PLANT PHENOLOGY & GROWING DEGREE DAYS   

In previous  posts we've discussed the importance of soil temperature for seed germination and have only briefly mentioned  the importance of air temperature as the furnace fueling plant growth.  Now, we'll spend  a little  time discussing why farmers (and probably only a few gardeners) are  interested, or should be interested,  in  closely tracking  the heating effects of daily air temperatures.

For most backyard gardeners just checking the planting directions on the back of a seed packet and noting that their favorite veggie matures in 67, 75 or 104 days probably fills the bill. Seed packet information usually summarizes how long a plant will take to develop within a region's growing season assuming average temperatures and moisture levels.  But plants do not, of course, develop at  constant rates after their emergence from the soil.

Source: intechopen.com

Their growth, photosynthesis, transpiration and enzymic activities - all occur at different rates given daily temperatures and lighting conditions,  but the growth of a plant's shoots, stems and leaves is mainly what we notice.  The chart to the right shows these differences in a plant's activities with respect to temperature. Although the temperature scale is not noted on the x-axis, the scale on the left side most likely starts at 50°F (below which most vegetables do nothing) and goes up to the high 90s°F on the far right side (when their activities mostly cease).

A plant's growth is  obviously sequential: a seed germinates, then the first  cotyledons appear, followed by the first true leaves,  then more leaves, budding, flowering etc.  Each of these stages only develops after a certain amount of heat has been received by the plant.  By tracking how much daily warmth has accumulated after planting a crop and monitoring weather forecasts, a farmer can  fairly accurately predict how quickly  a crop is developing.  This is important not just for predicting when to reap the harvest, but also for  knowing when to fertilize or when not to apply an insecticide etc. In fact, for farmers, the monitoring of Growing Degree Days (DDs) to anticipate when pests are going to show up is probably more important than knowing when their crop is going to develop its 3rd set of real  leaves.

Wonkish Break #1:  The identification and study of the stages of  plant life are part of phenology, the study of cyclical plant and animal activity in response to  weather and climate.  Other examples include bird migrations, insect hatching and the appearance of certain diseases.  The phenological stages of many agricultural field crops (including cut flowers) are frequently identified using the BBCH-scale (Biologische Bundesansalt, Bundessotmenamt und CHemische Industrie). This scale was developed about 70 years ago by four Germain chemical companies to provide farmers with better guidance relating to the application of fertilizers and pesticides. It's not the only developmental  classification system, but it is evolving into the most widely used. The BBCH-scale  provides  a common way to refer to plant development for both research and crop management. 

 Sweet Corn.  Source: publicdomainimage.net

In addition to phenological scales for crops, many scales have also been developed for both   beneficial and pest insects,  the emergence of unwanted plants, i.e. weeds, and even plant diseases.  These data  can be quite detailed  as  a chart for a tomato available from Wikipedia shows, but farmers don't need to track  every stage. They  just need to determine the key developmental stages for their  crops or to know when their favorite pest will appear.

And regarding pests, Cornell's website, Network for Environment and Weather Applications,  offers data on pest forecasts, DDs in general, and crop management along with detailed  pest forecast models specifically  for   sweet corn, tomatoes, apples and grapes.  Another resource from the same website, and a favorite of mine, is the Soil Temperature Map which provides temperatures at a depth of 2" across New England and the Atlantic Region states.  This is useful if you want to gamble on getting a head start on the season and are looking for validation of your soil temperature readings.

Sweet Corn. Source: Ohio State U. Small Farms

The  chart to the right  from Oregon State University's Small Farms website  for  sweet corn  clearly shows how the the crop's development varied in  different growing seasons because  the DDs needed for each stage accrued at different rates.

The Oregon SU chart provides any farmer with information about when sweet corn will be at a given stage by tracking DDs, but farmers need to track Growing Degree Days for their current year in order to manage their crops effectively. They can do this either by tracking daily temperatures themselves or by obtaining data from other sources.  The challenge  is to get current local data because temperatures vary within a region or even from field to field. Some Cooperative Extensions, including Cornell,  attempt to do this by offering data from a network of  weather stations and letting growers choose which station's data to use.

Wonkish Break #2:   What is a Growing Degree Day (DD)? It's just a practical way to count the amount of daily solar heating without dealing with any actual physics.  There are at least six  formulas for calculating DDs. Each one attempts to accommodate temperature fluctuations within a given day. The most popular one, shown on the left,  is referred to as either  the "simple average" or "single sine" formula. The Max  and Min Temps for a day are added together, divided  by 2 and then the Base Temp (the minimum temperature needed for  plant growth) is subtracted.  The result is a number that represents  a day's heating factor.  By adding these daily heating factors together  and comparing it with the plant's  phenological profile  matched with DD data, growers can predict within a couple of days when a crop will reach its next stage.  

Cornell University's Climate Smart Farming website provides  New York farmers with local DD data. Among the online tools available from the website is a  Growing Degree Day Calculator. By locating your property on its "Change Location" map  and entering a crop's planting date, the calculator will show you how DDs have accrued since your planting date as well as with  a 6 day forecast.   Two other useful tools available from this website include a Water Deficit Calculator and a Cover Crop Tool for Vegetable Growers. Of a more speculative bent, you can also get a projection for the outlook of the  entire current growing season.

Managers of commercial greenhouses, where  growing environments are carefully regulated, actually can  target particular dates for budding, flowering, fruiting etc.  Farmers in the real world outside of the greenhouse, of course,   control  fewer  environmental factors; but it enables those growing large field crops, grapes or  tree fruits and nuts to keep track of crop development in the farthest back-forty. Until recently phenological charts for vegetables and accompanying  DD data have been hard to obtain. This is starting to change as the Oregon and Ohio Cooperative Extensions' websites indicate,  but persistent Internet digging  is required to find DD data for many vegetables.  Vegetable growers, it seems, need to be ready to develop their own phenological charts and match it to DDs in order to make more extensive use of this technique. [This can be fun for those of us with obsessions about vegetables.] A good place to locate at least BBCH-scales for some vegetables is  Oregon State's  Croptime - Growth Stage Guide.  Charts combining developmental stages and DDs focussed on pest emergence and diseases are  readily available.  

And remember...

"Curiosity is more important than knowledge." - Albert Einstein