when Esther says: If you have some time, come to my Greenhouse to help me out a bit. When is the Greenhouse open?

I think the Greenhouse only exists as a link, so it isn't limited by hours.

Just click on the link in the text :)

I don't think anything's been released for her greenhouse this Vesnali, yet.

You can still do the old quest though. For the achievement with a coin reward :)