Sensing trees to trigger on/off valve

I want to build a unit that will sense orchard trees and then trigger a 12v spray valve to cut on/off. The distance between the sensor and the tree is what I am worried about, it will always be about 10 to 20 ft away from the tree. I'm worried that I won't be able to detect the tree or that the detection will be erratic. When the tree is detected all I need to do is trigger a simple 12v valve to turn spray on. In my mind this seem so simple but I'm wondering if I am overlooking some variables. I look forward to your responses and ideas...Thanks!

Hi,
Are the trees moving past the sensor? Or are you moving/driving past the trees? How big are the trees? Do they always have foliage when you are doing this?

Is this a spraying application of some kind?

You are right, sensing trees will be subject to errors. Image sensors will be confused by the varying shape and size of the trees, and ultrasonic sensors can detect only the trunk, provided it's smooth enough, and confuse it with upright humans as well. I'd put reflective tags on the trees, detectable by an infrared beam.

the sprayer would be moving past the trees. the trees themselves would be small, with about a 3-8 inch trunk size and they would always have foliage when spraying...I have seen similar "home-made" systems that use the ultrasonic sensors with much success. Here is a link http://ucanr.edu/datastoreFiles/566-8.pdf to a university of Florida research paper that details one of the systems I have seen before. These type systems, in my opinion, have been over-engineered causing them to be way too complicated and way too expensive. I would like to have just 1 sensor for now and then maybe later on maybe modifying the system to use multiple sensors.