Three words
Words without antonyms

That, This , Fast

Explanation:

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covert: hidden or secret

candid: truthful and straightforward

what i get from these phrases is covert means undercover or underground and that's where the flower is in april. on the other hand, may is the month where flowers start to bloom and thats the reason why candid is being used as a word to describe the blossoming period of a flower.

hope this answer you.