The thing is my lawn is perfectly fine except where it use to be a rock garden and was all weeds- those are dead and grass is actually starting to grow there. In fact, my other lawn grass is greener than when I moved in.

He wants me to pay to resod his lawn. Would it take over two months for the lawn to completely die?

A few things to note:

1) his lawn is over 30 years old and completely overgrown with clovers and multiple grass species.
2) he wants me to sod over my old rock garden area and said that "well, now you can get this done and it'll look nice."
3) we recently bought the house (decent fixer) and I'm working on the yard on the weekends, slowly rehabbing it.

I would appreciate anyone's advice. Should we get a soil test? I really don't know what to do.

You used a weed killer that does not kill grass.
The TARGETED area of this chemical is growing grass, so how could a peripheral area be worse?
How does he know you used that stuff in the first place?

Sorry, but it sounds like your neighbor is full of fertilizer.

What was the main ingredient of the spray? Unfortunately the most popular weed killer is glyphosate and that happens to be a grass killer, too. If you use ANYTHING else, he has no leg to stand on. He has insects or a fungus. He probably has a fungus if it happened very quickly and is targeted to one area.

Pictures would be very helpful.

Also if you have Saint Augustine and used Atrazine, and your neighbor has Fescue, Atrazine will kill Fescue, Rye, Blue Grasses, Bermuda, and Zoysia.

Personally I wouldn't worry about it and tell him to jump in the lake. Only if he serves you papers in small claims would I pay. Otherwise fences make for good neighbors.

why not ask him if you can try your hand at rehabbing his lawn? tell him you're learning about lawn care, and although you don't think you killed his grass, you'd be more than happy to try to help rebuild his lawn to get it healthier than ever. ask him if he wants to tackle it together. you might end up working togther on it, which could forge a good relationship.

trust me, a good friend in the neighborhood is worth more than being right. when you're out of town, locked out of your house, or out of milk, you want a friend next door. i've learned from experience.

I would consider myself to be very fortunate if I could have you as my neighbor. I'm sure your neighbors feel that same way about you. I appreciate the perspective you shared in your post. Thank you for sharing.

Here's a picture. My lawn on the right, his on the left.
Keep in mind that I sprayed EVERYWHERE on my lawn on a day that wasn't windy (I made sure)over two months ago. My weeds died within a week and his stuff was still very green.

My grass is very green and I actually have weeds growing back in all the areas I sprayed.

He also said that he had tried fertilizing his lawn to get the yellow to stop. It does seem like over the last two week it got very yellow, very quickly.

Another thing: Do you see the "island" in his yard? Yesterday, two of his plants were very wilted and his gardener cut off the top of them. I think that if my weedkiller was going to kill of his plants, it would have died at the same time.

We don't share irrigation. He has his own sprinklers and I have mine.

Here's what I used. http://www.homedepot.com/buy/outdoors-garden-center-lawn-plant-care-la wn-weed-control-grass-weed-killer/ortho-32-oz-concentrate-weed-b-gon-m ax-plus-crabgrass-control-133197.html#.UBwtgU1mRyA

I believe that our grasses are different. His has clovers and these tiny blue flowers growing throughout it. And, literally, the week I moved in he was talking to me about redoing our yards together.

I actually really like the guy but I just don't feel like it did this. The fact that his grass AND plants are dying two months later? I just don't think the chemicals would stay in his yard and not mine.

It looks worse than it is.

Does your lawn slope down toward his lawn, or vice versa, or are they about level with each other? Herbicides do not normally move uphill.

The yards are level with each other and slope towards the picture's perspective (to the street).

I am going to offer to help out with the rehabilitation (per ruthie's advice) but not take responsibility for what happened.

We'll see how he takes it.

Honestly, I think this is one of those situations in which it's impossible to tell what happened to his grass. It could have been killed/compromised by any number of things. I doubt it was your doing from what you've said. The important thing is that it's going to get fixed and he'll have a nice new lawn and someone he can ask for advice in the future.