Avoid picking up from the tips of the planters. In some cases, these are the fragile areas of the planter. 

Items you will need:

Glue, if it's a small crack. Most any glue will work well. I use gorilla glue for most cases.

Hypertufa mix, if it's a large or many cracks. Mix only the amount you need to repair using equal parts peat moss and portland cement sifted to a fine material and vermiculite, just a pinch. Add just enough water to the mix to make the consistency like gravy. Apply the mix to the inside of the planter where it is cracked to avoid having to restain the outside.