I had a lower back injury when I was a child; to the point that for many months afterward, the only way I could get any sleep at all was on the floor. While working my way through that situation, over the course of many years, I've learned a few shortcuts to save time on pain reduction-before working stretches. For me, it starts before I rise; even getting out of bed wrong can bring on an episode.
Take a day and think about your back throughout the entirety of it. Learn where stretches can be incorporated into the most mundane of chores/actions and eventually you will be able design a stretching routine that becomes habitual, and that eventually can become automatic.
One of the best adaptations I've been using lately is to do my morning wake-up ritual at a hip-high countertop. While I'm having coffee and checking in with ATP, I'll do leg extensions in every possible direction, using the countertop and my elbows to support my upper body. Using the countertop as support will help to lessen the risk of initial over-extension which can be worse than doing nothing at all. Try to lift and hold the leg to a height that's comfortable by using just the muscles of your hip and back -do a slow count hold to whatever number seems comfortable -maybe 10 or 15 to start -then exhale and extend and really stretch your entire leg like you're trying to touch the wall with your toes, while allowing the rest of your body to stretch back with the leg as you extend. Then follow with the same leg in the exact opposite direction. Repeat in every direction possible. When finished do a slow rise-to-standing-tall, and reach for the ceiling...exhale. A couple of cross-body elbows-to-the-ceiling stretches and you should be (safely) flexible enough to try to reach the ground.
I won't bend to the floor for anything until I've at least done this much, unless I've forgotten the risk.
As mentioned in previous posts...yes, make sure to take periodic assessments while gardening and take timeouts for stretching. Do a chore that's the reverse of what you were doing for a while; if stooping to weed, do something that requires you to reach up...etc. Alternating your usage really helps!
These exercises, (used very moderately) can help even once the back is flaming. Alternate with cold packs and utilize both several times a day...or as time allows.
I don't think there's any escaping the soreness of extended stooping, of course, but the incidences of tweaks that literally put us on the floor in agony can usually be lessened by stretching beforehand...and it only has to share the time you'd spend to drink a cup of coffee.
I have to add that I'd always wondered how you weeded all those roses, Zuzu. I'm full of admiration for all you've done, and still are doing! Hope this helps.