Temperature plays a key role in plant development, and understanding what growing-degree days are will help you give your plants the best possible start.
Dr. Ed Brotak is a retired meteorologist who applies understanding weather to growing a better garden. In the most basic terms, if the weather is too hot or too cold, plants will die. In between the critical high or low lies an ideal range of temperature for maximum growth. Of course, this varies by plant type.
We know how temperature affects plants, but what things affect temperature? Temperatures, especially at the ground’s surface, are a function of radiation, a means of transporting energy or heat. Radiation brings the energy from the sun to the Earth, heating its surface during the day. At night, the surface radiates heat back out to the sky, making temperatures fall. Because of the curvature of the planet, the sun’s rays are weaker at higher latitudes, making the climates there colder. Temperatures are also affected by elevation. On average, the temperature drops 3.5˚F per thousand feet.
What's More Important: Soil or Air Temperature?
When planting seeds in the spring, the major concern is not the air temperature, but rather the soil temperature. To germinate, seeds need not just enough moisture in the soil, but also enough warmth. The soil temperature will determine if a seed can germinate and how long that will take, and this will vary by plant type. Soil type also plays a role in determining planting dates. Well-drained, sandy soils heat up faster. Heavy clay soils heat up slower, especially if they are wet.
Once the young plant stem emerges through the soil surface, air temperature becomes the crucial factor determining growth—or damage. Even if the soil temp sufficed for germination, the threat of a killing frost may still exist. Although soil temperatures tend to increase steadily in the spring, air temperatures are much more irregular.
The young plant has both a lower and upper temperature limit for promoting growth, and between those two extremes there will be an optimum temperature for development. For example, wheat is a very tolerant crop. It will develop if the temperature is above freezing, will thrive with temperatures in the mid 80s and it will not shut down until the temperature exceeds 100˚F. On the other hand, a cool-weather crop like potatoes does best near 60˚F and won’t grow if the temperature exceeds the low 70s. The development of a plant depends on the accumulated effects of continued warm weather.
As a general rule, plant development will be delayed four days in the spring for every degree of latitude north you head. For every 100 feet of elevation gained, plant growth is delayed by one day in the spring.
You can manipulate surface temps by altering the radiation budget. By simply shading a crop, you will lower the temperature during the day; in this way, row-cover fabrics can stop cool-weather crops like lettuce from growing too fast in a warm spring and becoming bitter. Fabric can also maintain heat at night. Sometimes after days or even weeks of growth-promoting heat, temperatures can plummet one night. Watch the forecast for frost warnings and cover delicate plants.
Early season vegetables like lettuce can be planted with soil temperatures of 35˚F. Cucumber seeds, on the other hand, won’t germinate until the soil is 60˚F. Most seed packs will recommend early spring or fall planting for cool-weather plants or planting after the danger of frost has passed for warmer-weather species.
Many seed packages list the number of days after planting that a particular vegetable will be ready for harvesting. This is just an estimate. Growth rate will depend on the actual temperatures in your garden and can be better tracked by growing-degree days (see box).
Information on soil temperatures and growing-degree days is usually available through local National Weather Service offices and agricultural extension services. To get started, see weather.gov.
What Does Growing-Degree Days Mean
There is a way to quantify the effects of temperature on plants. It uses what are called growing-degree days (GDDs); one 24-hour period can count as any number of GDDs. To calculate the amount of GDDs produced on a given day, subtract the critical temperature (the temp below which the species in question will not grow) from the day’s mean temperature (its maximum temp plus minimum temp, divided by two).
GDDs accumulate over a season, with specific amounts relating to developmental stages in a plant’s life. GDD information can be used to determine which plants or crops are suitable for various climate types, to determine planting dates for minimal frost danger and to predict developmental stages, such as ripening.
Take corn as an example. Corn’s critical temperature is 50˚F. If the high temperature on the given day is 76˚ and the low is 52˚, the mean temperature would be 64˚. Subtracting corn’s critical temp (50) from 64 leaves 14. That day contributes 14 GGDs to the tally. To put things in perspective, corn should be planted when 400 GDDs have accumulated. Early silking will occur with 1,800 GDDs and full maturity, with 3,250 GDDs.
Dr. Ed Brotak is a retired meteorologist and a regular contributor to Horticulture.
This excerpt originally appeared in Horticulture April 2011 issue. Back issues are available at GardenersHub.com. Join thousands of gardeners just like yourself and get smart gardening advice from experts delivered right to your home—Subscribe to Horticulture.