Spring is coming, just not yet

Published 2:35 pm Wednesday, February 28, 2024

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I had an interesting call earlier this week, one that made me stop and think. At NC State Cooperative Extension, we are supposed to give research-based, unbiased information that comes from land grant universities. The caller asked, “In your opinion, how early is spring this year?” That is a question I had to back up and think about before I could give a reasonable answer.

My initial thought was that spring is a little late even though the groundhog said it would be early. We have a few indicators that we go by to judge but that can be shaky ground. I thought about when I have typically seen crabgrass emergence over the past few years and whether I had seen crabgrass so far this season (I have not, but I haven’t really looked either). Typically, the ornamental cherries (Prunus sp.), tulip magnolias (Magnolia x soulangeana), Bradford pears (Pyrus calleryana ‘Bradford’), redbuds (Cercis canadensis), and red maples (Acer rubrum) are among the earliest tree species we see.

Then I thought, what causes the trees and other plants to bud out? Have I seen many trees in flower? I certainly haven’t seen any of those stinky Bradford pears yet but I have seen some tulip magnolias, cherries, redbuds, and red maples that are in full bloom. The question remains, what is the proper measure of how far along spring is.

In the case of crabgrass, the indicator is when we have had 3-5 consecutive days of 55°F or above soil temperatures. Usually, we will see emergence soon after. The soil temperature triggers the germination of the seeds. Trees are a little more difficult. Most deciduous trees have a chilling and light requirement before they bloom or send out that first flush of leaves. In our area, when trees break dormancy is driven by the amount of daylight they are receiving. As the days get longer, the trees begin to sense that it is time to come out and play. However, they still need a certain number of chilling hours before they will break dormancy. It is a fairly complex process to count chilling hours so rather than do it myself, I use the NC State Climate Office’s model.

With this model, you select a base date and then it will calculate the number of hours we have accrued since that date. For instance, I like to use their peach model. The best hours are accrued between 36°F and 47°F. If we fall below 34°F there are no hours given and if we go above 54°F the hours are taken away. I feel this is a really good scientific basis for how far along spring is for any given year. Through yesterday, we have accrued 702 chilling hours under the Richardson Model. NC State University research suggests that we do not use any fruit trees under 750 chilling hours to keep them from freezing. It looks like we are in good shape so far this season.

Lastly, for a scientific basis, we can look at Growing Degree Days (GDD). This is a measure of heat units accrued above a certain specified base temperature and date. GDD is something that we use to predict when certain insect pests will emerge. GDD can also be used to assess how far along we are and then predict how long the pine pollen season will last. Pine pollen begins around 300 GDD with a base date of 2/1 and a base temperature of 55°F. So far under these parameters, we have accrued only 20 hours! The 10-year average for February 27 is 46 GDD. In the chart, you can see where we are with a base date of January 1st and a base temperature of 50°F. This is a good measure of where we are regarding spring. Currently, we are at 110 GDD. This time last year we were already at 221 GDD and in 2021 we were only at 34 GDD. Looking at the 10-year average of 136 GDD, we are just a tad behind but not far behind given Wednesday’s high temperature of 76°F. So, even though Punxsutawney Phil did not see his shadow this year, we are still a touch behind.

If you are having trouble with growing in your home landscape, call the Extension office at (252)946-0111 or email Gene Fox at gene_fox@ncsu.edu. Until then, Happy Gardening!