Sowing peas is an early spring ritual filled with exuberance for the new gardening season. In mid-April, I heard excitement in the voices of gardening friends, “Got my peas in this weekend!” By the end of the third weekend in April with still no peas in the ground, I started to feel hassled.
Finally, early in the morning on Saturday, April 25, I poured pea seeds into a bowl of water to soak for a few hours, then carried digging fork and rake to the designated garden bed. By the end of the day, my peas were in the ground. A new gardening season was under way!
My exuberance was premature. By May 9, two weeks after sowing, my peas were barely pushing green shoots out of the soil. As I write this, on May 19, the tallest are only 2 inches above the soil. Peas planted this weekend would catch up with mine in two weeks!
Last weekend Marjorie and I planted sweet peas, lettuce and carrots, all considered early spring crops. You could sow these crops this weekend as well, there is still time.
Meanwhile, forsythias that exploded into flower at the first hint of spring have dropped their wilted petals, while the fringe tree (Chionanthus virginicus) remains in tight bud, waiting for — what?
Gardeners learn the sequence of events in the garden, but the exact date each year that such events will occur cannot be predicted by the calendar. Plant development is largely controlled by the number of accumulated growing degree days.
You can track growing degree days, or GDDs, in the garden and, over time, accurately predict developmental changes in garden plants. All you need is a garden thermometer that measures the daily high and low temperatures, or you can use the temperatures reported in the newspaper or online.
To calculate GDDs for any day, begin by averaging the high and low temperatures for the day. A low temperature below 50 degrees F is recorded as 50; a high temperature greater than 86 degrees F is recorded as 86. (The assumption here is that 50 degrees is too cold and 86 degrees too hot for most plants to grow.)
From the average of high and low temperatures, you subtract 50 and the result is the number of GDDs for that day. If the average is less than 50, there are no GDDs for that day — plants cannot reverse developmental changes.
As an example, consider an early April day in which the high temperature was 54 and the low 40. Setting the low to 50, the average would be 52. Subtracting 50 gives a total of 2 GDDs for the day.
Emergence and development of insects that feed on plants are also controlled by accumulating GDDs. Thus the hatching of viburnum leaf beetle eggs is synchronized with viburnum leaf development.
By tracking GDDs in your garden, you will develop a keen sense of when to expect certain plants to break dormancy and when they should be flowering and fruiting. At the same time, you can predict when certain insects, friend and foe, should be around. Begin monitoring for specific pests (and the effectiveness of their predators) when the accumulated GDDs tell you they should be active.
So far, growing degree days have accumulated slowly, giving gardeners a little more time to get those peas and other cool-season crops in the ground. Of course, soon we will see daytime highs in the 80s, lows around 60, and we will be accumulating GDDs by leaps and bounds. By the first of June, you won’t be able to distinguish the peas sown in late April from those sown in the middle of May.
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