Ask Mr. Smarty Plants

Joe Marcus is one of the Mr. Smarty Plants team; he was answering that question about allelopathy and hackberries, not asking it. We will repeat the pertinent paragraphs from his previous answer.

"Every plant that puts down roots in the earth compete in some way with its neighbors; some compete more aggressively than others.  The rhizosphere, that is, the soil where roots grow, can be thought of quite accurately (if not a bit simplistically) as a war zone.  Plants, animals, fungi and microorganisms living within the rhizosphere continually struggle with one another for their share of the resources available there.

Many trees and other plants interfere with their competition through one of a multitude of processes known collectively as allelopathy.  Allelopathy typically involves living or dead and decaying plant parts exuding, volatilizing or leaching chemical compounds that are inhibitory or harmful to nearby plants.

Hackberry trees, Celtis spp. are known to inhibit the growth of competing plants through allelopathic processes.  The mechanism used by hackberries involves the release of a witch's brew of chemicals known as phenolic phytotoxins leaching from fallen, decaying leaves.  The leachates from rotting hackberry leaves have been shown to inhibit the germination of seeds and the development of seedlings.  So yes, hackberry can do harm to nearby plants, though it is unclear if it has any effect at all on established plants such as trees, shrubs or even mature perennials." 

In answer to your question, it is more likely that the shade and litter from the hackberry caused the problems with your (non-native) impatiens and other annuals. It also has a lot of roots up close to the surface, and those annual roots are competing with much sturdier and older hackberry roots. In a case like this, worm castings or fertilizer are probably not the solution, although the hackberry roots would probably love it. Sometimes you have to choose: do you want the tree or do you want the plants under it?

In defense of the tree, Celtis occidentalis (common hackberry), it is perhaps not the most beautiful tree, and deciduous, but did you know that it is a food source for the Wild Cherry Sphinx Moth, and a larval host for the Tawny Emperor, American Snout, Question Mark and Mourning Cloak butterflies? The mulch that you mentioned is a nice ground cover under a tree, and you won't have to replant the annuals every year. Perhaps there is room for a bed of annuals farther away, where there is no large tree defending its territory.