I'd give the linked thread a looking over.

Here is a link that might be useful: floppy Fennel

I have Bronze Fennel and it holds up better than dill in the heat but rue does even better. In my yard, Black Swallowtails seem to prefer Bronze Fennel until it starts to get a little crispy in late summer and then they use rue more. Bronze Fennel and Rue will both come back every year and have pretty foliage so that's a bonus. Bronze Fennel reseeds a bit too much though if you don't keep it deadheaded.

I have only grown regular fennel once and it didn't do as well for me as the Bronze. It didn't come back after winter.

I'm in southwest Missouri and we typically have lots of days in the 90's in the summer and a few over 100. Last summer was hotter than usual and we had a bunch of days over 100. My rue and Bronze Fennel both suffered but didn't die.

Most garden centers will have rue (Ruta graveolens) pretty cheap in the spring but you have to look in the herb section rather than with the perennials. They might have Bronze Fennel too. I get volunteers from both so I'm guessing that they would be easy to grow from seed.

I grow dill, bronze fennel, parsley, rue and Zizia for the Black Swallowtails. They seem to have different preferences each year. This past summer they used parsley, rue, and dill, but I didn't find an egg or cat on the Bronze fennel. Two years ago there were lots of cats on it though. They haven't laid eggs on the Zizia at all yet, but they do use it as larval food.

Fennel does take the heat much better than dill. The Bronze fennel was beautiful all season, although it did get a bit floppy. In my experience all the BST host plants are easy to start from seed.

I always grow a patch of carrots for the BST as well. When the other food sources run out, carrot tops are a good food source to fall back on, I don't even care about the carrots, they never grow very long or sweet, so I just grow them for the tops the cats. Easy to grow and you can grow lots of them in a small space, especially if you don't really care about harvesting the carrots.

Mechelle

Mechelle, what a fantastic idea! Since you are in Texas where the heat can be intense, do the tops seem to last longer without bolting than either Fennel, Dill, or Parsley?

Susan

Here in PA the dill doesn't last the whole summer; the fennel lasts somewhat longer. I also grow parsley for the Black Swallowtails and have 14 big rue plants and use that more than anything to feed the BSTs. I've been growing carrots the past 3 years and have only ever found 2 or 3 cats on the tops, so the least favored plant here would be carrots. It seems like each year is a little different, eg. one year they'll like dill the best and another year they'll like rue or parsley or fennel. It really does vary, but they always avoid the carrots here. I think each person has to use trial and error to find out what to plant for them, but honestly, I have all of those here every year because I'd be afraid that one year that I don't have something would be the year that they're looking for that particular plant to lay their eggs on! lol
Cathy

I forgot about the Golden Alexanders! I've been meaning to look for seeds for Zizia aptera or aurea. They are native perennials, so they'd probably do better than other non-native BST plants. I think a zizia patch would look pretty nice with some Verbena bonariensis. Too bad the BSTs don't favor them; I'm a little surprised at that. You'd think they would favor the native plants over the non-natives?

Thank you all for your helpful advice. I think fennel, rue, and Golden Alexander are all on the list for this year's additions.

To be honest, none of the Apiaceae family plants have held up extremely well in the heat. I haven't found one yet that does, that is. Rue is in Rutaceae family. I have heard of other Rutaceae members that do well, but the BSTs only use Rue and Dutchman's Breeches, and Giant Swallowtails use most of the other Rutaceae plants/shrubs/trees. Rue does stop growing for me in the heat of the summer, but doesn't lose the foliage it already has. I grow it in a bit of shade so the foliage lasts longer. I also do the Fennel and Dill. Rue roots well in water, so you might be able to keep it going by planting new starts along the way.

Another plant that may do well is Mock Bishop's Weed, or Ptilimnium capillaceum. Like Dill, it is an annual, but seems to grow well in Southern climates, so obviously can take the heat. The down side is that it likes a lot of water.

Another plant in the Rutaceae family that tolerates heat well is Thamnosma texana. It's a perennial native in Texas, so probably would grow as an annual in your location. It doesn't appear to have substantial foliage, so you would probably need to plant lots of seed. It can grow in very dry conditions. A common name associated with this plant is Dutchman's Breeches, but if you google name you get more results for Dicentra species, which is why I prefer to use the Latin names.

My zone 7 gets very, very hot in the summer. Last year year we had 60+ days 100+ degrees. The Fennel bolted quickly, but had lots and lots of flowers and the larvae LOVE the flowers. I have to harvest the cats quickly because otherwise the wasps that love the flowers, too, l
love them.

I grow Rue and Zizia and Dill as well, so I have at least something at all times to offer the Mamas for egg-laying.

Hope this helps.

Susan

I don't have problems with the carrots bolting, probably because it gets so hot here in the summer. I have never found an egg or cat on the carrot tops, but when I run out of their preferred food, they will eat the carrot tops. I have found that their least favorite food is Queen Anne's Lace. Most of my butterfly books list this as a must have for the BST's, but they seem to really not like it. Even when they have run out of food, they don't willingly go to the QAL, instead they chew on the remnants of the parsley, fennel, rue stems that had been stripped of leaves from their mealtimes.

Mechelle

I grow fennel, dill, parsley, and Golden Alexander. I've never found an egg or cat on either the Zizia or the Parsley. This summer, my dill didn't germinate very well, so I was left with primarily the fennel that came back from the initial planting last year. I had more eggs and cats this season than ever before. So, at least in my small part of the world, fennel does the job. The Zizia reseeds very enthusiastically, so is more irritating than I'm willing to put up with. I'm lucky to have relatively cool summers that are tolerated well by the dill and fennel. I think I'll stick to them.

Martha