It's easy to underrate yard-work, house-work, and other activity with a purpose when considering BG management. That's a basic mistake, of course. Instead, treat it the same way as a planned extended bicycle ride or mountain hike. When I was on a pump I would back off my basal by 20% or more. Set the temp basal duration for the expected time you plan to garden. If you overestimate you can simply kill the temp basal when you stop vigorously moving. Better to pull back on insulin that have to feed it.
For those of us on MDI, I would experiment with pulling back on the long acting insulin dose and planning appropriate BG checks.
Have fun in the garden! Getting your hands in the earth and reconnecting with raising your own food resonates deeply with our human DNA. Actively manage your BG with success and add to your pleasure. Enjoy!