Something that's matured is fully developed, at its peak of being alive and mature. Your dad's matured rose garden is full grown, blossoming with beautiful flowers.
In a garden, plants that have grown to their full height or fruit that's ripe are matured. An idea or plan that you've been working on until it's perfect is also matured, once it's truly finished. Children get more mature, but humans are never at a point where they are considered done the way plants and animals have. There's always room for improvement. The Latin root is maturare, "to ripen."