As with just about every family of flowering plants, not all orchids are fragrant. But many species exude an odor to lure insects to their flowers. The unsuspecting insect, looking for a drink of nectar, ends up pollinating the flower. This is vital to orchid reproduction. Non-fragrant varieties attract insects by visual means.
Only a few members of the Phalaenopsis Alliance are fragrant. Phal. violacea and its hybrids may be delicately perfumed. Other species such as Phal. lueddemanniana and Phal. mannii are also fragrant.
Many species of Cattleya possess pleasing scents and may pass this trait to their hybrids. Sometimes, the non-fragrant species can be coaxed to producing hybrids that are wonderfully scented. Some of these species are C. bicolor , C. forbesii , C. intermedia , C. warneri , C. violacea , C. maxima , and C. bowringiana.
The Encyclia group with their small flowers pack a powerful scent with a wide range of odors. From chocolate and coconut to bubble gum, numerous fragrances can be described. Enc. cordigera , Enc. tampensis , Enc. cilliare , Enc. fragrans , and Enc. citrina are a few that are odorous.
Some plants are very pungent and easily fill a greenhouse or even a home with their fragrance. The genus Brassavola and Rhyncholaelia are prime examples. B. nodosa will fill the night air with a wonderful smell. A large Aerides odorata can be detected over a great distance. Many of these strong-smellers can be overpowering at close range.
Miniature orchids sometimes produce an odor, but not as far-reaching as some of their larger cousins. There are some species of Bulbophyllum that smell downright nasty! Bulb. putidum emits a foul-smelling stink like decaying flesh in order to attract carrion flies for their assistance in pollination. Another type of Bulbophyllum smells like over-ripe cantaloupe.
Other genera that harbor fragrant species are the Vandas , Laelias , Coelogynes , Dendrobiums , Angraecums , and Zygopetalums , to name a few. Not all species are fragrant and many hybrids loose their scent even though the parents might be odorous. Some growers offer catalogs indicating whether or not certain orchids are fragrant. Your best bet is to take the time to pick up the plant, close your eyes and inhale deeply, and remember the name of the orchid with the wonderful scent.