Use good judgment to evaluate how fast your grass crop has filled in and matured, which will vary greatly in different conditions. If you have any doubt, try spraying just a small area first and watch it for a couple of days for negative impact. (If you don’t have any concerns about going ahead, you probably are not reading this in the first place?)
Just remember how much work you went to, putting in a new lawn. Is it worth the risk now? Be cautious.
Garden Counselor ANSWER:
Crab Grass Killer Is Different Than
Regular Weed Killer
Expect that the grass will show yellowing after application of this herbicide. This is quite typical of many chemicals on lawns. It should grow out of that after 1-2 mowings. (So don’t do it if you have a wedding or special event planned in the next few weeks!)
Weeds are more thoroughly controlled when you can hit them early. So I would recommend spraying first. You have plenty of time for new grass to grow, even with waiting 3-4 weeks. (Unless you have an event or activity on the calendar for which the lawn must be pristine as soon as possible.)
Spectracide products are good quality. If you have not already purchased, realize that they offer two versions of Weed Stop, one of them adding a crab grass killer.
Your choice will obviously depend on whether you merely have problems with broadleaf weeds, or if crab grass is also a concern.
Your approach to reseeding your lawn will be affected by which product is selected.
Weed control begins with proper management practices, which encourage a dense, healthy turf. A healthy turf shades the soil so that less sunlight reaches the ready to germinate weed seeds. A thick turf also minimizes the space available for weeds to become established.
Even when cultural practices are heeded, weeds can appear. If the number of weeds reaches an unacceptable level and pulling by hand is out of the question, you may want to turn to herbicides. At this point, it is important to know what weed you are trying to control. Local extension offices, the Clemson Home & Garden Information Center, the Clemson Plant and Pest Diagnostic Clinic, and publications can aid in identification.
Types of Weeds
Postemergence Herbicides: Postemergence herbicides target visible weeds. They are used primarily against broadleaf weeds, perennial grasses, and sedges. The chemicals 2,4-D, dicamba, mecoprop (MCPP), MCPA, carfentrazone, and triclopyr are broadleaf herbicides. They have been combined in many products that control many broadleaf weeds. Always check the product label to be sure that it can be used safely on a tall fescue lawn, that it will control the specific weeds in the lawn, and that it will be used at the correct rate. With many products, repeat applications in 10 to 14 days may be necessary for difficult to control weeds. For triclopyr, a repeat application may be needed in 4 weeks on some weeds.
Grassy vs. Broadleaf: Grassy weeds are true grasses which emerge from seed as a single leaf. The leaf blades are longer than they are wide and have parallel veins. An example is crabgrass.
Common dandelion (Taraxacum officinale) seeds blow in the wind and allow this perennial weed to become a frequent invader of home lawns.
Joey Williamson, ©2015 HGIC, Clemson Extension