[sblock=mleibrock]You are correct that the effect begins on your initiative; however, the duration can continue as long as you concentrate. Please be aware of the fact that Cantrips are very minor magic and that you and the other casters in the party have an unlimited number of them. Thus, I can't allow them to be too powerful. I suggest that you read some of the other Cantrips such as prestidigitation and flare to get a sense of what they can and can't do. You will need to roll to hit, verses the touch AC of 13 and the degree to which you beat that number will determine how much of an effect you have with a natural twenty perhaps being strong enough to spoil an attack or cause the target to waste an action removing the leaf. That's about the the very best result you can hope for. Some penalty to the next attack is the likely result of a hit with a high roll being perhaps as much as -3. Even then you are getting a bit of a bonus because the leaves are wet and sticky at this time. A given foe will get wise to your tricks if you try to duplicate them repeatedly.
If this is the sort of thing you really want to do with your character I would consider the dirty trick maneuver.
Advanced New Rules
Those rules assume you are acting at melee range and use an attack of opportunity as the counter to it if you don't spend a feat. If you invest one or more feats into this then I could certainly see using that mechanic as a way to resolve such actions. We'd certainly need to work out something that was fair to you such as a variation on improved dirty trick like maybe:
Magical Dirty Trick (MetaMagic)
You are skilled at using Cantrips to pull dirty tricks on your foes.
Prerequisites: Int 13, Cast Prestidigitation or Mage Hand.
Benefit: You can use the Dirty Trick Maneuver at range with the casting of an appropriate Cantrip. In addition, you receive a +2 bonus on checks made to attempt a dirty trick. You also receive a +2 bonus to your Combat Maneuver Defense when an opponent tries a dirty trick on you. Using the Magical Dirty Trick MetaMagic spell does not use up a higher-level spell slot than the spell's actual level.
Normal: You provoke an attack of opportunity when performing a dirty trick combat maneuver and can only use it during melee.
We could also use make a greater version similar to the Greater Dirty Trick Feat for a higher bonus and longer duration if you desire. If you like it we could probably figure out a Specialization and Focus for even greater benefits if you are willing to invest that many feats.