PaulaB, I'm not a person with much (make that NONE) experience with in-ground hosta. Just with container hosta.
What you mention as your mix sounds good to me.

I use MiracleGro potting mix, WITHOUT THE MOISTURE RETENTION. For this spring, I've also begun to use crushed crab shell as an additive to feed the good nematodes and bacteria that will then eat the bad nematodes. It surprised me to find that the 50 pound bag of Neptune's Harvest crushed crab shell--purchased from Neptune's Harvest in Glouster MA--was actually a product of Canada. So you may find the same crushed crab shell in smaller bags up in Canada....unless it is another case of spin out bags made in Canada, sold in the US, and shipped back to you in Canada. Value added I suppose? :)

Now, my DH is adding the crushed crab shell to his veggie garden...I mean, that 50 lb bag is going to last a long time.

And I expect the potting mix will last for a couple of years at least. Until the hosta needs to be repotted is what I anticipate. That is why I hope someone else can speak up here, because I haven't grown hosta long enough yet to even NEED to repot...except for my Sum And Substance, which is about to eat its pot. I'm going to try it in the ground pretty soon.

Your potting and planting work should pay off. They sure look nice in their new homes.

I am very interested in this topic as I too have many roots in my garden. I have also begun to use buried pots or spin out bags. Unfortunately, it has not been long enough for me to be able to comment on their effectiveness.

I am concerned though when you say that "I trimmed the extra fabric to the level of the top of the nursery pot." I believe that McTavish also mentioned that she cut the top of the bags for a better fit. I hope that you have retied the seams that have been cut so the roots cannot force open the seams. I think that the roots are very strong and are waiting for any weak point so that they can enter the bag.

Moccasin,
These are container hostas and I have just buried them in the ground (in the containers). I think I may have made a mistake and got the moisture retention one. I will check tomorrow and if I did, I will buy some of the non retention and add it to 'dilute' the mix I have so I don't have to waste it.

newhostalady,
I have several hostas that I have grown in 10 gallon containers for at least four years. A couple of these include fairly large ones like Jade Cascade, Earth Angel and Krossa Regal. They are all healthy and thriving so that approach has worked so far.
Pots were used not just for roots but in case I wanted to move them around and this way, I don't disturb them.
I wanted to test the spin out bags to see how effective they are because the hosta roots can escape the water holes or the maple roots can enter them. Generally what I do is lift (well my DH) lifts the pots up a couple of inches in the spring to break whatever roots have creeped in or out.
Regarding retying the seams, I am not sure what you mean. I trimmed about 3 inches off of the top all of the way around the top of the pot. If you look at the top pic, you can see how the fabric is higher than the pot - I cut of the extra so it is level with the pot. I left the plants in the pot which is covered with the bag. I hope that is clearer than mud! lol

Thanks for the update, good to hear the success. I need to try this also. If I would quit buying more hostas, I could probably afford to buy some of the bags, lol. I'd like to find a local place to buy them from and avoid the shipping charges.

I've had good luck with reversed spinout bags. However - I've had some problems with crowns rotting in winter. Apparently bags don't drain as well as plain dirt. Now I'm mixing in minibark nuggets to lighten the soil. (Menards has teensy tiny minibark nuggets that are useless as mulch, but are so small they make good soil conditioner.)

Also I've had problems with the roots of the hosta sticking to the inside of the bag. It can be very hard to get them out when it's time to re-pot. Still, for the ones that were getting choked out by tree roots, they're a lifesaver.

The pre-made bags are designed for trees. They're too deep for hostas, and they could be bigger around. Wish they made short fat ones!

As you mentioned, they are for trees so I cut the top off of the bags to make them shorter so they aren't as deep and I also used bark nuggets and vermiculite. To get them out to repot, I put a pot inside of the bag so the hosta slides out when I want to change the pot or move them.