
• Choose plants with healthy foliage and no sign of disease or insects.
• If possible, examine the roots of a containerized plant before you buy. Healthy roots will generally be white or light-colored on most plants.
• Shrubs and trees that have spent a long time in transit or on display may have gone through extended periods of heat and drought. Reject any plant with dark, discolored roots or a shrunken root mass.
• The root ball of a balled-and-burlapped plant should be round and firm — not loose — and there shouldn’t be roots growing through the burlap.
• Look for plants that are bushy and compact, with new leaves appearing close together on the stems (not leggy).
• Take an even closer look at plants on sale — they may be past their prime.
• Avoid plants with flowers that have opened, and opt for those still in bud.
• An annual or perennial with leaf scars indicates that the plant has previously lost leaves.
• Look for even and attractive branching from all angles.
SOURCE:http://www.diynetwork.com/how-to/outdoors/gardening/determining-plant-health