There is no single best free garden design app, but Gardena's My Garden is the strongest free 2D planner and Planner 5D has a usable free 3D tier. iScape also offers a limited free version on iOS.
Know the trade-offs before you commit hours to one. Free apps rely on generic plant icons and stock objects, exports are often watermarked or low resolution, and none of them show your actual garden, just an abstract version of it. They are good for thinking through bed layout and spacing, less good for deciding how a style will look on your real space.
Paid AI tools fill that gap by rendering concepts directly onto your photo; our ChatGPT landscape design page compares that approach with free chatbot prompting.