Abstract
Evolution of weeds resistant to herbicides demands new solutions to cope with the problem since economic losses generated by weeds can be higher than those caused by insect pests. Bioactive compounds known as allelochemicals have the potential to act as natural herbicides in weed management in agro-ecosystems. Laboratory, pot and greenhouse experiments were carried out to investigate the herbicidal effects of Datura stramonium aqueous leaf extracts on the germination and early growth of Tagetes minuta and Amaranthus hybridus. The laboratory and greenhouse experiments were arranged as completely randomised design, and the open field pot experiment was arranged as a randomised complete block design. Four concentrations of 2, 4, 6 and 8%, respectively of D. stramonium aqueous leaf extracts were used. Distilled water was the control. Data for germination, radicle and plumule length was collected within the first 10 days for the laboratory experiment. Root length, shoot length and biomass yield was collected 30 days after germination for both the greenhouse and field experiments. Results indicated that germination percentage, radicle length, plumule length and dry matter significantly decreased (P<0.001) as concentration of D. stramonium leaf extracts increased in all the experiments. This implies that D. stramonium has pre-emergence and early post emergence herbicidal effects on the two weeds. This study revealed that allelopathic sprays of D. stramonium can be used by resource poor small scale farmers or organic farmers for the control of Amaranthus hybridus and T. minuta in Zimbabwe. Key words: Allelopathy, aqueous leaf extract, D. stramonium, Amaranthus hybridus, Tagetes ereca, herbicidal effects, germination, early growth.  
Highlights
Allelopathy is defined as any process involving secondary metabolites produced by plants, algae, bacteria and fungi that influence the growth and development of agricultural or biological systems (Macías et al, 2007; Thi et al, 2015)
This study revealed that allelopathic sprays of D. stramonium can be used by resource poor small scale farmers or organic farmers for the control of Amaranthus hybridus and T. minuta in Zimbabwe
The results showed that the germination percentage as affected by D. stramonium aqueous leaf extracts was significantly (P
Summary
Allelopathy is defined as any process involving secondary metabolites produced by plants, algae, bacteria and fungi that influence the growth and development of agricultural or biological systems (Macías et al, 2007; Thi et al, 2015). According to Asaduzzaman et al (2014), Casimiro et al (2017) and Farooq et al (2011), the wise exploitation of allelopathy in the cropping systems may be an effective, economical and natural method of weed management These compounds are usually degraded in the environment due to their short half life as they contain fewer halogen constituents in their structures. Due to their impure nature, they usually contain a number of active compounds which can act on more than one site like a mixture of herbicides and may control a wider spectrum of weeds (Solts et al, 2013). Natural compounds pose as a potential source for the discovery of ecofriendly herbicides, so called bio herbicides (Solts et al, 2013)
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