Some important things real quick...
1. Only the OFFICIAL day gets the chance for these public works. That means forced Weeding Day does nothing to get these topiaries.
2. You only get the topiaries if there are NO WEEDS the day of the event. Leif will say how happy he is about it and say he'll tell Isabelle about special topiaries or something like that. You can then build them starting the next day (since you can't build during an event...)
Yes, that means you have to choose between unlocking all 3 Topiaries OR getting a piece of the flower series. If you really want the latter, it's honestly best to find someone willing to force the day (you need to get 50+ weeds to force it--Leif will show up randomly in town) because only 4 items can be gotten by whoever's town it is and the rest need to be gotten by visitors--the issue here is that visitors are required to pick 30 weeds in order to get an item. If they do not pick 30 weeds, they don't get anything--even if the person only has 10 or less weeds which makes it VERY hard to have enough for visitors to pick to actually qualify. The forced Weeding Day DOES NOT have this 30 weed requirement and just for picking a few will get the person an item.
3. Yes, the date varies per region.
-Last Friday in April = North American games
-Early March = Korean games
-Last Saturday in April = Europe games
-Early September = Japanese games