Growing rocks in our lawns?

​Growing rocks
​in our lawns?

​After mowing our lawns for over thirty years I pride myself in knowing every inch of the area that has to be cut. I know the indentations, the places where I have to mow around an outcropping of rock, and where I can just mow without any danger. A couple of weeks ago there was a bitter awakening for me! On a stretch with ‘no danger’, I hit a rock that in the last thirty years I had not noticed! I was puzzled. When I investigated I could pull the rock out. It was small, like a baseball, and I had gotten it on the top; the rest was still hidden in the soil. The small top of the rock had stopped the mowing blades and stalled the engine! Of course, I am always worried that I damage the blades or the gears. This time I was lucky. Despite the dramatic stoppage, the blades were not bent and the bearings survived.

I had not seen the top of the rock because the grass was high and I was in a “no danger” zone. So what happened?

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I learned that any place that has real winters that could freeze the ground, could face the magic of “growing rocks”, that suddenly appear in your lawn. It is like a miracle, but it is no magic. In the eastern United States they even have a name for them: “New England potatoes”. Most of the soils in our region are comprised of glacial till. These are composite soils made up of sand, clay and rocks of all different sizes, that were bulldozed through the valleys by the advancing glaciers.

Of course nature does its own thing and it all makes sense. We know that in the summertime the streets of Manhattan seem extremely hot. And they are, because the stone buildings bank the heat during the day and keep it much longer than- for example, Central Park. The rock on- or near the surface of my lawn is a better conductor than the soil around it. It conducts heat away from the cooler soil beneath it, saturating the soil under the rock. In the winter time that saturated soil under the rock freezes before other dirt at the same depth. We all know that when water freezes it expands. This why frozen water pipes burst in the Winter. The soil under the rock freezes, it expands and pushes the rock up a little.

When spring is here and everything thaws out, the space under the rock that froze will be filled with sediment and my rock is up a little higher. This process will be repeated every year and over many years the magic happens. The rock sees daylight and the blades of my mower.

The same thing happens in my vegetable garden. My “stone harvest” every spring is amazing and never ending.

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Each spring, we can see the evidence of how much the ground expands when it freezes behind our house. We use a snow blower in the winter to make a path across the lawn to our woodshed to get firewood to heat the house. The path is usually a couple feet wide and down to the grass. Snow is a wonderful insulator. Each spring when all the snow has left the lawn’s surface, you can still clearly see the path in a 1- 2 inch elevated mound leading to the woodshed. It subsides eventually as the ground thaws, but it takes about a month.

Soils that have high clay content will hold more water and therefore expand (heave) more. Therefore, when constructing a basement, attention to water drainage around the foundation is very important. Heaving soil can exert enough force to collapse a concrete block wall! This is also the reason that your local building codes mandate how deep the foundation footings for your house must be. Heaving soils can lift an entire house unevenly, cracking the foundations and endangering the whole structure, if they aren’t below the local frost line. For us, the frost line is 3- 4 feet deep.

I have come to the conclusion  that my lawn does not have any “no danger zones” anymore. We have too many rocks that are striving to see daylight!

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