So I reached level 25 and Merle Ambrose's face appeared, so I clicked on it. He told me to come back because he had something for me but when I did return he didn't have anything new to say. Am I missing something?
Also, when I started the gardening quest I thought the guy gives you a basic pest spell, but I don't have one. I have no means of harvesting as well.
For your first question, please refer to the Frequently Asked Questions section of the message boards:
https://www.wizard101.com/posts/list/29105.ftl Merle seems to be back to some bad habits of calling Wizards to his side when they level, and then not having anything for them when they arrive. Unless Merle has a symbol over his head, he's just checking to make sure you're paying attention to your studies!
For your second question, you can purchase spells for gold from all the different moles, and to harvest you simple leave the gardening interface and approach your mature or elder plant. If it is ready to be harvested, you will see an "Click X to Harvest". If you don't get this prompt, return to your gardening interface and inspect the plant to see if has needs, pests or still needs time to grow.
