Today was the first day of data collection for the season and it's only been two weeks since planting! (Two week old greens on the left, one week old greens on the right.) Like last spring I measure ten leaf lengths per quadrant and then measure the weight of all the greens in that quadrant.
So, why measure both?
Leaf length indicates when greens are ready to be harvested. With a quick glance at a tray of greens I can say 'those are big enough to harvest' or 'those are still too small to harvest.' I've found that between 6 and 10 centimeters is about the perfect size for baby greens. A lot of the greens I'm growing can, and will get bigger than that, but then are too big to be 'baby' sized.
Harvest weight, which I measure in grams per square meter, is important because salad greens are priced by weight. Right now the price of mesculun mix at farmers markets is about $12 per pound. So, knowing how the weight of the greens increases as they grow will be important for budgeting analysis (when I get to that step!). For example, 6 cm long greens will weight less than 10 cm long greens. So, if you let your greens grow more you could make a larger profit.