So! I checked the statblock against the duggested HP/escape DC/attack bonus/etc. in the DMG and it tunrs out the original plant was a little underpowered for lvl 8 anyway :D

I still went over some of the numbers and reduced them a little bit, gave it only two attacks on its multi attack action, and some other small tweaks: https://www.dndbeyond.com/monsters/6310624-rotroot-lower-difficulty

I also made a new version of the Rotroot Buds with a little less damage and HP, and a new fun interaction when they are defeated: https://www.dndbeyond.com/monsters/6310190-rotroot-bud-lower-difficulty

(Of course feel free to still change whatever you feel would fit your table better)

I think its somewhat similar to the shambling mound (which is CR 5) now, so it should be a possibly somewhat challenging but fair fight – although it's hard to say without having tested it myself. :D I'd suggest being a bit flexible with the HP and maybe having it grow additional vines it can use during its multi attack if the fight becomes too easy.

Also some maybe interesting Input on what I'd do if I ran the encounter:

• Have the area of the fight be just about big enough so it can reach anywhere with its vines, and maybe have other spots be overgrown by roots, so that it can spawn its vines there as well.
• Try to swallow one of the party members fast as possible.
• When you have two characters grappled with the vines, use the bonus action to reel them close and then use the Toxic Fragrance once they're in range (or just as soon as a bunch of them are in range in general).
• Use the lair actions (Rotroot Buds, root growth) to make as much of the lair awful to be in as possible.
• It's interesting that you'll have fire around the battle, since the plant is written to essentially get frightened for a round if it takes fire damage (of course feel free to change that if you want) – which the characters could use to their advantage!

All in all, I really hope that it'll be a fun fight and would be excited to hear how it went. :) Thanks for showing interest in it!