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How to Use Fennel (or Dill) to Keep Caterpillars Off Your Vegetable Plants

I don’t do a lot of companion planting. I’m not saying it doesn’t work (and I’d love to hear your success stories) but other than being impressed by mycorrhizal fungi, I haven’t felt the need to find my plants a buddy.

Oh! EXCEPT for growing fennel next to plants plagued by caterpillars. Flowering fennel is like a beacon for caterpillar-eating wasps. Think of veggie-munching caterpillars as the bad guys, the wasps as Batman, and fennel as the bat signal. Here they come to save the day!

(Okay yeah, if I distressed your geek-meter I do know that last bit came from Mighty Mouse).

Green fennel blooms are a beacon for beneficial wasps. Plant them next to crops that have caterpillar pests and watch the wasps carry away the offending larva.

Photo Caption: Fennel blooms are a beacon for beneficial wasps. Plant them next to crops that have caterpillar pests and watch the wasps carry away the offending larva. My tomatoes and fennel in this photo worked well together - I saw no tomato hornworms all season. Dill is a close relative of fennel and will work equally well.

(Much to my insect-loving daughter’s chagrin) I haven’t dealt with tomato hornworms for several years now. Since I know they are in my neighborhood I attribute the lack of them in my garden to the row of fennel next to the tomatoes.

When the fennel is blooming you can sit next to it and watch the wasps come in. Tiny parasitic wasps love the nectar-filled blooms but it also alerts larger predatory wasps. If wasps discover caterpillars in the area they quickly take advantage of this high-protein food.

You may be surprised but social wasps like yellow jackets and hornets are excellent garden guards. If their nest isn’t close by they aren’t interested in stinging you. They can clean out a caterpillar infestation in a matter of minutes — I’ve watched them do it! A hunting wasp will take a caterpillar back to its nest and then alert its roommates where the tasty larva was found. The other wasps go (like an army of flying monkeys) to collect the rest of the caterpillars. An hour later your plant is free of pests!

Parasitic wasps are also effective but the results are less obvious. Caterpillars that have had eggs laid inside them may continue feeding for a brief period but inevitably they will die. If you see rice-like cocoons pop up on the back of a caterpillar you can be sure its life is nearly over. Don’t destroy the cocoons! They will hatch into adult parasitic wasps that keep the cycle going.

The trickiest part is that you have to keep the flowers pruned when they begin to form seeds or the insects won’t come!

Fennel is gorgeous in the garden and the umbels of blooms seem to last forever… but it is misleading! Once the sticky, nectar-rich, pollen-sprinkled flowers start to elongate and take on a matte appearance they are actually immature seeds. These photos of undeveloped fennel seeds (taken by Anna at the Flower Garden Girl blog) show what you should prune off. Once they are at this stage by all means take them indoors to enjoy — then your plant will make the effort to flower again and attract more insects.

This is what fennel blooms look like before they start forming seeds:

Photo Caption: Fennel blooms like these attract insects until they start to go to seed. When they lose their petals and start to elongate you should clip them off. Your plant will make more.

This is also useful information if you are a butterfly gardener and you want to keep your caterpillars safe. If you don’t want wasps to carry away the black swallowtails eating your fennel, clip off the blooms the moment the blossom stalks appear. If you never let it flower you will reduce the number of wasps visiting your garden.

Photo Caption: This black swallowtail caterpillar eats fennel and turns into a popular butterfly. If you are butterfly gardening it is best to put your host plant fennel farther away from your vegetable crops so that you won't mind cutting off the flowers.

Photo Caption: Black swallowtail butterflies are a common garden visitor. The caterpillars eat fennel and other plants in the same family, such as parsley.

I love fennel! You can also eat the stalks of bronze fennel (like you would bulbing fennel) or you can use the seeds in curries and other recipes.

It makes a stunning ornamental — especially the bronze variety. I like to cut the feathery leaves to use as filler in heirloom rose bouquets.

If you’d like to get started with fennel near your veggies this year, you can buy the bronze or green variety. Bulbing fennel also blooms and serves a dual purpose as a tasty vegetable!

Alternately, you can use other members of the carrot family. Dill is the best choice, the wasps like it equally as well as fennel. Unlike fennel, (a perennial), dill is an annual, so it may be a better choice if you plan to completely empty your vegetable garden beds at the end of the season. Other options are Queen Anne’s Lace, cilantro, caraway, and parsley. Note that parsley is not as free blooming as the others and the flowers are what appeals to the insects you want.

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