When is the last time you fertilized? It may be too hot to apply weed killer for the clovers. Wait till it cools down in the fall. Is it getting enough water weekly?

Weed killer fertilizer doesn't really work at least for us in Texas where it is best to do both separately at different timing.

Where do you live?

When did you do all that?

What is your watering regimen (how often and how long)?

What was the guaranteed analysis on the seed you used?

What fertilizer did you use and was the temperature higher than 85 degrees F? How did you apply it? Did you water first and then apply or apply and then water?

Thanks for the replies.

I live in Canada. Our springs are pretty cool. Nothing over 25 degrees Celsius so far. (82 f)

I did all this in mid may.

Water at least once a week. 20-30 minutes.

Seed is generic all purpose lawn seed. Fertilizer is Scott's turf builder fertilizer with weed prevent 10-10-10, used a regular spin spreader.

I applied than watered.

Thanks for your help guys.

It's probably time to fertilize. This time, without weed killer. Go with high nitrogen kind like 24-0-11 with some iron or something similar.

I'd say go with organic fertilizer but I am not familiar with that type of weather on where cool nights would slow down the breakdown of organic fertilizer.

I was hoping for something more specific than Canada. I was born in Canada, so we must be from the same place! Okay, here's what you do for St John's, Newfoundland. First...

Anyway, Canada is good enough. At least we know it's northern grass.

May is a poor time to seed new grass. Depending on where you are in Canada, it can get too hot for new seed to survive. If you're temps never exceed 85, you have a chance. Check the lawn in August. If it is still thin, that is the time to reseed.

You applied the weed n feed incorrectly. The Scott's label says to apply to a wet lawn. That would be the reason why the weed killer did not work for you (assuming it did not). But it would not necessarily indicate why it is yellowing. 10-10-10 is a vegetable fertilizer not a lawn fert. Scott's lawn ferts have N values in the high 20s.

Watering
You need to understand your sprinkler system whether it is in the ground or a hose with sprinkler attached. Set out some tuna or cat food cans and time how long it takes to fill them. Rather than going 20-30 minutes, this new time will be your target for watering. It might still be 20 minutes, but for me it was 8 hours. And then you should adjust to the idea of watering only once every other week. If your temps never get into the 90s, then every other week should be fine. If you really live in St John's, then you can go to once every 3 weeks in the summer. Chances are you will have to teach your grass how to go that long, so start by figuring out your target time and working to stretch the frequency out. Where I live the daytime temps are in the high 90s but I have not yet watered all my grass yet for this year. It's a matter of teaching the grass to stretch. What this does is develop deep roots which can go much longer without water.

Mowing
If you are not mowing at the highest setting, you should be.

Fertilizing
Find some organic fertilizer and put that down. One popular brand is Milorganite. I don't use it, but many people I trust use it all the time. I use alfalfa pellets (rabbit chow) at a rate of 20 pounds per 1,000 square feet. The beauty of organics is you can use them as often as you want to without any fear of hurting the grass. On the minus side, they take 3 weeks to see improvement in color. But since it would be risky to use a chemical fertilizer this time of year, your only choice is organic.