Luxuriant and luxurious, both adjectives, have almost the same meaning—they are interchangeable in most cases, but there is a subtle difference.
Luxurious means “characterized by luxury” while luxuriant means “characterized by luxury or abundance.” (Luxury is a noun meaning “a situation of great wealth and comfort”—such as living in a big house, eating fine foods, and sleeping in comfortable beds.)
In most cases, either word can be used:
We enjoyed staying in Samantha’s luxurious/luxuriant house.
But when it means “abundance” in a way unrelated to fine living, luxuriant is the correct usage:
The greenhouse was filled with the luxuriant /*luxurious growth of jungle plants.
In simplest terms, luxurious means “all the comfort you could want” while luxuriant means “abundant.”