They can be harvested on day 10, I upgraded them as I can only make it to day 9. When you reach the harvest date you can click the mincart to get crystals or the wheat on the farm to get what you have grown. Shield upgrades are the most important so far for me as they determine which wave I reach.
I wanted to add explanations of what each upgrade is for but I thought it would be too much text, but I think you are right, I will think about how to solve it, maybe with a pop-up with “?” explaining each thing
