08 September, 2010

Autumn a Wolf in Sheep's Clothing in the Garden This Year

Drought damage to your garden happens faster now than mid summer. The end of summer and the arrival of fall, my favorite season, gives a false sense of security to gardeners when rain fall has dropped off. This fall will be a perfect example since we are on average about 5 inches below normal rainfall.

One would think that hot dry summers are the main culprit in drought damage. They certainly do contrbute. But summer has a built in drought defense system in the northeast: humidity. The relationship between temperature and humidity is a complex one but has a drastic effect on how much water plants transpire into the atmosphere.

To put it simply, as simply as complicated relational analysis can be, air like most matter expands when it warms. A larger volume of air can hold more water than the cooler air. This is irregardless to humidity level. Therefore an 80* summer day with 80% humidity has much more moisture in it than a 50* day with the same 80% humidity.

So how does fall weather with inherent lower temperatures hasten drought damage?
Humidity is the answer. Fall temaperatures are also accompanied by very low humidity levels. While fall days may be only ten degrees cooler in mid September than mid August humidity levels plummet by a much larger percantage. Fall humidity levels often drop to 40% or lower meaning there is much more room for moisture to enter the atmosphere as a percentage of volume even with lower temperatures.

Think of 80*/80% humidity like a fuel tank. Let's say an 80 gallon fuel tank is 80% full. That means that there a 64 gallons of fuel in the tank with room for 16 gallons more.
That leaves little room for more fuel entering the tank. (Or in the atmosphere little room for moisture from plants to transpire into the atmosphere so plant "breathing" slows down).

Now look at a 70 gallon tank that is 20% full. There are 14 gallons in the tank and room for 56 more. In the atmosphere a lot of room for plants to transpire moisture into the air so there breathing rates increase rapidly.

While there is no correlation between the weather with the following example it speaks volumes about the subject: 80x80=6400 whereas 70X20 only equal 1400. That illustrates in graphic detail the difference between 80*/80% humidity and 70*/20% humidity.

Here we have a small drop of ten degrees in temperature but a corresponding drop in humidity of a much larger percentage. Result: even with a lower temperature plants transpire much more moisture into the atmosphere beacuse of low humidity.

With normal rain fall this difference is not a matter. But with the lack of rainfall we have experienced this year it matters a lot

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