Weeding can really do some damage to the back. I overdid my limit on bending over last weekend and had to use ice, then heat and ibuprofen for several days.
My friend Mickey told me her trick to weeding which I had forgotten to use. It's the three poses of weeding, (no kidding).
Bend over and pull. If you do that too many times, you'll feel it for a week.
Squat down on your haunches. I can only do this for a few minutes.
Kneeling. This works great until you need to move.
You can add various yoga poses, like crossing your legs while stretching over to pull weeds, maybe meditate a bit too while you're down there. You can spread your legs and do a leg stretch in between weed pulling as well. The main thing is to change positions to prevent repetitive motions.
Back exercise. Here's a quick back exercise that is like the reverse of bending over. Lean back against a flat wall, press the small of the back against it with feet flat on the ground and hold for a minute or two. Repeat a few times everyday.
Weeding tips. It's best to weed when the soil is not too dry. You will need to water after weeding. A tool like the hula hoe which looks like a hoop on a stick or the mini rake or a pronged tool can help loosen the soil. You can use a hand trowel to dig up the big weeds.