The Short Version
This claim is well-supported by converging lines of scientific evidence. Multiple peer-reviewed studies demonstrate that leaf extracts and leachates from species such as Eucalyptus, Aegle marmelos, and Aizoon canariense contain identifiable allelochemicals that inhibit germination and growth in neighboring plants. University extension sources further corroborate that leaves of plants like black walnut contain allelopathic compounds. The claim's careful hedging — "certain plants" and "can affect" — accurately reflects the scope of the evidence.