Thanks, glad you enjoyed the game! The whole game dev journey has been fun, from prototype to mostly finished.
In order to harvest a mistberry, you'll need to get four correctly-typed arrows pointing to it in a single day. For instance, if you have a sunny-typed mistberry, you'll need four neighboring plants or rocks that send out sunny arrows. If a mistberry doesn't make it to 'fully grown' in a single day, it resets back down to zero growth - as I'm sure you've seen by now haha.
One hint: if you find a cluster of three or four rocks adjacent to a tillable square, you might be able to use certain potions to help get you closer to growing the mistberry...