This is all over Facebook among my friends right now. It seems somewhat plausible, but sells like BS given the breathless way its being reported.
Can WiFi Signals Stunt Plant Growth?
A group of 9th grade students in Denmark sowed seeds in several trays and placed some trays in a room with no router and some in a room with to routers. The seeds in the router room for the sot part failed to germinate.
The article linked above is one of the very few I could find that included this caveat:
Surely this isn't the first time this has been tested. We've had radio signals of various frequencies around us for a long time. What controls do you think were missed in the girls test?
My first reaction: why not put them in the same room, right next to each other, with one set inside a simple faraday cage?