Inula Royleana
Midwest Gardening
Frost Dates

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The Truth About Average Last Frost Dates

New gardeners may look for average last frost dates for their region to determine when to start planting.  But even a new gardener may quickly realize the date they find just does not seem right, depending on the source of the information.  You may even find conflicting dates from different sources.

USDA plant hardiness zones

Gardeners look to last frost dates in spring so they know when it is safe to plant.  They look to first frost dates in autumn to guide their selection of vegetable plants based on days to mature and to be prepared to protect plants from early frost.  But the information related to average frost dates can be confusing and contradictory.  You need to understand the source of the information and on what data that source has based frost date information.

Much of the average last frost date information is issued by USDA Plant Hardiness Zones.  These zones provide geographically mapped standards on which plant growers and nurseries can rate a plants likelihood to thrive.  General frost date information is often made available according to those same geographical zones.  But experienced gardeners know that this is only the beginning of what you need to know to determine if a plant will thrive in your backyard and when to expect the last and first frosts.  Large regional influences as well as local micro-climates will affect both the plants ability to thrive as well as temperature ranges and frost dates.

Frost date information published by various sources is influenced by a number of factors.  What you need to pay attention to is the source, and what may have influenced the date they selected.  Then you can determine your comfort level with the date based on your experience or lack of experience.  With experience comes the knowledge of micro-climates that exist within your personal property and gardens and how that will influence the frost danger.

  • The best source of good data is probably the least used tool, but it will also require just a little interpretation based on knowledge of the probability ratios.  The National Climatic Data Center publishes frost and freeze dates based on probabilities and statistics.  The information is available by state, so regional influences are largely accounted for in the probability data.  The data is further published by major city areas, so smaller micro-climates are accounted for.  The data is presented at temperature levels of 36 degrees for potential light frost, 32 degrees for heavy frost or freeze, and 28 degrees for hard freeze.  Note that temperatures are taken several feet above the ground level, but the coldest air settles to the ground.  The number of freeze free days is also in the table, excellent for comparing maturity days for vegetable and fruit selections.  The chart then displays the date for a probability of 90, 50 and 10.  The problem is sometimes extrapolating the data for your specific city or suburb.  In my case, I need to look at a major metropolitan area that is influenced by heat island effects and at a quite rural area that will be influenced by open fields subject to cold winds.  My specific suburb will fall somewhere in between.
  • Growers, garden centers and nurseries also display or publish average last frost dates.  Typically they display the date to protect their reputation.  The date they select is typically a very safe date to plant based on perhaps the 10 percent probability level noted above.  It is very unlikely that your plants will suffer from frost damage or death based on the probabilities.  This results in happy customers that wont blame the nursery for selling them a plant that died a week after it was planted.  I have found that garden centers in my area tend to go with a date that falls just after the 50% probability level.
  • Then there are the general charts that you can find on websites all over the internet, including my garden calendars.  Unless you are looking at a local or regional website, when addressing frost dates by zone there is a LOT you need to know.  First, the date is a simple AVERAGE, so your specific area could be much a much later date or a much earlier date.  Next, USDA growing zones stretch from coast to coast.  There is no accounting for regional influences such as inland, coastal, mountains, plains, arid, humid, etc.  The growing zone can also stretch a great distance from north to south.  So not only are regional influences not accounted for, but the simple difference of temperature from often several hundred miles north and south are not accounted for.  You could literally live a few feet from the demarcation line of the next colder zone.  And of course someone else lives right at the line of the next warmer zone too.  So the temperature difference from one end to the other can be an entire “zone’s worth”.  And finally, of course, it is not possible for an average general zone date to account for micro-climates in your immediate region, city, neighborhood and back yard.

Hopefully this information will help your to determine your comfort level with the frost dates you see published.  In time, you will learn the signals that nature gives to determine your planting dates.  And of course, some plants are more tolerant of frost than others.  That knowledge too will come with research and experience.

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