I trained this model by following a tutorial by a youtube creator called sentdex. You can find his videos here
https://www.youtube.com/user/sentdex
To train the model I took 150 photos of broad leaved docks in one of our grassland fields. I used a program called rectlabel for OSX to label all my docks in the image. This bounding box tells Tensorflow where to look for the weeds/docks. Tensorflow then trains itself to place accurate boxes around all the weeds in the data set by constructing and training a neural network.