Question
I have weeds and grass growing between my retaining wall bricks and also in planter. What can I use to kill and keep weeds from growing back? Nursery guy recommended KILLZALL or Over-the-Top. I have dismantled part of the wall and will rebuild but want to apply weed and grass killer between bricks. I also have a cockroach and earwig infestation in the planter and between bricks I would like to eliminate. (Phoenix area)

Answer
John,
Preen works well in the planter, and may work for a while in the wall.
The problem is that any material that you apply will not last forever.
I would recommend sealing up the spaces in the retaining wall and placing landscaping fabric behind the wall to stop further weed and grass growth. Using preen or another preemergent weed control will provide short term control. Use landscaping fabric in the planter with a layer of mulch to get longer lasting control. The fabric can be cut back when you wish to plant flowers or other shrubs without losing the control.
I hope this helps.
Good luck.
Ed Gulliksen