Fennel and Dill allelopathic - Ask Extension
I purchased Fennel and Dill plants in March on the advise of some speakers who said they attract hummingbirds and butterflies. I was going to plant t...
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Fennel and Dill allelopathic #933609
Asked May 31, 2026, 11:02 AM EDT
I purchased Fennel and Dill plants in March on the advise of some speakers who said they attract hummingbirds and butterflies. I was going to plant them today and in searching where they should be planted (ground or pot), I discovered they could hurt other plants in the garden. The speakers at the herb seminar did not talk about this. The flower garden will not have any vegetables but I don't want it to damage any plants either.. I have no additional garden that has no plants and they are too big to plant in a pot on my deck. Appreciate your help.
Baltimore County Maryland
Expert Response
The flowers of fennel and dill won't attract hummingbirds as a nectar source, but the small insects they do attract can be food for the birds, which consume a lot of insects (especially to feed their nestlings with). The butterfly primarily attracted to those two herbs is the Eastern Black Swallowtail, whose caterpillars use them as a host plant. (They also use native members of the same carrot family, like Zizia aurea, which might be a better choice if you have the right conditions for them, since they'd be a more "natural" food source that this butterfly species evolved with.)
Fennel and dill can have some degree of allelopathy, but so do a wide variety of other fairly common plants, including sunflowers, onions, and black walnut trees. Allelopathic impacts won't necessarily kill neighboring plants; sometimes they just stifle growth to a degree. (The sensitivity of other plants to allelopathic compounds also varies considerably, and there isn't data for every plant that fennel or dill root chemicals impact, or to what extent they cause those impacts.
Nightshades are noted to be sensitive to fennel's root compounds -- tomato, potato, pepper, eggplant -- but we don't have information on exposure level and how much distance should separate them. Field research (rather than lab conditions) on allelopathy is hard to do since a range of factors can interfere with how allelopathic chemicals reach and interact with other plants, including the soil microbe community, the soil's own chemistry, and weather conditions (how damp or dry the soil is, for instance). Allelopathic compounds do eventually degrade and become harmless.
We think it's unlikely that a lone plant or two of either fennel or dill will cause any noticeable problems, especially if they are separated by a couple of feet from nearby plants. Be aware that fennel can be quite weedy and self-seed readily, so you may want to dead-head the plant (remove developing seed clusters once the blooms fade) or be vigilant for seedlings to remove. Fennel is biennial, so it only lives for two years, and will go to seed in its second year, so after that, the parent plant dies and only the seedlings would regrow the following year. Dill is either annual (one year life cycle) or biennial, and it prefers cool weather, so can struggle in our summer heat.
If you want a more long-lasting butterfly caterpillar host plant, Zizia or another native member of the carrot or rue family would better serve that butterfly's caterpillars. (Although, to be fair, they do seem to prefer laying eggs on those non-native carrot-family herbs, although we don't know if the quality of that food is really as suitable for them as the native plants would be.)
Miri
Fennel and dill can have some degree of allelopathy, but so do a wide variety of other fairly common plants, including sunflowers, onions, and black walnut trees. Allelopathic impacts won't necessarily kill neighboring plants; sometimes they just stifle growth to a degree. (The sensitivity of other plants to allelopathic compounds also varies considerably, and there isn't data for every plant that fennel or dill root chemicals impact, or to what extent they cause those impacts.
Nightshades are noted to be sensitive to fennel's root compounds -- tomato, potato, pepper, eggplant -- but we don't have information on exposure level and how much distance should separate them. Field research (rather than lab conditions) on allelopathy is hard to do since a range of factors can interfere with how allelopathic chemicals reach and interact with other plants, including the soil microbe community, the soil's own chemistry, and weather conditions (how damp or dry the soil is, for instance). Allelopathic compounds do eventually degrade and become harmless.
We think it's unlikely that a lone plant or two of either fennel or dill will cause any noticeable problems, especially if they are separated by a couple of feet from nearby plants. Be aware that fennel can be quite weedy and self-seed readily, so you may want to dead-head the plant (remove developing seed clusters once the blooms fade) or be vigilant for seedlings to remove. Fennel is biennial, so it only lives for two years, and will go to seed in its second year, so after that, the parent plant dies and only the seedlings would regrow the following year. Dill is either annual (one year life cycle) or biennial, and it prefers cool weather, so can struggle in our summer heat.
If you want a more long-lasting butterfly caterpillar host plant, Zizia or another native member of the carrot or rue family would better serve that butterfly's caterpillars. (Although, to be fair, they do seem to prefer laying eggs on those non-native carrot-family herbs, although we don't know if the quality of that food is really as suitable for them as the native plants would be.)
Miri
