Ask an Expert: Spring Has Sprung, But Frost Still Likely

By: Taun Beddes, Utah State University Extension horticulturist, 801-851-8460, taun.beddes@usu.edu
 
Determining when to plant a garden can be especially confusing in Utah’s unpredictable, varied climate where last-frost dates can vary by many days within just a few miles. Many experienced gardeners have planted and later lost their plants to frost.
 
An example of how fickle Utah’s climate can be is in Cache Valley. Frost-free days vary from an average of 113 days in Lewiston and Trenton to 158 days on the USU campus. Similar examples are common around the state.
 
Geographic characteristics of where you live can help in determining when to plant. When a yard is located in a populated area or on a mountain bench, it usually has a longer growing season. Other areas located at slightly lower elevations where cold air drains and cannot escape have a shorter season. This is why local commercial orchards are generally located on benches. Additionally, urban and suburban areas are slightly warmer than surrounding areas due to the urban heat effect. Heat from buildings and warmth generated by sunlight reflected from roads and other surfaces increases temperatures and delays frost. It can be helpful to chat with a local farmer or experienced gardener in your area to determine what works for him or her regarding when to plant.
 
In addition to frost information, it is important to take into account the needs of the plants. Vegetables planted locally fall into four basic categories: hardy, semi-hardy, tender and very tender. Depending on which category a plant belongs to, planting dates vary from early spring until early summer. Consider the following:
 
Hardy vegetables, including asparagus, broccoli, cabbage, onions, peas and spinach, can be planted as soon as the soil is workable in early spring. This usually ranges between 45 and 60 days before the average last frost. These same vegetables can be safely planted until the average last frost date. 

Semi-hardy plants, such as beets, carrots, lettuce and potatoes, can be planted one to two weeks after the hardy group. These can be planted until the average last-frost date. 
Tender vegetables, such as celery, cucumbers, corn and most beans, should be planted on the average last-frost date.  

Very tender plants, such as squash, beans, melons, tomatoes, eggplants and peppers, should not be planted until at least a week after the average last frost. Even if frost does not occur before this time, these plants will not grow well and are more susceptible to disease until warmer weather. 
 
If you have lost plants to frost, you are not alone, and all you can do is try again. 
 
Average Frost Dates for Various Utah Locations. Note that these dates are averages and can vary from year to year.
     
                               Frost Dates     
City                           Last                         First                     Frost-Free Days
Alpine                       May 20               September 30              136
Blanding                   May 13               October 12                  153
Cedar City                May 10               October 5                    148
Delta                        May 17                September 28            134
Farmington              May 5                  October 10                 158
Fillmore                   May 16                October 4                   140
Huntsville                June 11               September 9               89
Kanab                     May 7                  October 20                 166
Lake Town              June 15               September 10             87
Logan                      May 14               September 25             135
Morgan                   June 6                 September 11             98
Moroni                    June 1                 September 18             109
Ogden                    May 1                  October 24                  176
Park City               June 9                  September 1                92
Price                     May 12                 October 7                     148
Roosevelt             May 18                 September 25              130
Spanish Fork        May 1                   October 13                  165
St. George            April 6                  October 28                  205
Tooele                  May 7                   October 14                  159
Tremonton           May 3                   October 10                  160
 
For information on areas not listed, contact your local county Extension office.