When to plant and why hoop houses might not work well in the winter.

Spring has sprung or is about to spring in some parts of the U.S. and some are wondering about what is the best date to plant plants. Often time seed packets have suggested months and dates or certain vague timing related to frost averages.  The true determining factor is soil temperature. All plants have temperature parameters in which they thrive. 50 degrees F. is a basic benchmark for plant growth. Below 50 degrees and above 32 degrees many plants can survive but will make very little active growth.  To bring a little science into planting equations and less guess work, one needs to buy an inexpensive soil thermometer. You might have a meat thermometer with a probe in the kitchen already. Begin to monitor what is happening with your soil. As they begin to approach 50 your getting close. On the internet you can observe ten day weather forecasts. Begin to combine the highs and lows. When the soil temp hits 50 and the ten day forecast averages 50, put your plants in the ground. If the ten day forecast is "good" except it might show one night with a hard frost, watch out. Some plant varieties require 55 and some 60 degrees before they should be planted. Lima beans need 65 degrees to germinate. Where I live at this moment soil temperature is 26 degrees. Do a little research regarding the various varieties. This concept affects hoop houses as well. Even with auxiliary heat,  surrounding soil temperatures outside can be 20-30 degrees. This cold soil mass surrounding the hoop house will migrate, perhaps three feet into the hoop house rendering the perimeter area unproductive. It probably is recommendable to dig a 2 to 3 foot trench on all 4 sides and put in vertically panels of insulation foam 2-3 inches thick around semi permanent structures to eliminate migration.