If you tap the button, you water one square. If you hold it down, it charges up and you will see three green squares extend in a line ahead of you. When you see that, release and you'll water those three. Another upgrade and you can do a line of 5... and when charging, you'll see the line of 3, then it'll charge more and you'll see the line of 5. You can let go when it is a line of 3 or wait for it to go to 5. (The same applies after further upgrades with larger areas too.)