You'll need a pot large enough for a really big root system if you expect a big climber. Although more compact, the root system will be as big as the top growth.
I'd get a large tree pot, minimum. Anything smaller won't allow he roots to develop and your climber will be stunted.I'm not taking about those bigger anchor roots you can see, I'm talking about the many tiny feeder roots that supply moisture and nutrients to the top growth.

When I had New Dawn, it took a while to mature. After it's 2nd year it was about 9ish ft tall, but far from mature. The initial spring flush of it's 2nd year was a pretty dense 6ft tall climber.

I would make sure to amend some peat in the soil.....you are going to need all the water you can get. As far as pots, I would make your own out of some wood. Something along the lines of a 4x4x2.5ft box and lots of good soil. I used Scott's moisture control soil in my pots, but get some peat in your case. You will have to fertilize and water A LOT more when the rose is mature. Even in a large box container, a climber will fill that with roots.

I've got a climbing rose in a container, but it's not 'New Dawn'. I have a 'Laguna' rose. This is what my 1st year Laguna rose looks like (Winter):

{{gwi:299208}}

It's over 2.5m tall. It could be a lot taller if I didn't wrap those arching canes around those stakes.

I agree that you need a really large pot for a climbing rose such as New Dawn because it can grow very quickly. I made the mistake of planting it into a small (50cm)container.

God knows what would happen to this rose next year. How am I going to transplant this rose? I've already trained it into a pillar. I don't think I will transplant this rose into a larger pot because it's too big now with sharp thorns and the canes have been staked and wrapped up.

I'm going to top mine up with soil and lightly prune off any growing tips in spring to prevent them from wilting.

The other thing you should know about growing a climbing rose in a container is, if it's a plastic container, it can easily be knocked over by strong winds. It's best to secure those canes.

Here's a link to (Susan's)Trivoli's matured NEW DAWN's rose:

Here is a link that might be useful: Susan's New Dawn

I have Blaze in a pot, a really REALLY BIG POT! It's been potted for about 5 years and is growing like a weed and is covered with buds right now. Make sure your trellis is VERY strong! New Dawn can be a monster when it's really happy!

Hmm... The general consensus is that I need a very large pot with soil that is able to retain water?

karl_bapst_rosenut: Do you have any idea what the actual size the root system will be? It would be a good measurement to know. That would help determine the minimum size requirement of my pot.

jeffcat: Peat... I'm remember to add lots of that when I prepare my soil mix. When you say I will need to fertilize a lot more when the plant is mature, what kind of fertilization regime are we talking about? Could you elaborate, please. Thanks for providing the dimensions for the planter. It gave me a better idea of the mammoth planter I will need.

jumbojimmy: Your Laguna must be gorgeous when in bloom! I chose New Dawn because I needed a climber that will grow to gargantuan proportions, is disease resistant, in one of the paler colours (light pink, white or pale yellow) and would survive my zone (5b/6a) with no protection (I hope). I'll have to see if there are any other easy care climber roses that fit my criteria. Maybe I could even locate a fragrant rose (fingers crossed)! Good luck moving your rose to a larger container! I suggest getting some armoured buff bods to do the job. Hmm... I was thinking of getting a concrete/stone container (expensive, I know), but now I will remember to stay away from light plastics. What kind of soil mix are you using for your Laguna, btw?

seil z6: How big is this really big pot your Blaze is in? Is there a chance of getting a photo? And yes, I want a happy monster rose! Large and healthy, making a valiant effort to eat my trellis and balcony! I'd like to know what medium you used to pot your rose as well!

The root system will be as large as the amount of top growth, although more compact. if the total length of all the canes is 20 feet, it'll have 20 feet of roots.
Remember, most of those roots will be tiny almost invisible feeder roots and will take up much less volumn than the canes. Confining the roots to too small of a pot will cause the plant to get root bound and severely stunt it's growth. If attempting to grow a large climber such as New Dawn, Get as large a pot as you can handle or your rose won't achieve it's potential. I'd use a very large tree pot for New Dawn.

This is not the best picture of the rose in bloom but it is a good picture of the pot!
{{gwi:299210}}

That pot is 20 inches tall and 24 inches wide. And I have bigger ones now but I'm moving Blaze into a new bed we're making right now so I didn't bother to up pot her.

Always, ALWAYS use good light potting soil, never dirt or garden soil, in pots. And make sure there are several good drainage holes. Drainage is key to growing roses in pots. However, that also means you need to water and feed them more frequently than roses in the ground because of that good drainage. Don't think that by keeping the water in more you'll avoid this. Roses hate to sit in water or soggy soil. You can mix in some of those water holding crystals but don't over due that either.

Before you put any soil in the pot cover the drainage holes with coffee filters. They will eventually dissolve but in the mean time they will keep the fresh soil from running out the bottom. Then put about an inch or two layer of styrofoam (not the new biodegradable ones) packing peanuts in the bottom. In your zone I also recommend lining the side walls of the pot with bubble wrap for two reasons. First, it will help prevent the pot from cracking in winter freeze and thaw. Second, it will also help insulate the rose roots in winter. After all that then you can put in the potting soil and plant your rose. And yes, even in pots I plant deep! Bud union at least 2 inches below the soil level.

I also keep all my potted roses on those pot trolleys. It allows the pot to drain better and it lets me move those heavy pots around more easily and without having to lift them. Believe me a pot that size full of wet dirt and a full grown rose weighs a TON! It also insulates it from the cement of my patio. Cement is like a sponge and will draw moisture out of the pot. And in the winter the frozen cement will suck any warmth out of the soil as well.

That particular pot is enameled steel but what I prefer now are the hick walled foam ones. They're lighter, hold up longer and have better insulating properties. The thin walled plastic pots will due but they will crack after a few seasons from UV exposure. Stay far away from ceramic, glazed or otherwise! Even with the bubble wrap they will crack from winter freeze. Or the pretty glazed finish will all crack and fall off. And if you think a plastic pot with a rose in it is heavy, HA! You'll need a crane to move a ceramic one!

And if you're planning on putting that climber in it get a good trellis that will fit inside the pot. I got that one at Big Lots and it was a tad too wide so the ends are bent in a bit to fit down into the pot. And the trellis is also staked down as well because when that rose is full of open blooms the weight will tip the trellis forward.

For winterizing your best bet is an unheated garage or shed. I don't have a garage and there's no way I can get my 50 some potted roses in my shed. His Nibbs would have a fit anyway if his precious lawn tractor had to sit out all winter! The only potted rose that gets to go in the shed for winter is my tree rose because the graft is at the top and not buried in the pot. The rest of my pots winter outside, against the south wall of my house, buried in leaves and wrapped in burlap. In 6 winters now I've only lost a handful of roses to winter kill. In fact I've lost more of the roses planted in the ground than the ones in the pots!

Yegads! This got long! Sorry, but I think that covers everything. If you have any questions you can email me and I'll send you the paper I wrote for my rose society about how I do all this stuff.

Karl: I think based on what you are telling me... New Dawn might not be the best choice for what I have in mind. I'm currently looking at "Felix Leclerc" from the Canadian Artist series as an alternative. I would have preferred a fuller, paler bloom... we'll see... I'll shop around some more.

seil: I would love to read the paper you wrote! I will keep your advice in mind. As I told Karl, I am looking into smaller climbers that I would be able to move in winter. Given that I need my rose to be as winter hardy as possible, I am looking at am own root variety of "Felix Leclerc" from the Canadian Artist Collection. Still not my final choice, but it does have promise!

And now I need to figure out how to email you using this forum...

Paper's on it's way, enjoy!