New York,
United States 
are forget me knots harmful for other garden plants, including lilacs. I have a large "crop" of them, hard to permanently remove some and they're in with hydrangeas, peonies, lilac trees, stella d'oros and mums. Are they the reason this particular garden doesn't produce many flowers ?
18 Jun, 2014
Answers
That's right, not in the least harmful. Are you saying the plants you mention don't have many flowers or is it other plants in between them? I'm wondering whether the tree roots are taking up a lot of the nutrients and the soil needs feeding, or whether perhaps the larger plants and trees are creating a lot of shade?
With more details we may be able to help on this.
18 Jun, 2014
No they aren't harmful but quite invasive. Just pull them out. They will free seed each year whether you want them or not.
18 Jun, 2014