Perry's Perennial Pages

Vermont Soil Moisture

Andrea Luchini, Graduate Assistant
Dr. Leonard Perry, Extension Professor
University of Vermont

Beginning in June 2003, soil moisture was measured at four locations at a site in Milton, Vermont, at numerous times throughout the season, until November 2003. The soil was a loam (Peru stony loam), relatively high in organic matter.  Soil moisture was also measured at the Horticulture Research Center in South Burlington in a Windsor sandy loam soil.  Since this area was under irrigation, seasonal effects could not be drawn, and plans this coming year are to monitor moisture levels in a non-irrigated area.  The following discussion relates to the Milton readings.

Moisture was measured at various depths using an AquaPro Soil Moisture Sensor (AquaPro Sensors, Reno, Nevada), a commonly used instrument in irrigation management. A special plastic tube was inserted into the ground to a depth of 24 to 30 inches and left in the ground.  The AquaPro sensor could then be inserted into the tube at any time.  The sensor uses low frequency radio waves to read the soil moisture and reports the results in percent moisture.  Readings were taken at several times during the season to depths of 6, 12, 18, and 24 inches.  Two of the four locations were in tree root zones with significant tree canopy.  The other two locations were in open perennial beds with little overhead cover.

In comparing percent moisture at a 6 inch depth (the depth at which many perennial plant roots exist) at all four locations, we noticed that the two locations in perennial beds maintained a consistently higher soil moisture content than the locations under trees, as you might expect.  All locations seem to follow the monthly rainfall data from the National Weather Service (NWS), as we can see dips in the percent moisture following drier months and rises in the moisture following wetter months.  However, the perennial beds varied in their moisture content much less than the locations under the trees.

From June until September, the perennial beds maintained a soil moisture level at 6 inches between about 70% and 80%. One tree location, under a maple, started at a high of over 70% in June but then dropped to about 45% in July, and went back up to over 70% in the middle of August for a brief time.  The other location, under evergreens, started at just above 60% in June, but quickly dropped to around 48% and reached a high point of about 65% at the reading in the middle of August.

After the very dry month of August, with only a little over 2 inches of rain according to the NWS, the soil moistures dropped dramatically, especially in the locations under the trees.  The overall low point for the location under the evergreens was about 35% and the overall low for under the maple was 45%, and these occurred in the middle of September.  Again, the locations in the perennial beds maintained a higher moisture content, even after the dry month, with a low of about 68% for one location and 75% for the other in the middle of September.  All locations had rebounded well by the beginning of November with moisture levels being almost as high, if not higher, than the first readings in June.

We also combined the data for the two locations under the trees, and the data for the two locations in the open perennial beds, to look at moisture changes at various soil depths-- 6, 12, 18, and 24 inches.  The open locations showed a high level of uniformity over the four depths.  The 6” depth varied the most, but all depths maintained moisture content between 70% and 80% for almost the entire season.  Readings from 12” and 24” were very similar throughout the season and had the highest readings overall.  These generally stayed between about 75% and 80% all season.  The moisture level at 24” varied the least throughout the season and stayed between about 70% and 73%.

The readings from the locations under the trees varied much more throughout the season, especially at the 6” and 12” depths.  Moisture levels also got much lower here than in the perennial beds.  The 24” depth readings dropped from about 75% in June to about 40% in late July and stayed at this level until the beginning of November.  The other depths had a brief increase in moisture content at the beginning of August (following about 3.5” inches of rain in July) followed by another dramatic decrease at the beginning of September.  All depths were similar to the 24” depth in the amount of time it took for them to achieve moisture levels similar to those at the beginning of the summer.  The depths 18” and 24” started and ended the season with the highest moisture levels, although they both dropped to the lowest readings in the middle of the season.

This data gives a quantitative look at the impressive effect trees have on soil moisture content, as well as the effect of high organic matter on maintaining moisture. In locations without trees and tree roots nearby, soil moisture in good soil is able to stay at a relatively high level, even in periods of low rainfall.  However, locations that have to deal with tree roots, as well as a tree canopy, are much more affected by low rainfall.  These sites have the potential to require supplemental irrigation for certain herbaceous perennial plants to survive in seasons with low rainfall.  Not only do the tree roots draw in an enormous amount of water, less rainfall reaches the soil level due to the canopy.

The data in the form of graphs can be found online on Perry’s Perennial Pages.  (  Hopefully with several more years of data, and observations on drought tolerance of various perennials in the field, such measurements can be useful in several respects.  They can help quantify at what soil moisture levels certain perennials need water.  Or they may show which perennials perform best at specific moisture levels, and so which may be suitable for certain sites or grouped with other perennials by such numerical water requirements. Thanks to funding from the New Hampshire Horticulture Endowment, a study over the next couple of years should also give a clearer picture of the effect of such moisture levels in the field and in pots on the hardiness of perennials.

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