If you experience pain when sitting and weeding, but are determined to continue anyway, you might gently explore the range of spinal movement between arching and rounding. Try varying degrees of each. If sitting on the ground to weed is for you, one or more of the stops between arched and rounded should feel comfortable enough to maintain for the entire weeding session. Keep in mind that your back will fatigue after a while.
Arching and rounding when on all fours is a relatively safe way to do this experiment. Therefore, warm up with an exercise like yoga's cat-cow stretch. Another possibility is to do pelvic tilts, lying on the ground or floor. The key is to pay attention to any warning signs, such as pain, that suggest your back may not be able to handle that position for very long. Also, sitting compresses the spine, adding further vulnerability. So let your warm-ups inform you of the amount of arching or rounding to use when you sit to weed.
Instructions for the Exercises