The scale of this diorama is 1/24 , or there abouts and I used a number of different methods for the foliage.
The vines / ivy , creepers and ferns are all brass etchings which I bought at a dolls house shop .
Some came ready painted and some I painted myself .
The other ivy is made of dyed birch seeds stuck on one at a time time to fine tree roots .
The leaves on the small oak tree and the dead leaves are 1/35th scale and are made by a Czech company called Plus Models.
They are available in The US from Mission Models ( I think).
There is another Czech company called Model-Scene ( www.model-scene.com) that have a similar leaf range as Plus Models (probably made by the same people) with the addition of 1:72 & 1/ 87scale leaves .
I think the creepers and vines that I used would be too large for HO unless you are doing a scene from The Land of The Giants!
However, if I were you I would look at the products aimed at fantasy wargamers and military modellers.
You know , it is really time you changed scales !