Oh no, not another weather column! by Bill Hayes

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Well , I’m sorry but it’s about time we discussed some weather issues as they apply to gardening. There are a lot of dried up gardens in Aiken right now along with some brown lawns and a lot of stressed trees. I don’t think that I have to tell you why this is happening especially if you happen to be outside on a daily basis. Two things come to mind immediately. It seems to be hotter than normal and we are not getting very much rain. Now you may be thinking to yourself; “Is this another quack with a global warming theory”? I guess my reply would be “not exactly” but I do believe that our temperatures have been rising lately. In 1989 I bought some plants from Woodlanders Nursery. One of the plants was a USDA zone 8 plant and Clemson, at the time, said that we were in zone 7b. I asked if my new plant could make it in zone 7b and Bob McCartney informed me that the plant that I just bought was grown in Aiken and he felt that we were in zone 8a.

We have two climate issues to discuss when it comes to growing plants. The first is the USDA hardiness zone which tells us the minimum temperature that we will normally reach in Aiken during the winter months. If we agree that we are in USDA zone 8a (10-15ºF) then the minimum temperature that we might see is plus 10ºF. Now all that we have to do is purchase plants that are cold hardy at that temperature. For example: A Cornus kousa (Japanese dogwood), is rated as a zone 5b plant so it might withstand a low temperature of minus 15ºF. We would never see that temperature in Aiken so this would be and is, a great plant for our area. On the other hand, a Nerium oleander (oleander), is rated at 8b to 9a. 9a zones will normally only drop to plus 20-25ºF and our low of plus 10ºF could harm the plant and might even kill it. Another consideration is how the plant is protected in cold temperatures. An oleander next to a brick wall that retains heat may prevent the plant from cold damage. Heavy mulch around the roots might also help.

Purchasing a plant based on its cold hardiness is only protecting it by 50%. We also have to protect plants based on the American Horticulture Society’s (AHS) heat zone map. This map shows us the heat zone that we are in and the days over 86ºF that we will see. Aiken is in AHS zone 9 and that means that we will see between 120-150 days over 86ºF. This is very tough on shrubs, trees and lawns especially when we are getting very little rain. So now you know that we can kill plants in two different ways. We can plant a hibiscus in the fall and kill it in one night when the temperatures reach freezing or we can kill it over several months in our hot climate by not giving it sufficient water.

Many of our favorite shrubs and trees are shallow rooted. Azaleas and dogwoods require extra water during drought conditions or they will show signs of stress. Dogwoods are “under canopy” trees and should not be planted in full sun. In times of drought the dogwood is among the first plants to show wilting and stress. Normally, an inch of water per week is sufficient to keep the dogwood healthy but these are not normal times. We also compound the problem by having sandy soil which drains very fast. Water the dogwoods in the morning with a soaker hose rather than a standard irrigation system. An irrigation sprinkler will not penetrate deeply enough and most of this water will be lost through evaporation. In times of 90º temperatures and no rain, it might be necessary to water the tree every other day. Keep an eye on the leaves to see if they are wilting. If not, skip a day or so until they wilt and then water as needed. Overhead watering can lead to disease and hot dry days can accelerate the infestation. Other plants that are shallow rooted and require extra water are azalea, Bradford pear, aucuba, redbud, crape myrtle, and Indian Hawthorne. All of these plants will show signs of stress during hot, dry conditions.

Are temperatures rising in South Carolina? The answer is yes and no and yes and no! In 1895 the average annual temperature was 61.6º F. By 1901 it had dropped to 60.6º F. Then the temperature started rising again and by 1925 the average was 65.1º F! A five degree rise in in 24 years is quite remarkable. But then it started to drop again and by 1966 it was back to 60.8º F. What this tells us is that we are in a constantly changing climate with no long term substantial highs or lows. For those of us with short memories, 2010 is an unusual year. Compared to last year, here are a few interesting facts. In 2009 we did not have a single day over 99 degrees. In 2010 we have had seven days over 99 degrees and two days reached 102 degrees. In 2009 we had 74 days of 90 degrees or above. As of September 22, 2010 we have had 104 90 degree days and we are still counting. Combine these high temperatures with a shortfall in rain and you can see why our plants are dying.

2010 had one final interesting note. In January we had a few days of very cold weather. December of 2009 was rather mild with only one night reaching freezing and that only lasted for an hour or so. In January, we started out with a bang! We saw freezing temperatures from January 2nd to January 16th. The high temperature on January 4th was 31 degrees which meant that out plants saw below freezing temperatures from 8 P.M. on January 3rd to 10 A.M on January 5th. This was a remarkable 38 hours below freezing with low temperatures reaching 16 degrees. For lawn lovers this was a disaster. Common centipede and St. Augustine grasses do not like cold temperatures and just a few hours of freezing weather can damage their root systems. 38 hours can cause severe damage and that was the case for many Aiken lawns.

If you have an interest in historical weather information, we have posted the historical weather averages for South Carolina.  See the Recent Posts section for that information.

The next Meet a Master Gardener event will be on October 2, 2010 at the Farmers Market. Come visit us between 8:00 A.M. and noon. The next Lunchbox meeting will be on October 18, 2010 at noon in the meeting room of the Clemson Extension office, 1555 Richland Ave., East. The topic will be Soil and Composting. This is a free service.