I am going to say you could as long as you eventually trimmed the roots back. I would look into Bonsai Forums and ask the same question. Eventually your roots are going to outgrow it's container it's held in and the over all vitality of that bush would decline.

The container is 5 ft x 2ft x 2ft and has 3 arborvitae in it. So, it isn't small, but since the arborvitae take up most of the container, I can't have a rose bush that is huge.

I have seen rose bonsai done but always with miniature roses. A rose needs to be able to grow to put on new blooms.

The smaller the rose bush the fewer the flowers. The point of a rose bush is flowers. It's not a foliage plant. 3'-4' is a small rose.

I was happy with how many flowers were already on it. I'm getting tired of annuals and getting anything with nice flowers that comes back and is smaller but not too small is proving difficult. I'm in zone 5.

It'll depend on the bush. Drift roses and smaller hybrid read or floribundas yes. But if you are talking a bigger bush you will have to work at it.

In zone 5 you would need to over winter the rose indoors.

Not necessarily, Patty. Temperate tree bonsai are many times overwintered outdoors, though there are steps which have to be taken to help them survive. Some folks overwinter them in an unheated garage. If the rose was treated as a bonsai, perhaps it would make it.

A rose hardy to Zone 5 may not be that hardy in a pot. Soil insulates the roots quite a bit, even with deep frosts. A pot full of soil is exposed to the air temps all around it, sides, top and bottom, so it may easily freeze solid and that may well kill the plant. Plus, how do you protect the portion above the soil from the drying effects of a freezing wind? Sunny days can thaw the soil and the plant, only to have it refreeze that night or one or several later. It honestly doesn't sound like a very suitable situation for a rose to be put on a roof in a pot in Zone 5. Simply treating a rose plant as a bonsai does not change how cold (or heat) hardy it is likely to be. That simply means you're going to torture it, inhibit its growth and stunt it into an unnatural shape. It doesn't change its genetic cold resistance.

Do you remember the name of the rose? Rose size varies greatly depending on location. A rose listed at 4x3 might grow to 3x2 further north, and 8x6 further south.

4x3 is pretty standard size listing for Floribundas and shrub roses, and once established that should be about the size to expect if given good growing conditions. At least this is what I have found in my Zone 5 garden.

You could keep the rose smaller through pruning, but pruning promotes growth and the rose might end up putting on new canes and foliage instead of flowers. Instead I would look for a similar, but smaller rose. Or else find a way to make enough space for it.

I don't remember the name. I know it had star shaped flowers that didn't look like typical roses.

Hey, I just saw that you were tired of annuals, but you wanted something small? Is your heart set on a rose or are you looking for anything that will flower? Because there are a freaking ton of smaller perennials that will give you loads of flowers and can be used instead of annuals. They usually won't flower as much as annuals, but if you get a good mix of them with different bloom times you can always have something blooming.

Take a look at the Ben Ledi Rock Rose (helianthemum). It is a shrubby perennial. It is the red plant in my picture. It comes in several colors. Stays small and is naturally mounding, no trimming. Heat and drought tolerant. I have it in a bed with ice plants where most things cannot survive. Spectacular blooms. Would probably do very well on a roof in direct sun constantly and with little water.

http://www.perennials.com/plants/helianthemum-ben-ledi.html

http://www.perennials.com/plants/helianthemum-henfield-brilliant.html

All the different colors:

http://www.perennials.com/results.html?findplant=helianthemum&searchbutton.x=0&searchbutton.y=0

While I am a huge lover of the sunrose (Heliathemum nummularia) pictured above, it will only be marginally hardy in a z5 climate and not at all winter hardy in a container! It also has a rather short bloom period, like most perennials, usually just a couple of weeks from mid May to mid June.