Comparative allelopathic effects of six medicinal and invasive plants on agricultural crops

Allelopathy is a biological phenomenon where plants release secondary metabolites that inhibit or stimulate the growth of neighboring plants. This study investigates and compares the allelopathic effects of six plant species—Eucalyptus camaldulensis Dehn. (eucalyptus), Piper nigrum L. (black pepper), Myristica fragrans Hout. (nutmeg), Lantana camara L. (lantana), Senna spectabilis (DC.) Sirin & Barneby (senna), and Chromolaena odorata (L.) Remaking & Hero. (Siam weed)—on the germination and seedling growth of selected agricultural test crops. Aqueous leaf extracts were prepared at concentrations of 2.5%, 5.0%, and 10% (w/v) and applied to seeds under controlled laboratory conditions. Results demonstrated significant, concentration-dependent inhibition across all donor plants. Chromolaena odorata showed the strongest allelopathic effect, achieving 80-100% germination inhibition at 8% concentration. Senna spectabilis inhibited root length by 52% at 10% concentration. Lantana camara showed species-specific effects, reducing black gram germination by 25% while paradoxically stimulating certain weeds. Eucalyptus demonstrated dual effects—inhibitory in dark conditions but stimulatory under light. Nutmeg and black pepper showed moderate to strong inhibition (46-85% germination reduction at highest concentrations). The inhibitory potency followed the order: Chromolaena odorata > Senna spectabilis > Lantana camara > Myristica fragrans > Piper nigrum > Eucalyptus camaldulensis (species-dependent). These findings have significant implications for integrated weed management and sustainable agriculture.