No. Adding your Constitution modifier to hit dice is a special feature of short rests.
The wither and bloom description tells you that you regain:
a number of hit points equal to the roll plus your spellcasting ability modifier.
So this is what you regain: the number on the die plus your spellcasting ability modifier. Adding the Constitution modifier is not a general feature of spending hit dice, rather, it is a specific feature of taking a short rest:
A character can spend one or more Hit Dice at the end of a short rest, up to the character's maximum number of Hit Dice, which is equal to the character's level. For each Hit Die spent in this way, the player rolls the die and adds the character's Constitution modifier to it.
Adding the Constitution modifier is only for hit dice spent in this way. Features that allow you to spend hit dice in other ways tell you what happens. Since wither and bloom does not tell you to add your Constitution modifier, you don’t add it.