The suggestion for curing new concrete to ready it for plants or pond plants is to flush with water 3 - 5 times a day with the hose for two weeks. Then allow to thoroughly dry and apply two coats of concrete sealer inside, allowing to dry over night between coats. Some will suggest a third step in between flushing and sealing: Mix up a solution of 1 quart of ordinary kitchen vinegar and 10 quarts of water. (you may need more with the length of your planter)
With a stiff brush (or old broom) and generous sloshings of the solution, scrub vigorously over every square inch of the vessels inside surface. "The surfaces may bubble up a bit as the vinegar works on the free lime, but don't worry, it will not weaken the concrete."
Rinse well, allow to dry, then apply the concrete sealer.

I have heard of people who apply a coat of rubber paint (but not to grow hydrangeas!) to some surfaces. Can you research that product to see if it would help in this case too?