I want to make a cast-concrete planter for the backyard. Would regular concrete be suitable, or is some specialised product available?

Regular concrete will work. However, use of a chemical additive called plasticiser will reduce the viscosity of the concrete without increasing the water content. Adding extra water to make concrete more fluid weakens it, and you should avoid the practice both when casting concrete and when doing standard concrete work, such as making sidewalks. Another benefit of plasticiser is that it reduces or eliminates the small holes left in the concrete after it’s been cast. In the precast-concrete business, these are known as bug holes, and they are the last things you want to find after you’ve gone through the trouble of casting something in concrete and then stripped the forms.

Remember to include a form for a drain hole. Otherwise, the planter is liable to hold water. Excess water contributes to root rot. If you’re unfortunate enough to live where winter temperatures drop below zero, this excess can crack the planter when water sitting in its base freezes and expands. The form for the drain hole need not be anything more elaborate than a well-oiled piece of wood dowel rod.