We determined the environmental requirements leading to germination by three common species found during the summer rainy season in a peri-urban site where construction of a university campus was underway. In particular, we evaluated laboratory responses to low-temperature stratification, day/night air temperature, and water potential for the native Onagraceae Lopezia racemosa and Ludwigia octovalvis , and the exotic Polygonaceae Rumex crispus . Low-temperature stratification had no effect on germination by L. racemosa , for which maximum germination averaging 88% was optimal at 25/15 and 30/20 ºC. Germination at 21 d was halved at –0.5 MPa and completely inhibited at –1.0 MPa. The seeds of L. octovalvis were also insensitive to low temperature stratification and their germination never exceeded 70%, with the two highest temperatures of 30/20 and 35/25 ºC being the optimum. For this species germination was maximal at 0.0 MPa, decreasing significantly under every treatment with a minimum germination of 21% for seeds incubated at –0.1 MPa. Germination for the exotic R. crispus was delayed by low-temperature stratification, although all of its seeds germinated regardless of the temperature or water potential treatment. While the environmental requirements for germination of ephemeral species often match the typical climate of their growing season, the differential responses found for the species considered in the present study provide some insight into the mechanisms leading to changes in species composition for communities from disturbed environments, including the displacement of native species and the proliferation of exotic, potentially invasive, plants.