and what I can do to stop them?

Hi, in my little front garden against the front of the house, I
planted alyssums, violas, and marigolds. The last week or so, the
weeds you see with the roundish leaves have taken over very rapidly.
When you go to pull them out, seems you get several lengths of the
weed. Also in the lower left of the pic, some kind of grass growing
right thru the alyssums, where removal is difficult, as the alyssums
often come out also when you pull on the grass like plants, and the
wispy thing that's laying on it's side also in the lower left and
middle, I get a lot of those. At least the ones with the roundish
leaves usually pull out fairly easily. Pic:

http://img830.imageshack.us/img830/3782/weedse.jpg

Thanks much.