If you choose to spray do it well before they flower. They have the ability to set seed even when the blooms are killed off, even when still in bud. Mow before they flower too. That at least will stop them setting seed. A weed killer designed for broad leaf weeds will not kill your grass.

Use a broad leaf herbicide at any stage and as soon as possible. One dandelion produces thousands of seeds. You will want to apply a pre-emergent as well to make sure all that seed doesn't make more dandelions.

Thanks everyone. So a broadleaf weed killer will NOT kill my new grass or prevent it from sprouting, even if we just planted it a week or two earlier?

What grass did you plant?

It was Scott's Sun and Shade mix. So a mixture of Kentucky bluegrass, perennial ryegrass, and fine fescue.

Ok, this is the wrong time of the year to be planting cool season grasses. It should be done in the fall, but I understand you may have not had an opportunity to do so. The problem with spring seeding is that all the summer weeds also germinate at the same time, and also that the grass doesn't have enough time to build a decent root system before the summer heat. Most spring seeding projects produce subpar if not catastrophic results.

Now, regarding the dandelions, having a newly seeded lawn completely changes things. The most common broadleaf herbicides require that you wait until the second mowing to spray on a newly established lawn. Pre-emergents are out of the question for 60 days. There is a herbicide called Tenacity which is safe to apply to fairly young lawns, but it may wipe out the fine fescue portion of your seed. It also can turn your grass white, though it recovers.

There are no great options. My advice would be to hang tight and see what happens with the grass you seeded. If it appears to be coming up in decent numbers, then wait until the second mowing and attack the dandelions at that point. If not, just let whatever weeds grow, mow them for a couple of months, and start to kill everything with glyphosate about July in preparation for a proper fall seeding/renovation.

When you say catastrophic results, do you mean that I could have actually made my lawn WORSE by planting seed now? Or just that the seed won't sprout at all? I'm fine with the latter but not the former :( We are new to the area and inexperienced with lawns here.

If I attack the dandelions after the second mowing, what herbicide would you recommend?

Also what is glyphosate?

I appreciate all of your help and advice.

When you say catastrophic results, do you mean that I could have actually made my lawn WORSE by planting seed now? Or just that the seed won't sprout at all?

It depends. Seed requires water 3 times a day, so if you're doing that right now you're going to germinate every summer weed known to man, and possibly cause fungus issues. That's going to make it worse. If you just threw seed and hoped for the best, it's probably not going to make much difference either way when it's all said and done.

Any broadleaf herbicide from the home improvement stores will do the job. Ortho, Weed-b-gone, Bayer etc. They pretty much all contain the same active ingredients.

Glyphosate is a non-selective herbicide, meaning it will kill anything it is applied to indiscriminately, including grass. As opposed to selective herbicides, which will kill some plants but not others. Broadleaf herbicides are excellent examples of selective herbicides - they kill broadleaf weeds, but not grass. There are also selective herbicides that kill grass but not bushes, like Ornamec.

Glyphosate is the active ingredient in RoundUp, but most home improvement stores sell it as a "generic" under store name brands for much cheaper. It is a great tool for wiping off everything to create a clean slate for seeding. Or when a selective herbicide is not available for the weed you're targeting.

I think it depends on where you live if you can grow grass in the spring/summer. I am zone 6. We can use grass seed with success anytime in the spring, summer or fall.

As noted, grass seed can be planted at any time.......it just takes more care and attention if outside the preferred window of time. IMO, the catastrophic results will occur if you do not address the dandelions NOW!! It is far easier to prevent further weed development than to have to address them after the fact. Do not let them flower if at all possible (mow) and certainly do not allow them to go to seed or produce the puffballs. And pretty much any broadleaved lawn weed herbicide will work to kill the existing plants, although it may take more than a single application. Get a liquid broadcast spray rather than a granular product - these are available anywhere that sells garden products.

Another issue with spring seeding is crabgrass. Crabgrass is a summer annual plant, which means it is highly adapted to sprout fast and grow well in the summer heat. What happens when you plant grass seed in the spring is the crabgrass seed sprouts first and has time to leaf out before the grass seed does. And as lawniac mentioned, the real grass does not have time to develop good roots to resist the summer heat, so what happens is the real lawn will start to suffer in June leaving thin spots where the crabgrass will spread to fill. It is possible to have a full crabgrass lawn by July. Still though, worse things have happened. You can live with that and learn how to care for your lawn until late summer when you nuke everything and start at the proper time of year. Fall is better because crabgrass seed does not germinate then and the new lawn has all fall, winter, and spring to develop hardier roots.

Gardengal48 thanks for your comments...are you saying I should mow the dandelions down now, while they have yellow flowers? And THEN spray with herbicide after all those little yellow heads are gone? Will that effectively kill them? I always thought the herbicide needed to be sprayed on while the yellow head was still there.

Dchall, I'm guessing you're from San Antonio. I'm in the midwest and am thinking things are a bit different here. I'm not going to nuke my whole lawn.

Thanks, all. I was following the "Lawn Care Nut's" plan and he is based in Indiana so I figured I was safe. I now know planting grass seed in the spring was not IDEAL and that fall is much better. Just hoping that I did not destroy my lawn.

The herbicide works on the leaves with or without the flowers. If you don't get rid of the flowers ASAP they will set seed. In the four days since you began this post you will probably already have missed some. Even buds can set seed after being cut off. Rake up cut off flowers or mow with the grass box on.