Depends on what garden effect you are going for.

If you want a "formal garden" effect, you keep plants well separated.

If you want a "cottage garden" or "mixed border" effect, you allow plants to grow into each other.

Roses lend themselves to either design approach. Roses are so diverse that you could follow either approach with roses exclusively, or you could mix in other shrubs and/or perennials and/or even annuals.

Personally, I think that generally the HTs, floribundas, minis and the like fit in better in a more formal approach, while generally the OGRs, shrub roses, species, etc.., fit in better with more of the cottage garden approach. There are exceptions in both groups, of course, and it is really up to you.

In a "traditional English cottage garden", old garden roses, flowering perennials, and herbs were/are the essential features. But you can modernize that, of course, and still follow the same basic approach.

In any case, if you want plants to be spreading into each other, you do have to work at keeping the more aggressive ones in check or they will take over. That includes those roses that spread aggressively from suckers. In general, coreopsis is considered a fairly aggressive perennial, so I would be keeping it under control.

This post was edited by nickl on Mon, Jul 1, 13 at 11:03

Nickl's advice is a good rule of thumb for gauging your preferences for how your garden looks. Another thing to consider is competition for water resources, and how much water your rose and companion plants want. A well-established rose might be able to share water with a perennial where a new planting might be happier with more space around it. If water is a low commodity (as in California), I'd think folks would keep the perennials farther away and save the water for the roses. In VT, you probably have enough water to go around.

You also want to consider the water conditions each plant wants, which is why lavender may be a poor companion for roses in some places, since they want it drier than roses do. Coreopsis is pretty adaptable so it should be fine if the rose is fine. I'm definitely on the cottage garden side of things, so there are perennials or other plants bumping up against virtually all my roses, or at least within the drip lines, and they're usually fine with that. The only thing I have to watch is to make sure the mulch doesn't gather up too much around the woody base of the rose, or it'll encourage canker.

If you like the look of the coreopsis under the rose, and the threadlead variety should stay low enough not to compete for vertical space, then you could try letting it share the space for a while and watch the roses for signs of water stress (or canker). If they seem happy, then you can suit yourself as fits your style.

Cynthia

Thanks. Right now, in Vermont, we're drowning in rain, but even under normal conditions, water isn't an issue. I guess I was more concerned about the coreopsis shading the bottom of the rose and hurting it that way. It is certainly within the drip line of the rose. It's not really an esthetic issue, but canker might be a possibility with the rain we've had. Thanks for the responses.