Fennel for the butterflies
Flowers & Herbs, Garden of Eatin'

Fennel For the Butterflies

In the back of my garden, I have a small patch of fennel (Foeniculum vulgare) that grows year-round and stands five feet tall. Sometimes I’ll harvest the fronds for a salad or a bulb for my favorite seafood stew, or even the pollen or seeds for my cooking, but for the most part, I let the fennel grow “wild” here.

Florence fennel in the garden

Aside from being edible and beautiful with wisps of anise fragrance wafting through the air, it also happens to be a beneficial plant that functions as a trap crop.

What does it trap? Parsleyworms, the striking caterpillars that eventually turn into swallowtail butterflies.

West of the Rockies, parsleyworms are the beginnings of the anise swallowtail butterfly. Its eastern version is the black swallowtail butterfly, and while the butterflies look distinctly different, their caterpillars and chrysalises are nearly identical.

Parsleyworms are so named because they feed on members of the parsley family, Apiaceae, including parsley, dill, carrot, Queen Anne’s lace, and of course, fennel.

They can be found in all stages of life on these host plants, and it’s truly fascinating to revisit your plants each week to see their natural transformation. As the parsleyworm eats and grows, it sheds an exoskeleton and emerges with a completely different appearance. In total, it goes through five instars (stages) of life.

Here’s the first (or possibly second) instar…

First instar of swallowtail butterfly caterpillar

The third instar…

Third instar of swallowtail butterfly caterpillar

The fourth instar (with the plump black-and-green striped body that you may be most familiar with)…

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And the fifth and final instar, also known as the chrysalis stage. It’s from here that the parsleyworm pupates and emerges as an adult butterfly.

Fifth instar of swallowtail butterfly caterpillar, also known as a chrysalis

Since swallowtail butterflies lay their eggs on any member of the Apiaceae family, you might occasionally find them on your carrots or parsley as well. They have large appetites, so unless you planted enough for yourself and the butterflies, it can be a depressing sight to see your food crop munched down to its stems by an army of hungry caterpillars.

Planting a separate crop of fennel (or any related member) can help preserve the butterflies in your garden. I especially like Florence fennel, or finocchio, a bulbing variety that gives you a vegetable, an herb, and two spices over its lifespan. It’s a cool-weather crop that can be planted after the last frost for a spring harvest, or mid to late summer for an autumn harvest.

If you find a parsleyworm feeding where it shouldn’t be feeding, you can simply relocate it to your fennel patch and at the end of the season, you’ll still have some bulbs to harvest. A win all around.

Florence fennel bulb

Florence fennel

Parsleyworm on fennel

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  • Michelle

    But isn’t fennel alleopathic? I’ve been dying to grow it, but I keep reading about the negative influence it has on all other plants.

    • Michelle

      *allelopathic

    • I grow a perennial bed of fennel next to a kumquat tree without any adverse effects, so it definitely depends on the plant.

      The entire Apiaceae family (including cumin, dill, etc) actually has allelopathic characteristics. It’s believed that the allelochemicals are found in the oils of the seeds themselves. Fennel seed has been shown to inhibit the seed germination and/or root elongation of certain plants when there’s a high concentration of fennel planted in an area (because presumably, where there’s a lot of fennel, there’s a lot of fennel seed that self-sowed).

      If you’re only growing fennel as an annual, you can plant it anywhere in the garden so long as you pull it up before it seeds. If you’re growing it year-round, it’s best to keep it somewhat isolated in the garden because it does self-seed easily and you may find yourself weeding it constantly from your vegetable beds.

      • Michelle

        Thank you so much! I’ll be sure to try it next spring, probably isolating a little in case I can’t get to the seeds in time. šŸ™‚

  • I have a small patch of fennel and a patch of bronze fennel, both were planted about 4 years ago. I only started noticing in the past two years that the butterflies seem to love devouring the plant. I enjoy watching them throughout their growth – especially when they cocoon.

    • The cocoons are so beautiful! Like little ornaments on the plants. šŸ™‚

  • yawningreyhound

    Guess we don’t want these in the chicken area or parsleyworms will be the daily treat! CONGRATS on the Where Women Cook issue! I just voted on the cover through Stampington.

    • Ohhh I didn’t know there was voting going on for the cover! Thank you!

  • Sheila

    Looks so pretty but it’s seriously invasive in my area of So Cal. I’ve never planted it but find it popping up where I removed the lawn and do my best to remove it!

    • Fennel can be invasive, but like any plant, you can grow it responsibly in your garden and treat it as an annual, if need be.

  • Laurie

    I always have bronze fennel planted for the butterflies. I have all the stages of growth, from eggs to butterfly, photographed (macro) of the black swallowtail butterfly. I also discovered that aphids like the fennel and that attracts ladybugs. I have many stages of their growth photographed as well. I was starting in on photos of lacewings when winter hit. My one huge plant was an ecosystem in itself! This year my fennel doesn’t have any critters on it. Not yet at least.

    • Isn’t it incredible how a single plant can be host to such wildlife? I love it. šŸ™‚

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