Answer: Eucalyptus leaf litter and root exudates are allelopathic, inhibiting growth in certain plant species and preventing establishment of most understory plants. I cannot find information specific to Jacaranda, but given the potential for competition, I wouldn't deliberately plant a Jacaranda beneath or close to Eucalyptus trees. Eucalyptus roots will compete for moisture and nutrients as well, leaving anything you plant near them high and dry.
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