The Importance of Average Frost Dates

Average frost dates are one of the most important dates every gardener should know. They are the key to successful planting, particularly in spring, but are also useful in the fall. You can see their importance in the simple instructions on almost every seed packets, plant tags and website: Plant in spring after danger of frost has passed.  Or, plant six weeks before the first frost in fall.  They are also important when it comes to timing various weed control measures, pest controls and pruning. But what are average frost dates?

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Everyone knows that the weather is unpredictable. That’s why gardeners use average frost dates based on historical weather data.  Planting instructions often refer to the average date of the last frost in spring, which is the date on which, in half of previous years, the last frost had already occurred. This means that 50 percent of the time, if you plant after this date, there won't be any more frosts and frost-sensitive plants should be safe outdoors. But it also means that half the time, there could be a frost, possibly damaging tender plants. That means you need to watch the weather and be prepared to provide protection.

The average date of the first frost in fall is also important. It is that date on which, in half of previous years, the first frost has occurred in autumn. It helps you time fall plantings, gives you the best idea of when you bring your tender houseplants indoors and lets you know when the harvest of tender vegetables is about over. But keep in mind, it is only an average. Half the time, a frost is likely to occur before that date and you have to be ready.

For more information, see Protecting Plants From Cold.

Different plants vary in hardiness and ability to withstand frost and cold temperatures. For example, it's best not to sow seeds or plant seedlings of tender, heat-loving plants, such as Tomatoes, Cosmos or Basil, before the average date of the last spring frost unless you are ready to provide protection. In fact, these crops are often planted later, after the soil has warmed.  Otherwise, they grow very slowly. Some plants like cool-season annuals and vegetables, such as Pansy, Stock, Snapdragon, Broccoli or Peas, can withstand frost. They can be planted as much as 6 weeks before the average date of the last frost in spring, often as soon as the soil can be worked. Many of these hardier plants are also planted in late summer to fall for fall harvest based on the average date of the first frost in fall.  

Many perennials, trees and shrubs are also planted according to average dates. Other hardier species, can be planted almost anytime the ground can be worked, but fall or spring is usually preferred.

Check with your Local Cooperative Extension System office or garden center to determine ideal planting times.

Finding Average Frost Dates for Your Area

Microclimates Can be Exceptions to the Rule

Microclimates are small areas that differ slightly from the overall climate of an area, usually due to proximity to buildings or topography. They can be warmer or colder that the surrounding area and can present planting opportunities, or restrict what you can plant and when. What creates a microclimate?

  • A windbreak that disrupts breezes, such as a building, fence or hedge
  • Elevation changes: Cold air rolls quickly downhill and settles in low spots like water. Plants on the side of a hill experience warmer temperatures than plants at the base.
  • Southern and western exposures are warmer than eastern or northern exposures. The north-facing side of your home is usually the coolest
  • Structures, concrete or pavement that absorb the sun's heat during the day and radiate it at night create warm microclimates. Overhangs or eaves can also be warmer at night as they heat underneath
  • Large ornamental stones or rock mulch trap heat during the day, releasing it a night
  • Bodies of water, such as streams, lakes, ponds and ornamental pools, provide cooling in warm weather and warming in cool weather

Experienced gardeners know that knowing the microclimates around you home can be very useful.  You can learn a lot about them by simply walking around your yard during different times of the day and in different seasons.  Use what you’ve learned to position your vegetable garden, tender plants and even outdoor living areas, such as patios and decks. Microclimates are key to getting extended harvests, providing frost protection and making your time outdoors more comfortable.

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