I'm not sure if we have a seperate hobby license here ( I don't think we do). You are either a nursery (Any person engaged in the production of nursery stock for sale or distribution, including own-use nurseries.) or a nursery stock dealer, plant broker, or agent (doesn't grow the plants but either buys to resell or acts as a go between for buyer/seller). The fees for a GROWER of plants are $25-$460 depending on the number of plants or acres. Even if you plan to just give away the plants you are supposed to be licensed; it's to keep pests, noxtious weeds (remember some of those pretty flowers we may have are invasive weeds in other states), and diseases out of states that don't have them and don't want them.
Now on the business end of the license that gets more complicated because you may not actually be a business, you may just be a hobby person. "The IRS presumes that an activity is carried on for profit if it makes a profit during at least three of the last five tax years, including the current year" plus a slew of other criteria.