Similar ideas have been tossed around, usually involving a bit more effort from the player to obtain that ql increase. From fertilizing to watering and whatnot. I think I could see a trellis crop's first few harvest rolling for a Q change again however. Here's how I think it should be handled if anything like this were to be implemented.
Trellis crops can as a standard, be harvested five times. Each time they are picked the quality can increase or decrease like we are familiar with, but with a twist. Each harvest will increase or decrease the Q in a different range. Younger harvest(1st and 2nd picking) would range from 1st:-1 to +1 Q change and -1 to +2, the third harvest would be -1 to +3. From there the 4th and 5th would actually favor a decrease in quality like so: 4th: -2 to +1 and the 5th being -3 to 0. Maybe this is too convoluted, but the idea behind it is plants tend to increase their highest quality stuffs at the peak(middle) of their life, not the beginning or end. To compound with this 'system' it would be neat if the amount of produce a trellis crop yields increases with each subsequent harvest.
With this system in place trellis crops won't grow eternally, won't have to be replanted every time they are harvested due to the best time being at the third harvest, and won't feel like a repeat of regular crops. After the final harvest trellis crops should be removed automatically.
![Trollface ¦]](./images/smilies/troll.gif)
Oh and I think trellis are too easy to build. They definitely need bone glue in there, and should maybe affect plant quality as well to make sure people can't just build them once.