I am trying to get a 100 inches sunflower, but I never manage more than around 70 inches until the time is over. How do you do it? Do you have some good advice how to go about?

Bookmark gardens that are attracting, especially if they have Superfood,  plant a couple of sunflowers and make sure you have nothing competing and use superfood yourself, have a copy and paste message ready to pist in gardens you visit, not sure what else would help. Be ready to switch your plants round you get near 100 inches

I got a 100" sunflower yesterday by using org food in mine using low tables where I could...got the food to 30 mins & then dusted the sunflower to FG. Beforehand I opened 2 or 3 windows of gardens in the wildlife guide going back several hours to hopefully find quieter gardens of summer critters so they would be ready for when the hour started. As I went through I discarded any that had recently had a visit & kept any that might be due. Replacing them as needed, fairy ring is so useful for this as it will let you open all the gardens an any one page. Once I had critters visit the org any time after the 33 min mark I also replaced these tables to get another 3/ 4 critters hopefully. As Jane says it's also useful to have a 2nd sunflower to switch too once you reach that magic 100. Good luck...

*either switch fully grown sunflowers around or removing food is another option xx

Not got there myself yet… lol

And the Sunflower must be *exactly* 100 inches; it cannot be any height over 100 inches. Is that correct?

Yes, that's correct :)

Can't wait to see what it is!

Thank you everybody for your help. I will give it a try again!

Good idea on the 2nd plant. I had mine at 100 with one minute to go and somebody watered in my garden and spotted a mouse. Now I have to start over :(