Hi Dan,
where you have ‘students make guesses and learn from what they see- – this is part of the explore section of the Framework for Inquiry. When the children are carrying out their investigation, if they have no clue what will be happening and are just guessing – then their science understanding isn’t engaged. They need to have a bit of background to be able to build on.
I’d offer children a class plant that needs to be looked after – and you can do this as a class for a month. Then they might wonder — does it need to be watered every day? What if we do / do not water it? Then you might grow some cress seeds (and treat the germination as a separate issue) that they can then use to test their ideas.