A Clean Start – Soil Solarization

One of the most difficult and unpleasant jobs in gardening is dealing with weeds in a newly planted area of the garden.  They pop up everywhere.  And they keep doing it.  Some weeds one can just pull up.  But others tend to linger.  The easy-pull weeds tend to be easily eradicated from the garden, but the ones that you pull up over and over again tend to be a real aggravation.  Perennial grasses are an example.

Last year I had a spot in the garden that was not yet planted with roses. I wanted to prepare it to host half a dozen roses in April.  It was full of yarrow and perennial grasses.   I dug up the soil and turned the grasses upside down in fall. In February some of those had started to grow again, so I  got out an old tarp and carefully covered the area, holding down the tarp with heavy stones so it didn’t blow away in the brisk winds of winter storms.  Every couple of weeks I looked beneath it.    In mid March I realized that the roses would arrive soon.  I pulled the tarp off.  There was a lot of yellowed grass there.  I was tempted to spray with glysophate, but I’d had so many problems with lingering effects in my last garden that I have banned its use within 15 ft of any plants. Instead I sprayed the yellowed grass with horticultural vinegar.

When the roses came, I planted them.  One of the curious things I noticed about this spot was how vigorously the roses grew.  It was as if the whole preparation process was perfectly suited to them.  The soil, for most of the season, had the rich musky scent of fertile forest soil.  And essentially none of the grass or yarrow that had infested the area the previous year was present. My tarp experiment was a success.  So now the tarps have become part of my ongoing system for reclaiming ground for the rose garden.  I have still to discover what role the horticultural vinegar played. Perhaps it helped neutralize alkaline soil.

Soil solarization is a method of preparing the soil for some cultivated plants whose purpose is to eliminate weeds (and sometimes other pathogens). It involves covering the soil with a membrane. Solarization takes two forms.  The first is the classic form of ‘baking the soil.’

  1. Moisten the soil to the level that would promote seed germination.
  2. Cover an area that is in full sun all day long with clear plastic in June or July.
  3. Wait for thirty to ninety days.

This process admits a lot of sunlight and uses the greenhouse effect to heat the soil to a high temperature.  It eliminates weeds by heating the seeds until they die. It is a popular method in California and Texas where bright sunlight and warm days and nights suffice to  raise the temperature high enough to kill seeds. It also works best over areas that are large – ten or twelve feet in each direction.   In smaller areas, in cooler or shadier locations, or in these same places at other times of year, such high temperatures are not achievable, and the method doesn’t work.

When it does work, it also kills beneficial soil flora and it can burn organic material from the soil much faster than they would otherwise disappear in hot climate areas.  This means that one will wish to pay special attention to reviving these populations with the application of organic material after the solarization process is finished.  This is especially true in drier areas where soil fertility tends to be low already.

Gardeners who live where the sun is not so bright, where it is difficult to bring the soil to the necessary temperatures for killing seeds, or where it is convenient to prevent winter weeds from taking over a vegetable garden might employ an analagous method.

  1. Moisten the soil to a level that would promote seed germination.
  2. Cover any area with black plastic or any opaque tarp.
  3. Wait for 60 to 120 days.

This process admits no sunlight and works by a different method of action.  It preserves some heat and soil moisture, causing seeds beneath the tarp to sprout.  But when they sprout, they receive insufficient light, and they die. The same thing happens with perennial weeds.  It’s a popular method in cooler areas. It can work where the ground is shaded.  And it works marvelously between fall cleanup and spring planting. Some deep seeds are not killed by this method. Subsequent tilling can bring them to the surface.  So this is a good method where one either tills in the fall or does not till much at all.

Now, there are fierce advocates of the first method who claim that the second method is not “solarizing.”  Some claim that it is nothing but “mulching.”   And so we have a disagreement over terminology.  Their point of view is that “solarizing” is defined by its method of action, namely heating the soil to a temperature high enough to kill weed seeds. But if only temperature mattered, then dampening the soil before solarizing with the first method would not be essential.

My point of view regarding the term “solarizing” is that it involves covering an area with a membrane which will be removed before planting.  The membrane may be clear plastic or it may be black plastic. Or it may be an opaque tarp.  But it stays in place long enough that most (essentially all) of the seeds die either from sprouting and not being able to continue growing or by being cooked out of existence.   What makes it distinct from mulching is that mulch is presumed to stay in place around a cultivated plant during the growing season, preventing weeds at that point in time. Solarizing, by contrast, is a method of soil preparation that removes weeds when cultivated plants are absent.

Whatever you might choose to believe about the terminology, the act of covering blocks of soil to choke out weeds is an old, time-honored one.  It preserves soil moisture. It kills weeds both annual and perennial.  And it can help make cultivation of any garden plants more pleasant during the growing season. It is an ideal way to prepare a new bed area for  a spring planting of roses.