Sedges are almost impossible to remove with one treatment of anything, due to the energy storage in the nuts. Two treatments done right should work for most people.
You are a bit limited in your treatment options if you have a new lawn. You may consider cutting back on the watering. Sedges love water, and if your seed is mostly germinated, you can cut back on watering the grass quite a bit.
Sedgehammer and "Image Kills Nutsedge" both are labeled for application on 'well-established' lawns. I would not apply either product to one month old grass. (Sedgehammer claims a 4 week old lawn is established - but it depends on what you call day zero and I would not risk it).
You can pull nutsedge. You won't get the nut but the regrowth will weaken the nut. It is certainly not the worst thing you can do and is actually one of my options below. By pulling, you take back the space and allow the grass to fill in, which is beneficial to the grass and helps slow (not prevent) regrowth of the sedge by shading the soil with the grass. You also remove energy reserves from the nut as the sedge regrows.
Below are treatment options for a new lawn and a few extra options for readers with established lawns.
First Treatment Options
Option 1) Pull out the sedge. Your grass will continue to grow stronger without the unnecessary competition. You will substantianally weaken the underground sedge nut and deplete a good deal of its reserves as a new sedge grows, even if it looks like it comes back unharmed or with renewed vigor. (The idea here is that a new sedge will come up, but it will be attached to a weak nut, so the second treatment options will work well. If you pull and then don't do a second treatment and the nut gets healthy again, then all bets are off).
Option 2) Put on a glove and get an old sponge, paper towel, cloth etc and soak it in some roundup mixed with dish soap (1 tablespoon soap per gallon roundup, approx). Sqeeeze out the excess. Coat the sedge leaves by grabbing the lower stem near the dirt and surrounding it with the sponge and pull up, coating the leaves with the roundup as you slide up along the leaves. This will kill the leaves and likely some of the nuts, but not all of them. The new sedges will appear, but again, will have weakened nuts. Don't spray the roundup - you will use too much roundup and you won't cover enough of the waxy leaf to make the impact that you need to make, plus you will have too much collateral damage to the surrounding grass.
Option 3 For Established Lawns Only) Sedgehammer or "Image Kills Nutsedge" treatment. I prefer Image as it is easier to apply and works in the soil through the roots as well as through leaf contact (so you can miss a few sedges and the overspray on the soil will get to the roots), and it is cheaper. You can spray it on the whole lawn, or spot treat. Sedgehammer is more expensive and needs to be spot treated but is excellent if you spray it on most of the leaves, and follow the mowing instructions (no mowing for a few days before and after treatment). You also need to use a nonionic surfactant with Sedgehammer (read the label). Sedgehammer applied correctly is the only shot of getting rid of nutsedge in one treatment, but I wouldn't bet the bank on it). Both Image and Sedgehammer work slowly - you need to be patient, it will take weeks. You can be a bit more cavalier with Image - you can use it in and around most shrubs and flower gardens and not worry about overspray - Sedgehammer on the other hand will cause damage to most ornamentals so be careful especially if it is breezy.
Image is about $25-30 for a 8000 sf treatment. It is readily available in the box stores and garden centers.
www.amdro.com/Image/Nutsedge/index.html Sedgehammer is about $15 for a 1000 sf treatment. Some garden centers carry it. Cheapest place I've seen for small quantities is on ebay.
www.gowanco.com/SedgeHammer/Default.aspx You can buy nonionic surfactants from garden stores for ~$5.
Wait two months. Now you have two more options. You will see some sedges, likely not as many as before, although possibly more - but they will be weaker due to the stress you put on them in the first treatment.
Second Treatment Options
Option 1) Repeat Roundup treatment as described above.
Option 2) For New Lawns: Treat with Sedgehammer. For Established Lawns Only: Treat with Sedgehammer or "Image kills Nutsedge". Sedgehammer is only active through leaf contact, and should be OK on a new lawn after 3 months. (Don't use Image on new lawns - although Image is milder than Sedgehammer, it is active in the soil, and your new grass roots won't like it, especially in the heat of the summer.
Done properly, two treatments will rid you of nutsedge from a lawn. Removing nutsedge is either time-consuming or expensive or both. So decide what is most important to you and go for it. Good luck!
