The above is the advice from the Colorado Chiropractic Association (CCA) and appeared as  a PRNewswire release on May 27, 2004. The article starts out by noting that bending, reaching, and digging in the garden can provide a great workout, but if you're not careful you can get hurt. 

The article suggests that a warm-up and cool-down period is just as important for gardening activities as it is for sports.  The CCA also recommends stretching before engaging in gardening.  They also recommend that if you feel aches and pains from gardening, and the pain persists, consider visiting a doctor of chiropractic.

The article gives several tips for stretching that should be done before gardening.

• Stand up and prop your heel on a back door step or stool with your knee straight. Bend forward until you feel a slight pull in the muscle at the back of the thigh, called the hamstring. Hold the position for 20 seconds, then relax. Do the stretch once more, then repeat with the other leg.
• Stand up and put your right hand against a wall or other stable surface. Bend your left knee and grab your ankle with you left hand. Pull your heel toward your buttocks to stretch the quadriceps muscles at the front of your thigh. Hold that position for 20 seconds, relax and do it again. Repeat with the other leg.
• Weave your fingers together above your head with your palms up. Lean to one side for 10 seconds to stretch the upper body, then reverse. Repeat two or three times.
• "Hug your best friend:" Wrap your arms around yourself and rotate to one side, as far as you can go. Hold it for 10 seconds, then reverse.