Great Idea--Up^cycling Water or Pop Bottles into Mini-Greenhouses or Bell Jars.  Leave the cap off during the day, if sunny, so they don't overheat and put it back on at night to keep them warm.  Simple, Inexpensive and Eco-Friendly.  Note:  It is best to use bottles that don't have Bisphenol A (BPA), a polycarbonate, which has been proven to cause health issues.  Use any bottle that has a #1,2,4 or 5 (The numbers appear inside a triangle of chasing arrows, usu. on the bottom).  I prefer to use the hard plastic bottles which are less likely to give off harmful gases when heated (Smart Water was the last one I knew of to be such a bottle).