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Growing Season: How To Know When To Plant

March 3, 2011

By WeatherBug Meteorologist, Andrew Rosenthal

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Spring is in the air! The snow is melting and the birds are flocking back to their summer homes. That also means that it`s just about time to start preparing the garden for summer. Impatience sets in, but if you plant too early, a frost may kill your garden before it can really get started. The growing season is the boss of all things gardening.

Technically, the "growing season" is the period between the last frost of the winter and the first frost of the fall. Numerous websites reference the average last frost for a given location, such as the link to the right. However, this should be taken with caution, as last-frosts can occur well after this date, with a light frost arriving in April or even May for many locations in the U.S. northern tier. Many plants can also survive a light spring frost, meaning they can be placed in the ground before the last frost.

If you don`t want to wait for the last frost, a better guide to determining when it`s safe to plant is to use an average crop-date formula. Certain crops can handle colder air, with the coldest-weather crops ready to go into the ground as soon as the ground is soft and pliable; others need to wait until after the temperature won`t drop below 32 or 36 degrees. Using a chart such as that produced by the National Climatic Data Center, match the temperature needed for your choice of crop against your city`s last date. Remember, like the last-frost, this is just an average date.

Once the crops are in the ground, it`s a matter of clock-watching to find out when the plants will be mature and ready for harvest. Since the weather plays a role in how quickly these crops will mature, a formula, known as Growing Degree Days, has been devised. It is determined by taking the average temperature (the midpoint between high and the low temperature) and subtracting a "base temperature." This base is typically 50 F, but can be slightly lower in sunnier climates. Add up each day`s Growing Degree Day index to get a running total. When this total passes a crop`s benchmark, they have matured. For example, corn can be expected to mature at 2,700 Degree Days.

The season winds down as the colder weather approaches in the autumn. However, the season can be extended past the first freezing night. Temperatures around 32 degrees will produce some frost, although a plastic blanket can extend the growing season later into the autumn. When temperatures drop below 28 degrees, the growing season is considered at its end. Temperatures this low can cause damage to leaves on many plants, and plants will shut down growth to prevent this damage. Overnight temperatures dropping to 24 degrees is the absolute low point for plants. Below these temperatures, the ground will freeze and damage the crop`s roots and possibly even kill a plant.

How do you know what to put into the ground so that you can get more than one year of harvest out of it? The U.S. Department of Agriculture has devised a formula called Plant Hardiness to determine what can survive in your area. Every location in the U.S. has been assigned a number, based on the average lowest temperature seen in the winter, ranging from the coldest at scale 0 to warmest at scale 11. Meanwhile, every plant or seed sold is given a Hardiness Number, with zero being the most resilient and 11 as the least hardy. According to the Department of Agriculture, you can expect any plant with a Hardiness Number equal or lower to your location`s value to survive and thrive in your climate.

Whatever you choose to plant this year, consult growing season guides for advice on when to plant. By following these guides, you`re sure to have a bountiful garden this summer. Be sure to check your WeatherBug for more information on gardening, and keep it active to receive the latest weather in your neighborhood. Get the latest updates anywhere on Twitter at WeatherBug WeatherBuzz.

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Story Image: Take care of your garden and abide by the growing season and it might be as beautiful as WeatherBug User Coleen Grisham`s.

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