Fennel is allelopathic to most garden plants, inhibiting growth, causing to bolt, or actually killing many plants. Like black walnut or elder, it produces some chemical or other which does bad things to its neighbours.

Although it causes coriander to bolt, the coriander does help to prevent fennel from becoming too much of a weed. It depends which one you want to sacrifice, I guess!

It will cross-pollinate with dill, resulting in a horrible-tasting plant.

I think fennel will be OK if grown in a pot, but you'll need to be vigilant to stop it from going to seed (a pity if you want those seeds!) - those seeds can travel far and wide, making it a very invasive plant indeed.

Here is a link that might be useful: allelopathic plants

Thanks for the info.

I've read about coriander (cilantro) and how it helps suppress fennel...I will be growing some cilantro as well, probably in between the tomatoes.

I'd like to see what a dill x fennel plant would taste like (just out of curiosity) but I probably won't bother with that. ;)

I will bag the blossoms when they go to seed, so the seeds don't spread all over the place. I do want to save some seeds for eating and planting next year (I have heard the seeds are good for chewing on).

I love the smell and flavor of anise, so fennel is one plant I don't want to pass up. I'll keep it in the pot and will bag the blossoms when they set seed.

Cilantro will not do well in heat and bright sun, which is fine for tomatoes. Cilantro is grown in mostly shade and cooler temps. Pluck off the seed head sprays (in flower stage) like ones that form similar to the ones on dill.

Just curious, but what other plants are allelopathic that I should know about? I know about black walnut but I never heard of fennel being such.

So many tricks up the horticultural sleeve....

There are many allelopathic plants, a lot of them trees. Below are some websites which mention some (also see the link in my earlier post). Others not included in the lists are sunflower, barley, rye, sorghum, wheat, lantana, some brassicas, radishes, some clovers, fescues, knapweed, sorghum, wormwood, mugort and other artemisias, oleander, elder.

http://desertwaterwisegardens.suite101.com/article.cfm/allelopathic_plants_loner_plants

http://www.sustland.umn.edu/implement/trees_turf.html

I didn't know about fennel being allelopathic. Never thought about it. I had it in a garden near sage, oregano, lemon balm, chives, and handful of other herbs with no noticeable negative effect on the other herbs. But then those other herbs are survivors. I suppose if it was next to the green beans or squash I would have noticed.

FataMorgana

Cilantro will not do well in heat and bright sun, which is fine for tomatoes. Cilantro is grown in mostly shade and cooler temps. Pluck off the seed head sprays (in flower stage) like ones that form similar to the ones on dill.

That's why I'll be planting it between the tomatoes, where they will be shaded. The tomatoes are planted close together so they'll shade out the cilantro in no time.

I have heard this about fennel before on these forums but have never witnessed it in my own garden. If you have Florence fennel (for bulbs) it might be ok in a pot but the herb fennel has a long tap root and might not be happy confined. In my own experience fennel has no effect on oregano, aquilegia, hardy geraniums and a host of other perennials which it intermingles with in my garden. I have several large clumps of self seeded bronze fennel in my vegetable garden and it has no visible impact on lettuce, favas, broccoli, arugula or chard all of which I have sown within a few inches of the fennel plants. I can't comment on dill or cilantro because I cannot successfully grow them anywhere in my garden irespective of where the fennel is. I think the soil is too heavy and the light levels and temps too low.

I'm growing Florence fennel, and the bulbs will probably get pretty tight packed in the pot. I will pick out some of the bulbs when they are big enough, and let the remaining plants go to seed.

I might pick up a couple bronze fennel plants I saw at a local nursery...I've heard Florence fennel doesn't have as much flavor as the herb fennel...I like the good anise flavor (has got to be one of my favorites) and I don't want something that's too weak.

Should I get some bronze fennel as well for stronger-flavored plants? I could grow it in a separate container of course.

small plants , due to their small roots, cannot be fatal allelopathic ,say, like Black Wlanut that seats in one spot for decades and affects the soil even beyond its canapy.
ONE more thing. If you plant fennels in an area, leave some buffer zone around it, such that their roots cannot go beyond that.I would say 6-8 inchs should be enough. I do not think that they can affect neighboring plants by other than through root system.
I am growing some from seeds right now and I will follow my own advice(hehe). I had never before planed fennel and did not now about being allelopathic. But I know black walnut very well.

Speaking of black walnut, I was digging the first hole in my garden in which I planted a tomato....and at the bottom of the hole, I found a sprouting walnut. I brought a lot of them home last year to try and cure them (didn't work)...thanks to the squirrels for incorporating them into my garden.

I have bronze fennell planted in three places. Under a pine tree and I never water it. It comes back every year for four years now. In a slightly shaded area along with other herbs and in a butterfly garden. The caterpillars will eat it right to the ground - but it always comes back. It has not been invasive or hampered the growth of the plants around it. I Love it.

It grows fine next to Penny Royal and knockout roses. It crosses freely with Dill and makes a foul smelling hybrid. It is not invasive. I disagree with the person about cilantro growing in cooler temps. It grows well here in Louisiana and we have quite warm spring days. Plant basil to improve the taste of your tomatoes.

I have had huge fennel plant growing for years in a flower bed because of the pretty 'air-y' look when it is growing. I do cut off the flower seed heads so don't have much of a problem with volunteers x the seed heads I miss & have not noticed it hampering the growth of any of the flowers in the same bed. The dill is in a far off garden so hadn't noticed the two crossing either.

I have two large bronze fennel plants in my flower beds..One is planted near a pink dogwood and coneflowers and the other is planted with daylilies and other perennials. I have never noticed a problem with them causing any issues with my other plants..Hope this remains the case as I love the looks of the bronze fennel amongst my other flowers..I do cut off the seed heads to share and a few for teas or cooking if needed..

I have a tall fennel about two meters and a meter wide, i have noticed that an avocado planted close to it had died, but what got my attention was a banana passion fruit that was fully established has died. I have an established apple also pretty close and that is florishing. I love the fennel its such a beauty to the garden but it might have to go before anything else dies.

I seriously doubt the death of your plants has anything at all to do with the fennel. Something else is causing the problems.

Brassicas, like kale, have no problem with fennel. They co-exist nicely. Same with Swiss chard