Hopefully the planter has not yet been built.

wood in contact with soil is not a good idea for longivity of the wood.
neither is allowing the wood to be constantly moist.

This is a major invitation for dry rot & insects.

Good outside woods would be; teak, redwood, mahogoney(sp?)

but you best bet is a liner; copper or plastic (thick ~1/16" min)

Do right this thing will last many years, done cheap a few at best

If you put the planter right on the lawn or concrete w/o an air gap &
"water resistant feet" the bottom will rot out in no time.

Wood can last for centuries, if kept dry, moist it can be gone in a few
seasons.

OTOH, if you use non-toxic finishes this planter can be mulch for the
next one

cheers
Bob