I don't know about you but I can't seem to find ANY studies besides the one done by the 9th graders on the effects of wifi on low order plants.
http://link.springer.com/article/10.1007/BF02861092 finding that under 60kW of radiation of the same type as wifi, 90+ hours of exposure is required to prevent plant growth over a radius of 50 metres. So say you're looking at 900 hours exposure (i.e. about the length of time the referenced expirement would have taken) and for simplicities sake 60mW (which is more power than a wifi router actually emits), the radius receiving plant-killing levels of exposure would be about 0.5cm. If you put your plants right on top of the router, they may suffer a touch. Otherwise, they'll be fine -- which suggests something went wrong in the reported experiment other than wireless interference with the plants.