If I could only have two plants for butterflies, I'd pick fennel (especially the bronze one) and Joe-Pye Weed (Eupatorium sp.). Since I've started growing fennel, I always find a good crop of caterpillers and when the Joe-Pye blooms.
I am astounded at how many swallowtails gather! They will leave the Buddleia for it. This year, I hadn't seen that many 'Tails in the yard, though I saw a good bunch drinking from the sand at the creek. But the word got out when the Joe-Pye bloomed: tigers, blacks, monarchs and assorted others.
I got my Joe-Pye start from the roadside, so it's usually 8-10 feet tall. I added a clump of "Gateway", but still prefer my tall Joes. Likes a good bit of moisture and lots of sun. Spreads from year to year.
Try it. And grow some fennel, too.
raydio.
Two favorite plants for swallowtails.
I love the green fennel. The bronze is ok, but is always the second choice of my BST's. It seems to get aphids easier here too.
I love Joe Pye weed, but I just can't seem to keep it alive!! I had a small one that died, again, and then found a tall one this year. It's brown and crunchy looking now, but I'm hoping it's not totally dead. It did have time to bloom a little and reseed, so maybe some will grow from that.
Do you have yours planted in full sun? What kind of water needs does it require in your area? It's HOT and DRY here this year, but I tried to keep it watered.
The *do* need lots of water and are often found in ditches that are always wet to moist. I have mine in almost full sun, and in August they will scortch some if the soil dries out too much. I think as hot as your zone can be, the moisture requiremant may be a factor in them not doing as well as you'd like.
The soil where I have mine holds water well, but still requires attention once the summer heat really sets in. You might think of adding some humus, peat, or lots of compost to enrich your soil and help it retain water. They're not picky as to soil as long as they get water, but I'd still amend sandy, thin soils with lots of organic matter and good loam.
I have noticed that a whole stem will suddenly die out and I've never been certain as to why. Most likely too dry.
raydio.
One thing you can do with the tall variety if you don't want them so tall, is to cut them back or pinch the leader early to mid-season. They'll branch and you'll have more bloom heads, though they will usually be smaller than they would have been unpruned.
They will produce lots of seed and they'll spring up whenever the seed land in a "good spot", usually moisture retaining or at least under other plants that shade or cover the ground , thereby making it moister in general.