March is a strange time in the garden, with half of the days feeling like spring and half like winter. The 70 degree days we’ve been teased with, naturally make me want to write about the most exciting part of spring: flowers! We have yellow, orange, and red Witch Hazel adding some much needed color to the garden, and some of our Snowdrops and Croci are blooming.
But at Wellfield, the flowers are used for more than just a spring indicator; they are also a pest indicator. We start looking for pests around this time of year and use the flowers as indicators of natural processes, so we can stay a step ahead of the insects and weeds.
We are able to track where we are in these natural processes by using Growing Degree Days (GDD). To simplify GDD, it basically tracks how long the average temperature has stayed above 50°F and how far over that temperature it goes. Sometimes a different temperature is used based on the application. It is possible to track the flowering time of plants and use them as indicators of what pests are about to emerge. For example, we use cut pine logs to attract pine weevils in the spring. They emerge after the forsythia in the garden begin to bloom. The forsythia is used as an indicator that we need to have the cut pine logs placed by this date at the latest. The timing of the forsythia flowers can also be used to judge the best time to apply pre-emergent (although we mostly prefer old fashioned weeding here at Wellfield). Another specific indicator we use is for magnolia scale. We spray an oil on the scale to control it when the redbuds are in full bloom. There are a lot of pre-made lists online that give a chronological order of what the GDD indicates.
There are many connections between flower times and pest emergence that can be used and there are lists online that give different examples of plants and pests. The one I linked was to a list by MSU. MSU also has a tracker for GDD that will help determine when to be on the lookout for pests and when to spray pre-emergent. It is also much easier to use an online tracker than it is to check the temperature each day and add it up throughout the season – who has the time? We have gardening to do! GDD is a useful tool, and if this is a newer concept for you, I suggest trying it out in your own garden.
Cody Hoff, Lead Horticulturist