Rue and bronze fennel for swallowtails

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In the past two weeks, I’ve caught two female giant swallowtails laying eggs on my rue, one of their host plants. It’s so exciting to see this aptly named butterfly fluttering around my yard… so big and beautiful! It is worth planting rue just to attract them. This year, I decided to keep my two favorite swallowtail host plants in pots again:

  • Rue (probably Ruta graveolens), host plant for giant swallowtails
  • Bronze fennel (Foeniculum vulgare), host plant for black swallowtails (from the Rockies on east) and anise swallowtails (from the Rockies on west)
giant swallowtail laying eggs on rue

Giant swallowtail on rue.


a bronze fennel in a pot

Bronze fennel in a pot. This plant is over two years old; being potted is probably limiting its size.

I initially started my plants in pots because I wasn’t certain where to plant them in my new yard. I am still learning how to keep these two plants happy (hint: they don’t like soggy soil), as well as learning about where water collects in my yard (apparently almost everywhere). Keeping them in pots allows me better control over moisture and sun.

It also occurred to me that keeping them in pots allows me some flexibility in protecting my caterpillars (cats) from birds and parasitic wasps once they hatch. I can move them closer to my house where birds are less likely to find them, or I can even move them indoors for a while (supposedly, bronze fennel does not do well indoors, so this might not be a good idea).


Once I dragged it out of my greenhouse, it took less than a month for a giant swallowtail to find my rue. I’ve mentioned before how astonishing it is that you can plant a single butterfly host plant, and the appropriate butterfly will find it. This year was no different.

Giant swallowtail female laying egg on rue

Giant swallowtail female laying an egg on rue.

Last year, I had black swallowtails in my yard all season, and my bronze fennel was devoured by two waves of black swallowtail caterpillars. The fennel grew back, and I overwintered it in my greenhouse along with my rue.

Black swallowtail cats on bronze fennel

Black swallowtail caterpillars on bronze fennel.

Of course, there is nothing wrong with planting rue and bronze fennel in the ground. The plants will grow larger, providing more food for hungry caterpillars. I will plant my plants once I’ve identified appropriate spots for them.

Rue is a native of Mediterranean regions, so it does well in sunny, well-drained spots, preferring poor soils, in zones 4-9.  An aromatic herb, many keep it as part of their herb gardens, but some online sources say it will inhibit the growth of certain other herbs, including sage, basil, and mint, so be careful where you place it.

Rue can cause skin irritation; the combination of rue oil and sunlight can cause painful blisters, so protect yourself by wearing gloves when working with this plant. It can also be mildly toxic if too much is ingested, so research how to use the plant safely as an herb.

giant swallowtail cat

Very small giant swallowtail caterpillar on rue.

Bronze fennel, another Mediterranean native, also likes sun and well-drained soils, but it only overwinters to zone 5. It can tolerate some shade. When happy, bronze fennel can grow to be a huge plant, up to 4 feet tall, so give it plenty of room. Fennel is a popular spice, although I don’t care much for it. That’s okay; I’d rather leave my rue and bronze fennel for my swallowtail cats, anyway.

Meet Leslie Miller

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