulch come out of the bag, it had a strong, nasty rubber smell. (That can't be normal! It must dissipate and leach out after being on the ground for a while.)
I guess it's not a big deal since you are probably not eating your trees very often!
Opinions do differ, and obviously lots of people do buy rubber mulch or it wouldn't be in Home Depot.
For effectiveness at stabilizing soil temperatures, I'm guessing that rubber is not as good an insulator as anything fluffy or chunky with air spaces.
Bark chunks or wood chips work very well and I think they would last longer than a
finely-shredded coir. If you can find coir "chips", they might last a long time. I think that wood chips are probably what arborists choose most often, but I am guessing.
One enormous advantage of organic mulch is that it enriches the soil as it turns back into soil. And the things that they release ARE things that plants and soil organisms HAVE evolved to digest over billions of years.
(Anyway, that's what I focus on.)
>> Do mulch rings protect from frost?
Here's my concern. If the roots need frost protection, wouldn't that mulch ring need to cover an area as large as the drip line of the leaves (or some other estimate of the size of the root zone)? I'm thinking that most mature trees would need a mulch ring hugely larger.
But if you're protecting a small tree, or a young tree for it's first 5-10 years, I understand.
(While I was looking for a borrow-able photo, I stumbled on this article. This guy is passionately against mulching trees in ANY way, mainly for his own aesthetic reasons. But what an unusual passion!)
http://landscapeofmeaning.blog...
