Ripeness Test

• Test the ripeness of a plum by giving it a gentle squeeze. A ripe plum gives a small amount under gentle pressure between your fingers and feels slightly soft. Plums that feel firm or hard when squeezed are not yet ripe, so leave them on the tree.
  
  

Color

• A ripe plum also has a uniform color, and the fruits typically deepen in color shortly before they reach peak maturity, the Ohio State University Extension reports. Plums come in many colors, however. Ensure you know what color the fruit on your tree produces, because the color often darkens and intensifies immediately before it is ready for picking.
  
  

Other Signs

• Before picking a plum from a tree, give it a quick once-over to ensure it appears healthy. Look for fruits with a smooth skin, and avoid picking plums that have cuts, dents or grooves. Also, stay away from bruised fruit, although this more commonly occurs when ripe fruit falls to the ground.
  
  

Ripening Plums

• If you find yourself with slightly under-ripe plums, ripen them by placing them in a paper bag and leaving the bag at room temperature. Check the fruit once a day until it feels and appears ripe. Then the fruit is ready for eating, or placing in the refrigerator, where it keeps for at least a few days. If kept at temperatures between 31 degrees F and 32 degrees F, with humidity levels between 90 percent and 95 percent, plums stay fresh for up to four weeks, the Ohio State University Extension recommends.