Plants taller than one block should be able to trigger tripwires

Any plant that grows beyond one block size after planting should be able to trigger a tripwire.  (Sugarcane, bamboo, cactus, chorus, vines (going down), ferns and grass (when bonemealed into 2 high varieties), trees (both logs and leaves), mushrooms (when bonemealed on podzal or mycelium).  

Yes, you can use observers, but running string is much simpler, less visually overwhelming and can be done more compactly if you are running it over a big distance.  Currently, as far as I can tell it blocks growth instead.  Since some people use string to regulate the height of plants this behavior should only apply if the string is attached to a tripwire- otherwise it should still block plant growth.

You need some rules for governing this behavior.  There are two general types of block behavior to deal with.  Logs and mushroom sections...  If a log or mushroom section grows into a block it should break the string and trigger the tripwire.  These solid blocks and you'd have to focus on that particular block to reenable the trap anyway, so it makes sense they should apply enough force to break the string.  Other plants are less substantial and shouldn't break the string unless, when you harvest them you specifically break the block the string is on the string stays- all of those blocks can be harvested by harvesting blocks above or below them.