Many things affect the plants in an Oklahoma garden. Our Oklahoma climate is one that is hard to understand especially if the gardener is from another area, state or even country. They don’t understand that our temperatures can go from extremes in a matter of hours — too hot to too cold.
While it is true that most places will support some type of plant life, it isn’t true that every plant will grow in your landscape or garden. Plants are a lot like humans and even animals, they have particular environmental requirements to survive. They sometimes can adapt, but not too often.
Temperature is one of the most important aspects of the environment and should be the starting point for selecting plants. Since temperature is an element that normally the gardeners cannot control, they must choose the plants that can survive the temperatures they have in that location.
Oklahoma is a state with varying weather conditions. The gardener can add two weeks differences in temperature as you move from the southern one-half of Oklahoma to the northern one-half. That is only a generality, but it makes it important that when the gardeners are studying temperatures for planting that they know where the source of information comes from. Books and many magazines can be misleading because many times they come from sources from the Northern part of the county.
Remember, too, that temperatures can vary widely even within the same landscape. Plants in sheltered locations can survive extreme freezes while plants exposed to the cold winter winds will freeze quicker. On the other hand, plants on the western and southern side of a home or building in the summer will have to tolerate higher temperatures than if they are on the northern or southern sides.
Every plant that is grown in this area has three critical temperatures: Minimum, maximum and optimum. These temperatures or temperature ranges will be critical in determining the success or failure of specific plants in the landscape.
All plants have a certain temperature below which they cannot survive, sometimes called “ minimum temperature range.” Bougainvillea, a tropical vining shrub that comes in bright colors, freeze at 32 degrees, while junipers shrubs can stand temperatures below zero. Minimum temperatures, then, is one of the major criteria in selecting a plant for a certain locale.
Few plants have a maximum temperature above which they cannot survive. The problem is not just the high temperature but the prolonged extreme heat, when the plant must struggle just to stay alive. Many times, stored food in the plant is consumed faster than the plant can manufacture more. Eventually the plant runs out of stored food and dies.
Every plant has a certain temperature range in which it grows best. Mountainous plants may thrive between 30 degrees and 60 degrees while plants at seashore tropics prefer 60 degrees to 90 degrees. Move a plant into the wrong temperature range and plants will usually just sit there and eventually die.
There are two terms that relate to temperature that are used in the landscape retail business: hardy and hardened.
Hardy refers to a temperature at which a gardener can reasonably expect a plant to survive. This is the low temperature at which a plant, when properly conditioned, can be expected to survive. They are indicated by the USDA Plant Hardiness Zone Map (Oklahoma is Zone 7).
If a plant is gradually conditioned to colder and colder temperatures, it becomes “hardened.” It is far less damaging to a plant to be exposed over a period of time to temperatures of 40 degrees, then 35 degrees, then 15 degrees than it would be to go directly to 15 degrees. Going the other way, similar progressive hardening also helps plants survive extreme heat.
Jim Coe lives in Lawton and writes a weekly garden column for The Lawton Constitution.