I’ve grown Basil on our windowsills for a few years. Version 3 is now growing in Utz Cheese Ball containers.
With flower pots, I used a moisture sensor and watered the plants a couple of times a week.
Version 2 of my window planters was less pretty but massively more functional gallon milk jugs. Their square shape provided far more volume in the same sill print. In that extra space, I also integrated some physics to reduce maintenance. I put an inch of gravel in the bottom of the jugs, a layer of fabric, and then filled the jug with potting soil. The fabric just keeps the soil out of the gravel. The gravel provides drainage when the soil gets saturated. As the soil dries, water wicks up from the gravel via capillary action and moistens the soil from below. When I can’t see water in the bottom, add some. It worked very well.
While v2 worked great for the plants, the gallon milk jugs were hard on the eyes. I watched for substitute containers. Last years make-the-walls-7-inches-deeper-project left us with 9″ deep window sills. That permitted use of containers in many more sizes and shapes.
Version 3 was born when the last jug of cheese balls was empty. I nipped off the top couple inches and assembled it the same way as v2. I dropped in a few basil plants and set them in my West window. They couldn’t be happier. Since I only have to water them every few weeks, I’m happy too. It’s time to buy more cheese balls!