setting post in quikrete in a plastic planter
When setting posts in the ground the concrete mix will absorb extra moisture from the surrounding soil. I pour some concrete mix in and stir water in.....repeat till the hole is filled. I also know of fences and 4x4 posts that were set the way you described and they are still plumb straight and strong many years later. Only reason I would say to not just pour water in on top, in your situation, is because there is nowhere for the bottom mix to get extra moisture from if needed. I would either pre-mix the concrete beforehand or mix it after you put it in the planter.