In the picture there is one plant with tiny white flowers.  Under that there are green plants with big serrated leaves.

This is Bishop's Weed or Gout Weed.  It can be very difficult to control. The link provided will give you details and guidance on how to control. 

Most importantly you could clip the flower heads off before these go to seed and spread further. 

Let us know if you have further questions. 

Emily

Thanks Emily,

Can you tell what the green serrated leaves below the Gout Weed is?   Could it be poison ivy?

Muriel