I'm sure I could find the answer to this question if I spend enough time searching through this forum, but I am really pressed for time right now. Any quick help would be appreciated.

I "inherited" 2 large (about 6-7' each) juniper spiral topiaries from a friend. I had the perfect place for them on my sunny deck. I got them late last summer and they weren't looking extremely healthy. I could tell they were very rootbound, dried out, and malnourished. My intent was to repot them in the fall, but I didn't get to it. So to get them by I dug out a bit of the old potting soil and added a little fresh soil. I also sprayed them a number of times with water to eliminate some spider mites which are now gone. They are in very thin cheap plastic pots that can't help when the sun warms them up.

Fortunately, they are looking much more healthy, and I am now ready to transplant them. I've never grown conifers in pots before. I use a great potting soil mix for all my other deck plantings, which is a "water-hold" mix which uses coco fiber instead of peat. Other ingredients, I think, are compost and maybe some sand. I don't see any perlite or vermiculite, but it's not too heavy. It works very, very well for my mixed deck planting I do each year with a mix of tropicals (cannas, phormium), annuals and perennials.

Now I need to repot these junipers. My gut feeling is this soil will be too heavy for these large pots. I'm thinking of buying a large bag of perlite and adding a generous amount to the above mix. I have new decorative plastic pots, but they are not cheap. They are double walled, which will provide some insulation.

Here's where I need help. Do you think this potting mix would work as is? Should I add perlite? Should I add sand (which I think would make too heavy)? Should I root prune when I transplant? The original pots were about 18" wide; the new pots, about 20".

Thank you all in advance!