New Invader for the Don?

Seed pods. They are covered with bristly hairs that stick to your clothing

Seeds line stalks that appear at opposite sides from leaf axils

Typical leaf from plant

This summer I've noticed a new plant in several places in the valley. I am not sure if it is new or maybe I am just more observant. One thing that leads me to believe it is new is that I can't find a listing for it in my Peterson's Field Guide to Wildflowers.

I have taken several pictures of the plant, some of them shown above. I took some pictures of the flower but none of them turned out. Here is my list of features:

• the plant grows about 40-80 cm tall
• leaves grow opposite each other and alternate in pairs up the stem
• a typical leaf is about 8-10 cm long and 4-5 cm wide
• leaves are dark green in colour and roughly toothed
  
• flower stalks grow from leaf stalks on opposite sides
• flowering period was for the month of July
  
• flowers are small (about 5 mm wide), white, with 4 petals and 3 stamens
• the seeds each about 5 mm in length appear along the flower stalks, alternately placed
• each seed is covered with fine hairs which stick to clothing or fur like velcro

If anybody has any clues as to what it might be, please let me know.

** Newsflash! **

Thanks to a tip from Michou, this plant is not an invader but is actually a native perennial called Enchanter's Nightshade (Circaea quadrisculata). It is listed in Peterson's guide. I was misled because the plant is not listed on a page with flowers of 4 petals. This plant has only 2 true petals. But it also has 2 reflexed sepals which look a lot like petals. Only a botanist could figure this one out!