Growing-Degree Days

ChiloquinNews article 3/7/11

Spring is just around the corner, albeit a long corner, but still thoughts are turning to spring plantings, and no doubt we’ll try tomatoes again this year, only to have our hopes dashed once again at the end of summer. Hope springs eternal though and remembering the one year when my garden was overflowing with ripe tomatoes, I always try again, thinking that maybe this will be another good year. If you’ve wondered why the garden does better in some years than others, then it probably has a lot to do with growing degree days, which is simply a way of measuring how much heat your garden gets each day.

Growing degree day (GDD) measurements are much more meaningful, especially for growing vegetables, than looking at a plant’s hardiness, which is just based on a minimum temperature that the plant can survive. Of course the minimum temperature is vital, because if you choose a plant that is not hardy (tomatoes, corn, peppers) then the late spring freezes will kill it back. We are all familiar with this scenario. But when we diligently wait until there are no more freezes and plant out the tomatoes, and they still never manage to ripen fruit, why is that?

GDD are measured by adding the high and low temperature for the day, and dividing by 2 to get the mean. Any result over 50 is considered a growing degree day. So, if the high temperature is 66, and the low is 38, add them and you get 104, divide by 2 for a mean of 52. So, that day had 2 GDD, which means that pretty much our whole spring limps along accumulating about 2 or 3 GDD each day.  And then when summer arrives, even though our days can be hot, the nights still plummet to the mid-40s, reducing that mean temperature, and hence, the GDD. The Agro National Growing Degree Map currently shows Klamath County with an aggregate GDD of 1 so far this year! To put this in perspective, dandelions need 50 GDD to flower.

The GDD requirement can be measured for all plants, and as well, for the insect pests that infect those plants. Some GDD examples:

Forsythia flowers at 1-27 GDD, lilacs 80-110 GDD, elderberries 330-400 GDD

Peas require 1200 GDD for mature pods, and early corn varieties require at least 1750 GDD to ripen.

Getting back to tomatoes, early season varieties require about 1400 GDD to ripen fruit, and late season varieties require about 2000 GDD.

So this year, instead of praying for warmer days, let’s hear it for warmer nights!

Flowering this week in my garden: Crocus sieberi tricolor (0-1GDD)


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