To turn "buy big fruit trees" into a "proper text," it helps to know your goal. Here are a few ways to rewrite it depending on what you need:
When buying big, ensure the root ball is healthy. Home Depot recommends digging a hole twice as wide as the root ball for proper planting. buy big fruit trees
This sounds more professional and specific. "Established" or "mature" is the technical way to describe "big" trees that are already a few years old. If you are asking for advice: To turn "buy big fruit trees" into a
Make sure the "big tree" you choose is native or suited to your specific area to avoid it dying shortly after a costly purchase. buy big fruit trees