The adjective luxuriant means lush, fertile, prolific, or abundant.

The adjective luxurious means extremely elegant, expensive, lavish, or elaborate.

Examples:

• The chateau was surrounded by a grove of the most luxuriant foliage, except in front, where the prospect opened on an extensive lawn.
• I stepped cautiously into the large, luxurious bathroom, with its deep tub, fancy toiletries, and polished silver fixtures.

Usage Notes:

• "Luxuriant means rich in the sense of profuse (luxuriant vegetation) whereas luxurious means rich and lavish (a luxurious house). But the most common adjective from luxury now is--luxury (luxury apartment, coach, cruise etc.). No room for confusion here."
  (Wynford Hicks, Quite Literally: Problem Words and How to Use Them. Psychology Press, 2004)
• "Luxurious is the adjective that belongs in sense to luxury & conveys the ideas of comfort or delight or indulgence; luxuriant has nothing to do with these, implying only rich growth, vigorous shooting forth, teeming; as luxurious to luxury, so luxuriant to exuberant. Luxurious houses, habits, life, people, climate, idleness, times, food, cushions, dreams, abandonment, desires; luxuriant vegetation, crops, hair, imagination, invention, style. The points at which they touch & become liable to confusion are, first, that abundance, essential to luxuriance or exuberance, also subserves luxury, though not essential to it; &, secondly, their common property in the verb luxuriate, which means both to enjoy luxury & to show luxuriance. A luxorious fancy is one that dwells on luxury; a luxuriant fancy one that runs riot on any subject, agreeable or other."
  (H.W. Fowler, A Dictionary of Modern English Usage: The Classic First Edition [1926], with notes by David Crystal. Oxford University Press, 2009)

• "The two adjectives are not interchangeable: luxuriant is principally applied to things that produce abundantly; luxurious to things that are very comfortable, expensive, opulent, self-indulgent, etc. The noun luxury is also used as an adjective, meaning 'desirable but not essential'--luxury goods. Some people dislike its use as a synonym for 'luxurious,' especially in advertisements--a luxury car, a luxury hotel, luxury flats, etc."
  (Good Word Guide: The Fast Way to Correct English, 7th ed., edited by Martin Manser. Bloomsbury, 2011)

Practice:

(a) Sir Richard Burton's _____ prose is generously ornamented by fine metaphors and similes.

(b) After striking it rich, the Clampetts moved into a _____ mansion in Beverly Hills, California.

Answers to Practice Exercises

Glossary of Usage: Index of Commonly Confused Words