There are a number of plants which release chemicals toxic to other plants. The process of releasing chemicals that either benefit or harm other plants is allelopathic, but most of the plants which are allelopathic are harmful to other plants which is why the term has become almost synonymous with the term ‘killer plants’, or ‘natural herbicides’.
Fennel, a great herb for digestion, has an adverse effect on most other plants except dill, with which it may cross pollinate so best not to grow the two together. In turn wormwood has a detrimental effect on fennel.
Black walnut releases a chemical called juglone which affects a number of plants and it is interesting to see that all three are common ingredients in herbal remedies for internal parasites.
Fennel and wormwood are natural flea repellents, and fennel is attractive to beneficial insects and birds, but keep them in their place.