Yeah this is a similar case to the Belongongs and Positioning question. Use your own judgement to roleplay these effects.
So for example, I've had a caster who was made 1/10th their size, climb up into the pant leg of an enemy to distract them, which gave their party members a Buff when attacking the enemy who was frantically trying to get rid of the little wizard who was crawling up their leg.
In case of the poison ivy, this can pose a pretty annoying situation for characters and enemies without proper footwear, resulting in itchy legs and feet, which can be pretty distracting during combat.
As you can see, this is just another thing that can influence creative problem-solving or create problems for the players they have to adapt to.