Answer: Neither Hackberry (Celtis) or Hawthorn (Crataegus) are reputed to emit toxins; perhaps you're thinking of the growth inhibiting secretion from walnut trees. If plants struggle to survive beneath large established trees it's generally because the tree's extensive root systems are quick to take up moisture and nutrients, leaving precious little for annuals and perennials. The shade cast by trees can also starve light-loving plants. If you'd like to grow plants beneath a tree's canopy, why not put the plants into containers where they won't have to compete with tree roots? Or build raised beds and line the bottoms with weed-barrier to slow the intrusion of tree roots?
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