Looking for some help here. We are to install irrigation and St. Augustine sod on a yard that is mostly weeds and crabgrass. Pre m needs to be applied now but we are to start irrigation in 2 weeks. Is this safe....us working in the dirt the chemical is in, and putting in pipe the water is going through (backflow preventer, but I'm still concerned)? Also sod will be installed in late March/Early April, how will chemicals affect this? We need to keep weeds and crabgrass form taking over our sod. Thanks for any help.