Warm La Nina effect weather along the Front Range this year has many people anxious to plant. In this post I want to discuss the common wisdom of planting by holidays and the science behind it. As always, individual decisions on planting date are based on risk tolerance. In some years early planting may prove a good bet and in others, an impatient action.
The holiday rule of thumb is to plant peas and lettuce on St. Patrick’s Day and peppers and tomatoes on Memorial Day along the Front Range.
The science behind this is that cool season vegetables are planted when soil temperatures are sufficiently warm for seed germination. These vegetables are able to withstand cold air temperatures. Warm season vegetables require warmer soil temperatures for seed germination and root growth, and warm, stable air temperatures for plant tops that are generally intolerant of freezing air temperatures. Many but not all warm season vegetables are planted as transplants and not direct seeded.
Soil temperatures for vegetable seed germination*
Cool season vegetables –
35 degrees - lettuce and onions
40 degrees – peas, radishes, spinach, cabbage
Warm season vegetables –
50 degrees – tomato, peppers, corn
55 degrees – beans
60 degrees – cucumbers, squash, eggplant
* soil temperatures measured with a soil thermometer at 4 inches at 8:00 a.m.
How does the planting by holidays prescription stack up with the science? It turns out to be a relatively safe guide when used in conjunction with observable weather trends.
Dates for soil warming for years 2005 to 2008*
40 degrees - Mar 28, Mar 28 and Mar 4, Mar 24
50 degrees – May 5, May 6, May 8, Apr 28
60 degrees – Jun 11, May 13, Jun 10, Jun 14
* recorded at the Lory Student Center weather station on the Colorado State University campus in Fort Collins (monitored by the Department of Atmospheric Sciences)
For more information on vegetables and their temperature tolerances, see CSU Extension Garden Notes Planting Guide #720.