dirtyhandscleansoul:
The sensors either need to fully penetrate the canopy or read only the canopy.
That seems to be the critical problem. I guess the typical ultrasonic sensors are waiting for an echo that is strong enough to exceed some threshold. If you can get an ultrasonic sensor that has a narrow beam, and look for the longest distance over a short interval, that should give you a height above ground signal. If you're trying to detect height over crops you need to look for the shortest height over the interval and it might help to have a sensor with a wider beam. (If you intend to support both modes then I guess the sensor width will be fixed.)