Degree Days: Anticipating Your Growing Season

Many of those in a field related to horticulture pay close attention to degree days. These degree days are important because they allow us to anticipate blooming of certain plants, what pests to expect, and can anticipate when a plant will reach maturity.  Degree days use temperature to calculate these estimations and are highly accurate, unless effected by other causes such as drought because water provides turgor pressure and is needed for growth, with a lack of water in the environment, flora will have difficulty growing to full potential. As a landscape designer, I use degree days to plan flowering times within the plants I choose. This allows me to pick flowers that I can time blooms for all season or to have certain colorways at certain heights. As a landscape consultant, I use degree days to determine what plants are in bloom (this can tell you the difference in similar looking plants for identification) or to help determine what is currently crawling around that could be destroying your blooms.

Degree days can tell me when the seed should germinate, the cotyledon’s sprout and when the fruits shall arrive, plus tell me senescence (death) of desired plant.

How degree days are calculated may seem a bit tricky, but I guarantee it is easier than it looks. In equation form Growing Degree Days are calculated as; GDD = (Tmax + Tmin) / 2 – Tbase .  Temperature is represented as “T”. First thing is to take an average of the days temperature, add the high with the low and divide by two. Your “Tbase” temperature is generally set, following the averages of previous years or what your desired degree stage is considering your plant or insect you inquire about. To have accuracy you have to do this as soon as the season starts and follow through daily for the entire year. Thankfully, there are many websites already tracking and providing the math for us, so as long as you follow them you should be good. Try this site as it breaks it down simply, The main thing is staying up to date with the degree days as every plant and insect has a specific day of sprouting, for its specific region. Red Maple up in the northeast will have different degree days than one in the southeast due to differences in climate.

If you as a gardener have ever wondered what pest was eating your plant at a certain time every year, degree days can help. Insects will go through a process of “over wintering” during the colder months because they are not like most animals who can generate their own heat. Insects are ectothermic, meaning they have to rely on the outside temperature to control bodily functions and metabolism. Those that stick around in the winter find places to hide (generally within the soil) and will pop their heads out when it is warm enough again for them, much like thawing out that protein for your dinner table. Keep in mind, that this isn’t just for pest insects but for beneficial insects as well such as lady bugs, assassin bugs and mantids. The benefit of knowing these degree days for your insects is that this will allow you to keep up to date for treatments. If you noticed a ton of beneficial insects emerging then they could be all you need for a biological reduction of pest insects. Likewise, if you saw a ton of pest insects emerging but haven’t seen as many beneficial, then a bigger problem can be occurring in the environment that you will be able to stay ahead of.

Now that you have an understanding of what a degree day is and how to calculate where you are in the season, lets see if we can improve the green sanctuary around you. Remember you do not have to do the math on your own, look to local agriculture websites and Co-ops as they will have done the  math for us already!

Happy Gardening!


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