There are a few different ways to approach this question, and the answer may depend on what type of garden design app you are looking for. If you are simply looking for an app that will allow you to plan out your garden and get an idea of what it will look like, then there are a few different options available. Garden Planner 3D is a great option that allows you to input the dimensions of your space and then add in various plants and features to see how they would fit. Another option is PlantSnap, which can help you identify plants based on pictures that you take with your phone.
If you are looking for an app that will provide more detailed information about plants and gardening, then MyGarden by Burpee is a good option. This app includes a plant encyclopedia with over 7,000 entries, as well as tips on gardening and growing specific plants. It also has a feature that allows you to track the progress of your garden throughout the season.