I have read to put it down when the crocus bloom. I'm in zone 5 and my perennials are all starting to come up already. Not sure about annual weeds, but I think the window is fast closing on putting down pre-emergent unless you are quite a bit further north than we are. We use corn gluten meal, and while the jury is still out on its effectiveness, we already put it down a week or two ago.

Sorry. This will be a long one. It always is with me. (blushing) I'm with the OP on this one. Even if no one but me ever saw my lawn, I would want it to be dark, rich green GRASS...and nothing else. (I'm known for wanting everything to be beautiful. I'm also a bit of a home body, and enjoy spending time in my yard. Many guests have remarked that, no matter where they look (on my property), there's always something beautiful. I'm always taken aback by those comments, since while that was MY goal, I never expected others to pick up on it.) Case in point about the intentional act of putting down grass, and not other (lower maintenance) plants. I own a small, remote island. I made the decision to create a lawn area and, once you include the slow-moving barge fees (3 days to get from its port to the island), labour, equipment, and materials, that lawn cost me $14,000. That obviously begs the question, why, on earth, would anyone spend a nickel putting in expensive, high-maintenance grass, only to let it be taken over by clover and other weeds??? My husband and I recently purchased a 19th century farmhouse in need of roof-to-foundation renovation. As such, I inherited a lawn that is EASILY 70% weeds, including clover so thoroughly interwoven, it would be impossible to pull by hand. I pride myself on my beautiful lawns and gardens, and can spend/have sent hours, each day, ensuring everything is healthy and how I want it. At my last home, I developed a bit of a reputation for being out weeding my lawn by hand for hours, plus how awesome my expansive lawn was. My point is, I want my lawn and gardens to be a certain way; plant the plants I want; pull the plants I don't want; unafraid to put-in the effort to achieve that. I prefer not to use any kinds of "chemicals", other than the ammonia and mild soap I include in my homemade fertilizer recipe. (My last home was a beach house. So, that only reinforced my desire to go chemical-free.) But this new-to-me house is demoralizing me...practically to the point of tears, much of it because of the state of the (laughing) "landscaping". Sure, mowing can cut off 60% of the sea of clover flowers that make my lawn look white (or dying...can't decide which). But there are still lower flowers that seem to sprout up to full height within the next 24hrs after a mow. These flowers last from late spring to fall. In short, I'm plagued by clover flowers throughout the entire season I want my lawn to look great. It's just such a shame the previous owners didn't nip the clover in the bud, when each plant was jut a little, round cluster. I love catching them early, knowing I've stopped additional infiltration. But I'm helpless, overall. Looks like I'll have to replace the entire lawn; start from scratch; do what the previous owners SHOULD have done, and pulled weeds when I first see them. I'm not really buying the whole "benefits of clover" thing, amidst a grass lawn. If I wanted a clover lawn, I would plant a clover lawn. Putting down and maintaining grass is an expense in time and resources. Clover is virtually free (since it shows up on the wind), and is seemingly mantenance-free. Since most sod and grass seed are sold with some guarantee of being weed- (and clover-) free, I just don't believe that most people who spend the money and effort to put in a grass lawn, are okay with losing any percentage of it to other, invasive, undesirable plants. I kind of liken it to suggesting that a Ferrari owner would be okay with someone switching it with a Cavalier, and just saying, 'They both get me from point A to point B, so I don't mind.' I can fully envision putting in other kinds of ground cover, and have considered it at past properties. Not being my first rodeo, I already have substantial plans to reduce the amount of lawn area, and increase planting beds. But much of this current property begs for a beautiful, thick, dark green lawn...of grass. No dandelions. No clover, but particularly no clover flowers. (I'd be more okay with clover, if not for the flowers.) The desperation was so great, I did something I thought I'd never do, and tried the iron-based WEED-B-GONE product. (Harsh, chemical herbicides have been banned in my area for years...resulting in public parks being taken over by weeds, for the most part. It's such a shame, in the sense that a healthy, well-maintained lawn is not only beautiful, but fantastic underfoot.) That is really my philosophy, beyond the aesthetic. A lawn should be cool, soft, and luxurious under bare feet. Weeds, even overgrown grass, also tend to make barefoot walking less than a joy. So far, I haven't found a way to get rid of mats of clover. The WEED-B-GONE did nothing I could notice. I'm at my rope's end on ths matter, and likely will have to pull up the entire lawn, to put down new, almost-weed-free sod, where I can physically manage pulling individual weeds as they appear. I guess I'd plead with people to pull unwanted clover plants when they're small... if not for yourself, then for the next property owner. Sure, it means time, every day, or every few days, combing the lawn, looking for weeds. Yes, it can be back-breaking and time-consuming. But that grass lawn didn't get there by accident. A grass lawn is intentional.

Wait, wait, wait. Don't spray borax on your lawn unless you have a soil test that shows you are short on boron. Boron is an essential micronutrient, but there is a sensitive threshold on how much a plant can take. If you go over the limit, you would essentially sterilize your soil and nothing but a few select weeds will grow. The best soil test to determine your boron levels is the $25 test from Logan Labs.

Prodiamine 65 Water Dispersable Granules (Generic Barricade) is my pre-emergent weed killer of choice. A 5 lb jug costs about $60.00. It should last many seasons. It does a great job at keeping weeds at bay. Amazon has got it listed $57.46. Also bear in mind that it might not be allowed to be used in certain states. Example New York state EPA does not allow its use. So sellers will not ship to a New York address. Further, application of Prodiamine is not easy. You have to apply with a sprayer. I use a tow behind electric sprayer (14 gallons) which I hook up to my John Deere riding mower just for applying Prodiamine. One acre application still takes a full day because I have to do some spot spraying where the lawn mower cannot be driven to. Tenacity is another more expensive option of pre-emergent weed killer.