Not the best person to answer this, but my understanding is that either you replace all the soil where the new roses are to go, or you wait 6 months before replanting.
My guess about the big cardboard box thing is that it just keeps the new rose's roots out of contact with the old soil for an equivalent length of time. You could probably get some old cardboard boxes of a suitable size from the supermarket.
Comtesse :¬)

As far as I know, rose replant disease is more of a problem in Europe than in the USA (I don't know about Australia at all). I gather it's worse in some countries over here than in others, but I deduce this mainly by instructions given from different nurseries about re-planting roses where others have grown; for example, at Peter Beales in England, they say it's even a possible risk if you have ever had roses growing in a given spot! whereas an Italian nursery takes a more laid-back attitude,saying if it's necessary,plant the new rose next to the spot where the old one was. PB definitely recommends the cardboard box thing, but in my experience you do have to be careful to make sure that you don't create a "bath-tub effect" ;i.e., with the soil in the box being too uber-amended in comparison with the surrounding soil: you want the rose's roots to be happy to venture outside the box when it's big and strong enough.In the USA, it's not supposed to be an issue, but I'd still replace the soil and amend it heavily, since it's probably pretty tired out by now. Think about farmers rotating their crops: they don't want to have, say, tomatoes growing every year in the same spot, because that spot's soil will become drained of the stuff that tomatoes in particular like. They also let fields go "fallow" for a season or two, to let the soil replenish itself, maybe planting a green manure crop to help things along. For roses, marigolds are said to be excellent soil purifiers; also it is said to add lots of organic matter and mychrorizza funghi.In other words, if it was me, I'd get rid of all the old roses, trying to remove as much of the roots as possible. Then I'd heavily amend the entire are with LOTS of organic matter-depending on the size of the area, but I'm talking about like a pick-up truck full of goodies.Personally I am beginning to think that the best type of organic matter is stuff you gather from the floor of the woods, because there are already mychrorizza in that,but I don't deny that this is a very tedious job. Then maybe leave it alone for a season, just sowing it all up with marigolds. Then, at planting time, I'd make sure I had LOTS of native soil on hand, taken from some part of the garden maybe,but where roses have never grown,and I'd plant the new roses in holes about 2feet by 2 feet,filling said holes in with the new soil plus some amendments.But that's just me...

I don't believe that it's a problem in the US. In Florida, nematodes can harm roses and eventually they will fail. Many Florida growers use roses grafted on Fortuniana which is resistant to the nematodes.

David Stone who retired as head gardener of Mottifont Abbey in 2014 was convinced rose replant disease was actually condition of soil that had been robbed of beneficial microbial life by overuse of chemical fertilizer. He believed that you do not have to replace the soil, but rather should restore its health with compost. We know now how important mycorhizzal fungus is for healthy plant growth.

I've seen his garden, and I believe him.

In any case, whether he's right or whether he's wrong, it has never been a problem here in the United States.

Rosefolly

It's never been a problem in my area, and we have certainly removed roses, and replanted in the same location.

(OTOH, EVERYTHING we plant here must be planted in a nursery squat pot in the ground -- with many holes drilled. If not, gophers will eat the roots.)

I would be VERY dubious about the cardboard box thing. Here's why:

Some years ago, there was a big promotion of nursery pots made of compressed cardboard. You could plant a bare-root in the pot, and grow it up -- THEN plant it, pot and all.

The idea of it was that once-underground, the pot would disintegrate, and the roots would push out. In practice, what happened was, the pot did not disintegrate AT ALL.

When some roses failed to thrive, we dug down, and found that the roots were just swirled-around inside their cardboard prison. What a mess!

Jeri
Coastal Ventura Co., SoCal

I've done the cardboard box thing - I actually had to move the rose again, not even nine months later, (due to fencing issues) and the box had utterly disappeared, no trace left at all. I'm on the proverbial fence, as it were, but generally, I've decided that if I move a rose I will do so with as much of the soil that it is currently growing in as possible, and add rootgrow, new soil and compost to the old spot. I do use cardboard, but as a moisture retaining addition for the dry spots or those where there are competion from tree roots.

The American Rose Society has (at least) 2 articles on this subject, replant disease. One by Dr. Rankin is a 2009 article with a very long link so I put it in link box below.

The second is very recent by Professor Zlesak:

http://www.rose.org/advances-in-understanding-rose-replant-disease/

Here is a link that might be useful: Dr. Rankin 2009 article

..thank you for those links Mr Kuska... it tended to confirm what we already do.....

..I replace roses quite often and have experienced this on some occasions due to hurried replantings without changing the soil sufficiently.... you can usually tell quite soon if your rose is not thriving and a simple move to another slot solves the problem almost immediately....
...however, I have recently been using this mycorrhizal fungi with only minor soil amendment, and the results are promising.... it's early days and I'll know in a few weeks I think if it has worked..

...I've never used the cardboard method, it doesn't appeal, although with all our rain and damp it doesn't surprise me that this soon rots away.... like almost everything else does here..

...in fact, I wouldn't mind a bit of Californian sun right now...

Thank you all for the wonderful information and taking time to answer my questions. I do want my new babies to thrive so it looks like the new soil will cost more than the plants but will probably be worth it in the long run.

Marlorena, I will gladly take any and all snow and put it up in our mountains so we would have adequate water in the summertime. Wish that would happen.

I've done the cardboard thing and for me, too, it rotted away very quickly. I wonder if those "cardboard" pots Jeri told about were the same type of cardboard as that of common boxes; I sort of doubt it...

In my estimation, those of us who live in the arid West cannot depend on any kind of cardboard, newspaper, leaves, etc "rotting away very quickly". It just doesn't happen, at least for me. Way too dry. Diane

That's the idea I'm getting, too, Diane; did you see the thread about mulching with cardboard? Kim's comments made it clear that, at least in his area, it's a big no-no,yet for me, it's been a big help.We get awful drought in summer,but it's been raining and this year even snowing in fall and winter, so one can use the c-board to help hold in and save the precious moisture,but it certainly would not be good in a climate in which all it ever does is sprinkle a bit,I'd imagine...

This thread brings up several examples of things which act differently in different places/climates. It is not just the roses which act differently! Just a reminder to take specific advice about how a rose does, how to garden, etc. ONLY from those who garden in the same conditions that you do.

Jackie

Great advice. Looks like I will be replacing the soil. Sigh! Hopefully this will be the last time! Thank you so much.