The only thing that comes to mind is cosmos. They flower for me prolifically all season. And they come in every color imaginable. Just look for one with the right color and height that you desire.

Tracy

I see ageratum still blooming here.

Creeping zinnia! Still blooming here in Minnesota!
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A liitle hard to see here, but red ratibida. Also small zinnias:
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I forget the name of this -- strawflower? Amazing bloomer. Hard to photograph because it's so shiny:
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Nasturtiums, especially these variegated ones:
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But, now, take a look at these marigolds with cannas (this is from a garden in England -- the others are my gardens in different years.)
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Be sure to post photos when your marigolds are blooming!

What beautiful pictures! For companion plants to marigolds there are lots of things to choose from. I like blue ageratum with yellow marigolds, the blue and yellow are such a nice combination. Cosmos is a good choice too, you can get the orange, red-orange and yellow types, or the pink and white kinds. Zinnias are a good choice too, they come in all sizes and colors. Petunias are also an option, although they get leggy late in the season, if you cut them back they'll keep going. I assume its a sunny spot, so vinca works well. My vinca went through the terrible summer drought and just kept blooming. Nasturtiums also look great with marigolds, the red and orange colors complement it well, although mine stopped blooming when the temps got in the 90's here. I assume you'd prefer to not have to water often, since it's such a large border. Cosmos, vinca and nasturtiums can take it if if it gets a little dry. Being a lazy gardener, those things are important to me. Good luck!

You know a lot of people pooh-pooh marigolds but Im a big fan also. They are easy to grow...keep blooming all summer...likewise they are still going here...and help keep aways pests. I use them to fill in spots of color all over the yard.

I second the vinca....another favorite of mine. Mine have kept going in the worst of conditions. They actually came up volunteer by last years seed in one of my poorest/driest gardens. Stawberry Fiels Gomphrena would be cool with marigolds. In fact I think I might do those together next year!