Signs that spring—and planting season—is on the way

What are good indicators that it is time to plant crops in spring?

Forsythia bushes are often used as a phenological indicator that spring is here. Image courtesy of Eric Anderson, MSU Extension.

Forsythia bushes are often used as a phenological indicator that spring is here. Image courtesy of Eric Anderson, MSU Extension.

Ask a number of older farmers about what signs in nature tell them spring is here and it’s time to get out the planter and you will hear many different stories. “When the robins come back.” “My wife starts to get after me about getting the taxes done.” “I get stuck waiting for the ducks to cross the street.” “The groundhog saw his shadow…or he didn’t see his shadow, I forget which.” “The porta potties start to line up at the seed corn plant.” “Fertilizer prices go up.”

Of course, we all know day length and soil temperature play the most important roles in determining when it is time to plant crops. For corn, the minimum soil temperatures should be 50 degrees Fahrenheit to allow germination and emergence to be as uniform as possible. Corn will germinate at colder soil temperatures, albeit with inconsistent results, and soil temperatures between 60 and 70 F will usually result in quicker germination if soil moisture is not limiting. We use 50 F as a good “starting point” to ensure the most uniform stand as possible.

Soil temperatures for soybean germination are similar to those for corn. A key time window is the first 24 to 48 hours following planting when the seed gets its first “drink” of water—water temperatures are key at this time to avoid chilling imbibition, a well-researched topic (see “Soybeans and corn seed rot – seedling blight and damping off” by Michigan State University Extension; “Imbibitional Chilling and Variable Emergence” by Iowa State University Extension and Outreach; “Early-Planted Corn & Cold Weather” by Purdue University; and multiple other authors).

Forsythia bushes (Forsythia spp.) are often used as a phenological indicator, i.e., using the annual/seasonal cycle of plants or animals to tell you something else is soon to occur. Why is that? Is that “an old wives tale” or is there some truth to a correlation between forsythia bushes blooming and ideal planting conditions?

Forsythia are part of the Oleaceae family (olive trees) and are comprised of several species. It is a deciduous shrub that grows to a height of 3 to 9 feet and can grow as quickly as 2 feet per year. It grows well in USDA hardiness zones 5–8, which includes most of Lower Michigan and parts of the central Upper Peninsula. It is most widely known for the brilliant yellow blooms (see photo) in early spring before the leaves are produced.

Timing of forsythia bloom is strongly affected by soil temperature, but it is also influenced by variety, day length, air temperature and sun exposure. Researchers have attempted to time pre-emergence herbicide applications for crabgrass (Digitaria spp.) based on withering time of various flowering perennials, but results varied and ended up being only better than basing timing on “a routine calendar date or a best guess.”

Crabgrass can also be used as a good indicator of soil temperature as germination generally begins when soil is 57 to 64 F at 1-inch depth for one or two days. But who wants to watch for crabgrass to germinate just to gauge soil temperature? Better to buy a decent soil thermometer, or bookmark your local MSU Enviro-weather station and keep tabs on the 2-inch soil temperature.

For example, using the station data for Mendon in St. Joseph County, the maximum and minimum 2-inch soil temperatures on March 30, 2017, were 46.1 F and 43.6 F, respectively. On the same date in 2016, the maximum and minimum were 48.0 F and 43.7 F, respectively. So, even with the unusually warm February this year, soil temperatures are actually slightly cooler than last year. In 2016, soil temperatures (maximum and minimum) at the 2-inch depth remained above 50 F for three consecutive days by April 20. If we assume a spring warming trend similar to that in 2016, we can expect favorable soil temperatures for planting in southwest Michigan towards the end of April in 2017.

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