The introduction to the book tells about gardening in Oklahoma, including a short discussion on plant hardiness and ecosystems made up of similar climates and geographies. That discussion got me to thinking and researching a little bit more. Most of this post is about that subject.
The majority of the Oklahoma Gardener's Guide consists of plant profiles. The profiles include information on how to grow these plants in Oklahoma, as well as different named varieties that are available and which are best choices for our location. The profiles are divided up into several different categories:
- Ground Covers
- Ornamental Grasses
- Great Plains Plants
A large appendix includes maps and information from the following topics:
- Planning and Starting a Lawn
- United States Ecoregions Map
- Oklahoma Frost-Free Map
- Oklahoma Freeze Map
- Oklahoma Precipitation Map
- Oklahoma Northern Counties Map
- Perennial Ground Covers as Lawn Alternatives
- Beneficial Insects
- Other Plants for Oklahoma
- Natives as Ornamentals
- Oklahoma Public Gardens
- Reference Publications of Interest
- Your Garden Plans
Discussion of Plant Hardiness
In addition to the USDA Plant Hardiness Zones map, Steve Dobbs includes a map of Ecosystems in the continental United States. There are six ecosystems identified in the United States: Pacific West, Mountain West, Desert Southwest, Great Plains, Continental East and Humid South. The last four of those ecosystems intersect in the center of Oklahoma. The center of Oklahoma is a sort of "four corners," dividing the state into quarters. Lucky for me, I live right in the center of the state. This means that gardening outdoors can be a little tricky where I live. The map below shows a much more detailed view of ecosystems in the US.
[caption id="" align="aligncenter" width="355" caption="USDA Ecological Subregions map. This is a more detailed map than the one shown in the book, but still demonstrates the diversity of Oklahoma's ecoregions."][/caption]
Oklahoma is known for its extreme weather, usually in the form of severe storms and tornadoes. But Oklahoma made national news at the end of 2007 when we were hit by a horrible ice storm that tore down many trees and left others severely damaged. It is also not unusual to have fairly long droughts during the mid to late Summer, when much of the state is behaving more like the Desert Southwest, rather than the Humid South. Temperatures in my home town range from 10 F to 105 F in an average year.
The USDA plant hardiness zone map was updated in 1990, dividing many of the zones in two (an "a" and a "b"). Those zones have been consistently used by growers to indicate the cold hardiness of plants for several decades now. However, the map indicates only the lowest temperature that could be expected during the course of the year. Steve Dobbs writes that a single indicator is not enough information to convey whether a plant will survive in your yard or not. Just as you need to take into account how much to water a plant and what sun exposure it needs, you also have to consider the relative humidity required by the plant and the heat that it can endure. This is often described by saying that a plant is drought hardy, but being able to withstand a week without rain is different than withstanding a month's worth of high temperatures above 95 degrees Fahrenheit. This happens in July and August in Oklahoma.
As it is now, plants that are hardy in my zone (zone 7) are suggested for here as well as Delaware and New York City. Clearly these are very different climates and the same plant would require different care in these two locations, assuming the plant could live in both places.
There is some discussion about the USDA creating a heat hardiness map that would indicate the maximum temperature that can be expected in a year. This map would show most of Oklahoma hitting a maximum temperature of 105-110 F.
The American Horticultural Society has produced a US Plant Heat-Zone Map that categorizes locations based on how many days each year the temperature reaches above a threshold value (86 F). On that map, the majority of Oklahoma is in heat zone 8, with 90-120 days above 86 F. By contrast, New York City is in zone 4, with 14-30 days above 86 F! Now all we need is for growers to start labeling their plants by heat hardiness. For instance, a marigold plant might be hardy for cold zones 4-9 and heat zones 7-8. I just made that up as an example, so don't get worried if you're trying to grow Marigolds in heat zone 6. I don't really know where Marigolds are hardy.
Most people that have gardened for more than a year or so understand that there is more to satisfying a plant's needs than just matching it with a cold hardiness zone. But the more information that becomes consistently available for all of the plants that we see, the better for all.