For Release January 8, 2002

Plant Hardiness Difficult to Rate

by Chuck Otte, Geary County Extension Agent

I have a good friend who has a strong horticultural urge. He likes to grow lots of different plants, especially a lot of fruit trees. Likewise, I enjoy trying to grow a lot of different things, my real passion being chile peppers. In both cases, we may very well be trying to grow plants that arenít well adapted to this wonderful Kansas climate.

A term that is often used in the horticultural trade is hardiness. The trouble is that there are no clear cut standards for hardiness, as it can mean many different things. When used in general plant growth terms, it often refers to cold hardiness. The USDA has spent a lot of time developing an excellent map of temperature zones so that plants can be rated on cold hardiness.

In this system, northern Kansas is a zone 5b. That means that in any given winter, we can expect absolute cold temperatures in the 10 to 15 degrees below zero range. It doesnít mean that it will always get that cold every winter, and in some winters, it may well get colder than that. As recently as 1989 we hit 21 below zero. But, in most years, weíre going to experience nothing colder than 10 or 15 below, if that cold.

What doesnít get figured in to this equation is a whole lot of other weather factors. How much humidity and wind is associated with that absolute temperature? What are the maximum expected high temperatures in the summer and how much humidity and wind are associated with that maximum reading? How fast will it go from high to low temperatures?

January 31, 1989 we had a high temperature of 73 degrees. Less than 72 hours later we were at -10 degrees. We dropped over a degree per hour for 72 hours. The average temperature for January 1989 was almost 10 degrees above normal. Plants were not hardened off and ready for that kind of temperature swing. Ten below was not out of the normal expected range of cold temperatures. But, a little more gradual acclimation would have been helpful.

Now, my friend gets upset because his fruit trees wonít bear. His thinking is that they claim that these trees are hardy to zone 5 (our own) or in some cases even zone 4, so why arenít they bearing? I ask him if the trees are still alive. He tells me they are. So, the plants are hardy, as advertised, but hardiness has nothing to do with productivity.

My chile peppers donít worry about plant hardiness because they die with the first frost anyway. But I try to push the season and grow varieties that need a little more heat or a little bit longer season. The plants grow very well here in Kansas, but about the time they really start to set on chiles, is about the time the weather starts to cool down in September.

The truth is, there is more to a plant surviving, thriving, and bearing than just an absolute high or low temperature. There are soil conditions, there are humidity conditions, there are the extremes to every weather impact we face. There is no place else that is exactly like your yard, your garden or your orchard. You can use plant growth and survival in nearby locations as a guide, but the micro climate in your particular growing space is unique.

For many common plants, they will probably survive just fine in your growth space. Thatís assuming you give them good care, you donít drown them and you donít expect outrageous growth or production. But once you start pushing the envelope you are going to have to rely more and more on your own experience and expertise. You are going to have to learn more about the plants you are trying to grow and the environmental effects that impact them. But donít stop trying! Thatís what makes gardening fun!


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