Question
my backyard is about 10 feet x 30 feet (i am in the city), with no lawn, and a few large flagstones every few feet.  it is currently wild with weeds that sprout between the stones. can you tell me what to spray there to kill all the weeds, and how often i need to respray to keep it from growing back. also, there is a 20 foot high tree in the corner - will what i spray effect it?

Thank you very much for taking the time to answer my question.

Answer
Round Up or any other commercial weed killer will take care of the current growth.  You wont have to worry about a mature tree - Round Up works as a systemic, that is the material is absorbed through the leaves and fleshy tissue of the plant and will not penetrate the bark.

To keep weeds at bay, you may want to add a ground covering of crushed stone or mulch to cover the soil.  You can also plant ground covering plants like Pachysandra or Liriope.  A pre-emergent herbacide like Preen will prevent new annual weeds from germinating and taking over.

Good luck!