Allelopathy of fennel?

My question is in regards to allelopathy. I have just cut out a huge fennel about 2meter high by about 1 meter wide, in fact it was three plants. So i was wondering if this fennel had affected the growth of two trees. One a young avocado, and the other a very well established banana passion fruit vine both dead. The banana passion gave fruit last year. If this is indeed the case that it was the fennel that affected them, does that mean that anything i now plant in that soil will be affected ?  What am i able to plant there, otherwise how can i fix my soil? As i was removing the roots i noticed that there were very few worms 3 or 5. Thanks in advance

Im not sure why there are no comments to this post should I change the subject line? im quite new to gardening and i thought here would be a great place to pose my question as there are alot of knowledgable people here…

Hi Angela...sometimes it just takes awhile to get an answer especially on the weekend I think.  
It's a good question and I think in the right forum. I'll add it to 'plants' and 'trees' also though.

It makes me wonder about the large bronze fennel I have growing on the garden fence.  No trees near by or other vegetation for that matter so I don't know if it's causing any problems or not.  It seems like I remember reading in one of those 'companion planting guides' that fennel didn't get  along with anything.  

EDIT to add a quote...

Fennel Companion Planting. Fennel is a highly aromatic and slightly odd plant that is pretty much a poor companion plant for everything. Fennel companion planting generally only works when growing dill. However, even dill is a poor choice because the two herbs tend to cross-pollinate.

Thanks Judith, I forget about the time and day of the week, am living quite remote and working and living for/by myself for some months now (not that that should be an excuse for my social skills or lack of). I will use this to practice more patience. Permies.com is a wonderful resource.

Just saw this post, sorry for not catching it sooner.

I have not found any references to fennel being severely allopathic to any plants.
The lack of worms might be an indicator of lacking soil biology, particularly fungi which both of your trees needed to be healthy and long lived.

Have you inspected the tree trunks for small holes in the bark? This is an indicator of borers being the culprits of their death.
If there are none, then you might consider taking soil samples to a lab for a biological tests as well as a mineral test. That might give more clues as to what went wrong.
One other thing to consider is voles or gophers, they both leave tell tale signs if they are there.

I do know from experience that other vegetables seem to show that fennel can be mildly allopathic to them but I have two pear trees and two plum trees that I grow both fennel and garlic around with out any ill effects (three years of doing this).

Redhawk

I have read that fennel is a poor companion plant, but have never had that experience. It grows very well here, and I have planted it with all kinds of different plants. It grows wild here as well, and it doesn't seem to have a negative effect on the surrounding vegetation.

Thanks for replies, i have had a close  look and find no bora holes in the trunk of the dead trees, as suggested i can look into taking a soil sample. Thak you again

hau Angela, please do let me know what you find out about your soil conditions, that way I can give you ideas on how to make improvements fast for the long term.

The avocado might have gotten sunburn (I had this happen to a year old tree and it died from it).
Root disturbance is something that can also wreak havoc to young or old trees, so if there was any of this going on, that may have contributed to their demise.

Redhawk

I must admit having grown this plant and seen it many times in the wild here in France I have not noticed it being allopathic rather a sign of soil issues . It's often found on railway sidings for example , poor rocky/ sandy soil etc to me it's a sign of soil issues rather than a cause of them

David