(this happened earlier in the semester when we had just started on the roof)
Before we started planting, we had to do some major weed pulling in the succulent modules. While pulling them, we had to keep a record of the number of each weed we pulled. This excluded the grass, since there was so much of it. Some of them were tricky since they looked similar to the succulents.
It took the whole class to pull and record them. Although recording them took a while, it was important to know that plants could find their way up to the roof and grow there. It was also important to know which plants could grow on their own, even if they were weeds. It was meticulous work, but it paid off and I learned that recording plants, even weeds, is very important and contributes positively to the green roof.