Horticultural definitions. Has a great ring to it, doesn’t it? It sounds like the opening line of a bestselling novel — or maybe it doesn’t. But if I do this correctly, you’ll soon realize how important these terms will be to your success as a North Texas gardener. That could happen quite soon.
Hardiness zone. Beginning decades ago, the USDA started conducting a long-term survey to determine the average lowest temperature each tiny corner of America might expect to experience in average winters. They drew lines connecting like temperatures, first in 10-degree increments, then refining it more, down to 5-degree differences. Just looking at those maps can be fascinating.
The next really big step would be to determine the minimum temperature at which each plant species would be able to survive. When you stop and think about it, you soon realize what a huge undertaking that would have been for the thousands of landscaping trees, shrubs, vines and ground covers we grow in America.
Every 15 or 20 years, the climatologists revisit the temperature map. Warming or cooling trends are taken into consideration and the lines are redrawn as needed. The map of 1990, for example, was amended dramatically when a new map was released in 2012. A decade of mild winters pushed many of the hardiness zones to the north by several counties. Tarrant and Dallas counties went from being at the very south end of Zone 7 to being solidly in the middle of the theoretically more “tropical” Zone 8.
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Still with me? Good, because that shift of the map suggested that plants like gardenias, pittosporums, loquats, oleanders, Confederate star jasmine and holly ferns should be good investments for us in the Metroplex. But historically that hasn’t proved to be the case. There’s always “that” winter, when record low temperatures cause us to realize that we’ve taken too many chances. Wise gardeners will limit the numbers of Zone 8 plants that they use. Stick with Zone 7 selections here in DFW, and your heart will be broken far less often.
Hardy. This term is a continuation of what you’ve just read. If you have a shrub that is rated for Zone 7, and if you live in or near the Metroplex, you would say that that plant will be “hardy” to your area — that it will survive all of your winters. That would be true, at least as far as winter low temperatures were concerned. You want to choose and use plants whose hardiness zone numbers are Zone 7, Zone 6 or maybe Zone 5.
The smaller the zone number, the more winter-hardy the plant is, but that can also work conversely. It may be a plant that doesn’t do well in heat. In spite of attempts, we don’t have a really reliable heat hardiness zone map. There is no specific temperature at which plants will die. That depends on how suddenly it gets hot and how long it stays there. Let your Texas certified nursery professional be your guide.
Hardened. This is one more variation of the same word, but in this case it means that the plant has been exposed to gradually lower and lower temperatures as autumn has progressed. Remember how cool 60 degrees felt at a late September football game? By February, that same 60 degrees was almost a tropical temperature. You had become hardened to the cold.
Plants that normally can survive temperatures of 10 degrees can be killed by 28 degrees if it occurs without prior conditioning. They wouldn’t have become “hardened.”
Acclimated. When a plant has become hardened to cold, it is acclimated. But “acclimated” refers to much more. Plants can become acclimated, or adjusted, to soils, wind, sunlight and even moderate drought. They become toughened, better able to survive the adversities. It’s a progressive happening.
Native. My dad was a Ph.D. botanist who worked in range ecology for Texas A&M. As a young child, I remember his saying, “A plant is only ‘native’ where you find it growing in nature.” As we gardened together over the years, he explained that plant species have very specific needs in terms of soils, sunlight, drainage and temperatures, and that it was possible for a plant to grow successfully on one side of a town and fail pathetically on the other.
Just because a plant is native in one part of Texas, that certainly doesn’t guarantee that it will thrive in another. Plants from West Texas may not be able to survive prolonged wet spells, and plants from East Texas may not handle drought or alkaline soils. There is no magic in the term “native.” It just means we need to do our homework and see what conditions it had where it was found growing in nature.
Adapted. This is a far more useful term. This refers to a plant that is happy to have a chance to prove its worth in your garden. It doesn’t matter if it’s native to Texas or if it came from China, Japan or Europe. It has proved its ability to adapt to what you have to offer. These are the plants you want to feature.
Killing frost. I’ll admit it: This one doesn’t really belong here, but since we’re at the time of year that we have to be concerned about the first freeze of the fall, I thought I’d throw it in.
There’s a difference between a “killing frost” and a “killing freeze.” A freeze will only happen when water freezes — at 32 degrees. But frost can form at temperatures above 32. You’ll see frost on windshields and meadows at 36, 38 and even occasionally 40. It happens when the dew point is reached, and it will always be on clear nights and with calm winds.
So if you hear that temperatures are going to drop into the 30s, and if there are no clouds and no wind, it’s probably a night where you’ll want to move tropicals indoors and cover the tomatoes.