A few weeks ago I described how to calculated growing degree units. Remember? Basically they are a measure of heat accumulation, and pretty useful when you're dealing with changing seasons, night and day, or greenhouses heated to different temperatures. Here's how I use them.
We'll use lettuce as an example. I planted lettuce nine time throughout the fall last year. Using all the growth rate data I collected I figured out how many days it took to reach 8 cm. (8cm is a good baby green size.) In the graph below you can see that it took more days to reach 8cm as the season changed from fall to winter, especially in the cooler greenhouse.
Still on board?
I did the same thing, but changed growing degree units for days. So, I calculated how many growing degree units it took for the lettuce to reach 8cm. The number of growing degree units accumulated by lettuce at 8cm stayed about the same in the warmer greenhouse and slightly decreased in the cooler greenhouse - even though the actual amount of time to reach this size increased. Make sense?
So, now on to interpretation. Why does this happen and what does it mean? I'm not totally sure yet, but here are some ideas:
- It takes about 600 GDU to have a harvestable baby lettuce crop. How long it takes to accumulate that heat can vary.
- Why does the number of GDU decrease in the cooler greenhouse? Maybe because there is so little light late in the fall. Even if greens were in a warm environment, their growth would be limited due to limited light.
* As usual, this data pertains only to our greenhouses this fall. It is not complete and should not be used elsewhere. If you are looking for complete data pertaining to winter greens growth, and growing degree units you'll just have to wait until I'm done with my thesis! *