These troughs are called a funny name and I can't think of it but they use to appear in the European gardens and cottages.
Last year I was lucky enough to do this concrete project with my neighbor's Susan and Jim. They take a year to 'cure' and they had experience with this project before. Not sure of the recipe. It had perlite and concrete fiberglass fibers along with water, and sand. I think Martha Stewart has a recipe somewhere. You mold it over boxes, bowls, and buckets with a layer of plastic to aid in the unmolding. It is a slow curing process where you have to spritz it with water and keep it under plastic. I lucked out here - Jim and Susan did all that.
When it is ready for outside, you have a lightweight concrete 'bowl' ready for planting the next year. You can burn off the fibers sticking out but I am leaving mine as I am lazy that way. I planted mine with succulants mostly where they will survive a shallow planting.
I even made a small fountain which I am awaiting some sawsall cutting by Marty yet. To make these hold water, you have to either add a concrete smear inside after it's cured or I poured in Thompson's Deck water Seal and let that puddle up , dumped it out into a plastic pail. Let it dry. Poured it back in again. Dumped it out. Let it dry. It mostly holds water now. It may need a couple more times yet. But it should be ready to plug in soon. I bought a tiny little water pump in the pond section of Menards and I have some shiny stones just waiting to get wet. I may have trouble with the chickens using it as a watering hole but I am willing to share.
Take care and happy gardening on this beautiful day - Patsy