The effects of seven constant temperatures (10–40°C at5°C intervals) and seven after-harvest periods (30–540 days after harvest) were evaluated on seed germinationof nine Amaranthus species (A. albus, A. blitoides,A. cruentus, A. deflexus, A. graecizans, A. hybridus,A. lividus, A. retroflexus and A. viridis). Seeds ofA. blitoides and A. viridis were also tested at alternatingtemperatures of 10 ⁄ 30°C (12 ⁄ 12 h thermoperiod) incontinuous darkness and in an alternating12 ⁄ 12 h dark ⁄ light photoperiod. With the exception ofA. blitoides and A. viridis, germination increased astemperature increased from 20 to 35°C; the latterrepresenting the optimum temperature (70–100%germination). At 10 and 15°C constant temperatures,no significant seed germination occurred in A. albus,A. deflexus, A. graecizans and A. lividus, while inA. cruentus, A. hybridus and A. retroflexus there wasno germination at 10°C, but at 15°C more than 60%germination occurred. Germination was influencedstrongly by after-ripening period in A. cruentus, A. hybridusand A. retroflexus, partially in A. deflexus, andbarely in A. graecizans and A. lividus. Seeds of A. blitoidesand A. viridis required alternating temperatures andlight to achieve high germination percentage (>90%).Primary dormancy in Amaranthus plays a fundamentalrole in extending germination over a longer period, sothat the probability of seedling survival is maximised.The present study adds to the understanding of theenvironmental control and germination ecology ofAmaranthus species and provides data that can contributeto predicting weed emergence dynamics.