Here in the HOT inland valley of CA I normally pot-up something small so it can build a root system before putting in the ground in the Fall, otherwise the chances are good it won't survive unless planted in a shady spot. If you do this, just remember that it might need to be repotted several times as it grows.

It's hard to kill Comfrey around here. In fact if you aren't careful it can be invasive.
But whatever you do, once you decide to plant it make sure you put it where you want it, because if you try to move it, you may find yourself with Comfrey in multiple locations. Comfrey is fascinating in that out can send a taproot down add deep as six feet into the soil. This its of course one of the reasons it its so rich in nutrients that other plants need, because it is able to draw them from deep in the soil where the other plants can't reach.
Many people assume that such a deep root system makes it more drout tolerant, but my experience has shown that it isnt any better at drawing and holding water than plants with much shallower systems.

It is a fun plant to grow. Hope it does well for you.

I have read that comfrey is generally only "invasive" if you dig around the roots or move the plant around. All the little broken off root bits sprout. I have not moved my comfrey and generally have avoided digging around it and it has stayed exactly where I originally placed the two plants about 10 years ago. I have no special varieties.

As for the original question, I live in a cold zone and so can offer little useful advice other than if you are going to plant them, do it now before it gets any hotter. Otherwise, baby them in the pot in a partial sun location throughout your blistering summer and plant when it is cooler.

FataMorgana

I currently have my comfrey in a pot and it's doing just fine. Like you, I live in a hottish area, but I intend to transplant it into an herb garden before it gets too hot, that way, It'll be easier to keep it watered. I guess it all depens on what you feel is best. In a pot, you'll be able to better regulate the temp and keep it watered, but it would do okay in the ground too, whatever you think is best.

~Ino***

In choosing where to plant it, since it is so difficult to remove, keep in mind that although at first it grows straight up, at a certain point (usually while flowering) it flops down in a circle all around its center, effectively smothering any smaller plants within its reach, All told, it then occupies a circle of at least three feet in diameter.