Growing Degree Days
I previously asked a 3-part question, all related to GDD (question #112872), but only got an answer to part-1, so I'll re-ask the rest! If I'm limited to one question per submission, please let me know that's the case.....
1. In my research about GDD, it seems like it is primarily applied only to farm crops & to woodies (trees, shrubs, and vines), not to herbaceous perennials found in home gardens - is that correct (because I haven't found any web sites focused on pests of herbaceous flowers)?
2. If I find GDD numbers for a particular insect from another geographic area (say California), wouldn't it still be applicable here in Michigan, as long as our Michigan accumulated GDD count is used - is that correct (e.g. a GDD number for a specific insect is good and useable in other areas of the country, as long as the GDD of that other area are used)?
Advanced Master Gardener
1. I have not seen or been told about any growing degree days for perennials. If this exists, it has not been advertised.
2. There may be some variation, but it should not be significant. Just make sure that your problem insect is identified correctly and it is identical to the other state's one...genus and species.
If you are gathering information, do some looking a plant phenology. That can give you information about insect emergence or seed germination timed to a specific plant blooming or it may be like crabgrass germinating about two weeks after forsythia stops blooming. Cornell University had information on phenology awhile ago. Good luck.
Keep in mind that GDD are usually applied from emergence forward in time. Prior to emergence, the soil temperature is very important. The minimum temperature for emergence/growth to occur has been catalogued for many plants including some common garden plants, see http://www.ianrpubs.unl.edu/pages/publicationD.jsp?publicationId=1457
Also, GDD for various plants are not calculated the same for each plant. The base temperature for corn is 50 or sometimes 55 F while the base temperature for wheat is 40F. If you borrow the information relating GDD accumulations to phenology stage from any web or literature source make sure that you apply the same base temperature (ie. 50, 55, etc) that was used by the source and the upper limit if that was used.