Is it pronounced steevia or steffia?
No, it's actually STEH-veeya. Do a Google Search for "(pronounce OR pronunciation) stevia" (without the quotes) and you'll see several more official sites than this indicating the correct pronunciation. Unfortunately this incorrect pronunciation (post by heathen1) comes up first in a Google Search, and has been here for two years apparently. Gotta love the web.
Actually, I'm going to trust Marriam-Webster over your random Google results, which confirms the original "stee-vee-a":
You can even hear it pronounced here:
The plant is from South America; Latin, Spanish, and Portuguese almost always pronounce the letter "e" as "eh". Anglos have warped many words in the romance languages so the common pronunciation has come into predominant use and acceptance; for instance, Amarillo and coyote. I hate to see another word lost to the arrogance of the master race.
Money and might make right???
In this case you can thank the marketers.
Steeeevia sounds prettier than stevia, so that is the way they market it. And thus why everyone starts thinking it is steeeevia.
Merriam-webster.com is WRONG and is bowing to popular ignorance.
Other examples of marketing influence:
Althea being marketed as Rose Of Sharron (for obvious reasons). ROS is actually the double blossom version of Althea and looks a little like a rose.
And tartar substituted calculus, again, marketing.
It's unusual enough that the botanical (Latin) name is still being used in Stevia's case (and that it hasn't been renamed "sweetie-plant" or something else deemed suitable for marketing).
It would be hopeless at this point to try to "educate" the public into pronouncing it "steh-via". And there are plenty of cases of English words incorporated into Romance languages (like French and Spanish) which are "mispronounced" by speakers of those tongues. You'd have to retrain them too and it hardly seems worth it.
You pronounce it Stevia.