If you are wetting the leaves, it would be best to water in the morning. Otherwise, it really doesn't matter. Depending on the size of the containers, the density of the root mass, the type of plants and the potting soil, you could have to water more than once a day during the extremely hot, windy weather we are experiencing now. Containers can dry out very quickly.
Carole
Johnson County Extension Master Gardener
When to water container plants
If you are wetting the leaves, it would be best to water in the morning. Otherwise, it really doesn't matter. Depending on the size of the containers, the density of the root mass, the type of plants and the potting soil, you could have to water more than once a day during the extremely hot, windy weather we are experiencing now. Containers can dry out very quickly.
Carole
Johnson County Extension Master Gardener