Ta da! Experience to the rescue! I found this last year and love it, love it, love it. I only water my hybrid pairs now because this is so easy and it works! Please read carefully, though, and don't bother if you don't/will not time travel. Btw, I rarely time travel, but
I will for this! I don't know who wrote it...
GRASS REGROWTH TRICK STILL WORKS! 
Hi guys, today I'll help you recover your town's grass by time-traveling without losing any of your villagers.
Simple as that. If you are against time-traveling and you consider this method cheating then please do not read any further. FYI, I am not a time-traveler in general myself and this is the only occurrence where I time-traveled because my town had transformed from a forest to a desert.
First things first: I didn't discover this method and I do not claim any credit; it has been posted and mentioned in many sites.
Ok, let's start and
be sure to follow the following steps carefully.
1. Have beautiful town ordinance active.
Very important if you don't wanna lose your precious flowers and have your town flooded with weeds.
2. Find a villager that wants to leave your town.
The moving villager must ping you and walk toward you to trigger the conversation about leaving. If you find any difficulties finding the mover, talk to your villagers many times; they tend to tell you if someone is thinking about moving.
3. Tell the mover Yes/No.
If you want to keep the villager tell him to stay; if you want him to leave' tell him to leave, simple as that. Sometimes they want to stay even if you tell them to leave. If this happens you have to reset and try again. Even if you reset the mover for that day is the same.
4. Time-travel
After you are sure you've done all the above steps, save and quit.
Press the home button on your 3ds and go to settings then "today's date".
Change the date to the previous month.
Do not change it 1 month forward as many people claim is the best way, it's not.
If you time-travel 1 month ahead it might spoil your game, such as town aesthetics, colour of grass etc.
So change the month to 1 month before. Exit the settings and load your game.
As soon as you load your game, save and quit instantly. Do not talk to anyone or wander around. Nothing has changed yet, this is normal. The mover who you convinced him/her to stay/leave is still in your town. (This happens because time-travelling any amount of time backwards counts for only 1 in-game day).
After you save and quit, go to your 3ds setting again and change the date to the current date. That means 1 month ahead of the current settings.
5. Enjoy.
Load your game and wander around town.
The mover is still here if you had told him to stay or he's gone if you had told him to leave.
Explore your town and witness the regrown shining grass. That's it!
Extra:
After you've done this and you are back to your current time someone else may want to move that day, so check around. Even if you've played that day before attempting this method, after you've done this it'll be a completely new day with new fossils, items in shops, etc. This is normal. There may be a few weeds, some extra flowers and your character will have bedhead.
Also, you will be able to easily tell where the "dead spots" are. Some ideas to cover them are: plant trees, bushes, flowers bamboo. Put a PWP on them, or a characters house or plot a villager on them. Make an orchard over them (not easily seen in a grove of trees or bamboo).