The simplest way is to buy a water meter from the nursery for about $8 and leave it in the container and use it for estimating when to water. If you use the same soil in the containers, you can use one meter to gauge when to water all the containers of the same size.
Different sized containers will be watered differently, the smaller ones more often. The meter will register wet when wet and dry when dry. You water it before it registers dry, about halfway in between the two. Also, the container must drain and not be closed unless you use a pot in a pot container, a smaller pot inside a larger one. If you are using one container inside a larger container then inside container must drain into the larger one.
Put gravel in the bottom of the larger container to a depth so that the smaller one can rest on top of it without sinking. Then you can use coarse mulch to surround the smaller pot, filling the void between the two pots. 4 You should always water so that some water comes out of the bottom of the container each watering to flush salts. Our municipal water carries a lot of salts compared to many other communities since it is derived from the Colorado River. This will require that you check periodically that you are not building up water in the larger pot if it does not drain. It is always best, though, to have both pots drain.
By Robert Morris