There are lots of different types of Verbena. Lobelia, frogfruit, bronze fennel, 'Autumn Joy'Sedum, partridge pea.

Do you want real low growing, that stay under a foot?

Verbena, 'New Look' Penta, Aristolochia fimbriata, are all ground huggers and attract lots of butterfly traffic.

Also you could let passion vines ramble along in front of the milkweed as a groundcover.

There are low growing stokes asters that would do - they're about as easy to grow as you can get!
As I recall, there are some decorative oreganos that stay small and attract butterflies like crazy - the cultivar I'm talking about has a German sounding name - Sangerhausen?
MissSherry

Sorry for the delay... Thank you for the suggestions... The verbena and asters are especially interesting to me, because aren't they also host plants?? Can't wait for the ground to unfreeze, so I can start diggin' in the dirt!!!

I plant Purple Homestead Verbena as a ground cover for butterflies. It is a perennial here in Louisiana. Stays low and hugs the ground with constant flowering. Also very beautiful and atractive for any kind of garden

Leubafr

Stokes asters/stokesias are a different type of aster - I don't think they host anything, but they sure attract the butterflies with their nectar, and they make a big, beautiful flower.
MissSherry

I don't think that the Verbena is hardy in Philly, unless it is planted every year as a annual. There are low-growing phloxes that are perennial, like 'Nicky', and you could intersperse with some low growing liatris like 'Kobold', which is shorter than other liatris. I like Misssherry's idea of the Stokes Asters, and some of the newer cultivars have a longer bloom time. Some of the shorter salvias are also good, and often, if not perennial, will reseed. Monardas that are shorter are also good nectar plants.

I don't know which butterfly weed you're growing, and whether it is one that likes a lot of moisture or is pretty drought tolerant. You would probably want to choose something that would tolerate the same conditions as the milkweed. If you wanted to stick with something in the milkweed family, there are shorter growing milkweeds, like verticillata, that has feathery foliage that could make a nice contrast with other milkweeds.

Susan

I am growing A. tuberosa, and I need something to hide the unsightly aphid attacks, which I generally don't do anything about, because I find that, except for making the plant look pretty ugly, they aren't really hurting it--- they seem to co-exist peacefully with the cats!

Cydonia33 - a girl after my own heart! I don't either. If I leave them, I get more beneficial lady bugs and lacewings, who do the job for me. If I blast them with water, I would probably knock off tiny cats and eggs before I would do any good getting rid of them. So, I just leave them.

Susan

My Lantana 'Pink Dawn', a soft pink/yellow variety, looks beautiful with my A. curassavica. I also fill in with some low-growing Zinnias of all colors. The Homestead Purple Verbena is in there too, and it's the first thing to bloom in my garden each year! This year I'm adding Borage and Phacelia to attract the beneficials (my wintersown seeds have already sprouted), and they're going to go in front of the Bloodflowers too.

Do all of these low-growing things (like the verbena and phloxes) die all the way back in the winter, because I have drifts of crocuses (blooming now!!) that I wouldn't want to cover up...

You can trim back the dead foliage on your plants before your crocuses start blooming. I would wait until right before they bloom, though and use mulch.

Susan

It's not native, but I always seed lots of Calendula seeds in front of my Butterfly Weed...the flowers are beautiful and they attract all kinds of beneficial insects. They also have lush leaves which cover up any caterpillar damage to the milkweeds :-)