Fennel and Dill -- do pollinators keep them separate?

Fennel is a ubiquitous weed around here. I like growing dill in my garden. And I find that, although they are in two different genera (Foeniculum vs. Anethum), they seem to look just alike. So I searched the internet for how to tell them apart. Guess what? Even articles purportedly about how to tell them apart, didn’t actually say how to tell them apart, other than by scent – fennel has an overpowering licorice scent, dill does not.

More worrisome was Wikipedia, saying that they can hybridize, and when they do, the fennel predominates. Worrisome because I don’t want my dill crop to become “fennelized.” Either this is a rare case of intergeneric hybridization, or, like previous purported intergeneric hybrids, they get reclassified as the same genus.

But that raised a question in my mind: how have they remained separate species? They seem to have both been originally native to the Mediterranean region, i.e. sympatric, so given that they did not merge into a single species in ancient times, there must be some barrier to hybridization (albeit not a perfect one).

I wonder if I have found it. The other day, I was observing a large field of weedy fennel, and its flowers were full of honey bees. My dill crop’s flowers draw hover flies and greenbottles. I did not see any flies on the wild fennel, and I have not seen any bees at my dill. So that got me to wondering if perhaps their barrier to hybridization is different pollinators.

But then, it may be that my dill plants are too few to attract honey bees, which tend to work only the most abundant flower in the area when there is one; and that if the dill was as extensive as the fennel, bees would work it. Or, it may be that the density of bees on the fennel deterred flies which would otherwise have come. Still, it is a suggestive, preliminary observation. And an encouraging one, since it suggests that my dill plants are not receiving fennel pollen.

I also saw one pollinator shared by both: the European paper wasp (commonly mistaken for a yellowjacket). So even if the bees vs. flies distinction is ecological reality, the wasp makes it an imperfect distinction.

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There’s fennel honey, but no dill honey, so probably it’s true, I personally never seen a honeybee near dill.
Should be added that dill is also shorter than fennel, and bees are not generally visit small Apiaceae that really as a group evolved to use non-specialized pollinators like beetles and diptera, maybe it produces enough nectar to be used by bees.

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It seems like it is a bit unclear whether they really do hybridize, this blog post argues that they probably don’t [http://living-mudflower.blogspot.com/2016/12/do-dill-and-fennel-cross-pollinate-with.html] and I couldn’t find any discussion about dill x fennel hybrids in the scientific literature (at least in English, but I didn’t look too deeply).
If they do hybridize in nature, their species boundaries may be maintained by selection against hybrids, where the hybrids may have have reduced germination, survival, or reproductive output compared to pure fennel and pure dill. In general, species that do readily interbreed can’t have ranges that overlap too much or else they will either merge into a single species, or one (or both) will go extinct. Usually the only species that are able to substantially overlap their ranges either don’t interbreed, or interbreed so rarely that it won’t cause the species to merge, and usually the hybrids are less fit than the parental species.


As far as I know hybrids are fairly rare in Apiaceae and I´ve never heard of hybrids between Anethum und Foeniculum. The genera (and species) are usually easily told apart in an empiric fashion by size both of the whole plants and the leaf-segments (and of course in the field by the different smell). If that doesn´t seem enough you have to wait for ripe fruits (courtesy of the pollinators be they bees or other insects): Anethum has flattended and conspicuosly winged fruits, in Foeniculum they are almost circular in section and the marginal ribs (forming the “wings” in Anethum) are just a bit more pronounced than the lateral ones and therefore don´t form wings.

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No scientific advice to offer, except one year i planted both in my yard. The next year, the dill did not taste good. I was told by a neighbor that they probably crossed. Dunno. Maybe 2nd year dill just doesn’t always taste good anyway.

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The two genera are fairly different in their seeds, and the smell reflects their chemical composition. Apiaceae in general are very difficult to tell apart from general appearance, you often have to use minute details of the fruit.

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