When planning a strategy against weeds, it is important to think of the soil as the weed’s bank account. The actual number of live weed seeds are sort of like their cash balance. If some manage to germinate and reproduce, it credits the account. When mice, insects or birds munch on them there is a withdrawal. Occasionally, even our efforts can help reduce the number of seeds in the ground by removing plants.
One tactic is to create an environment for weeds to germinate in, then eliminate them in a single control. You can think of it as swindling a weed into making a foolish investment with its seeds. If done properly, we can reduce the weed bank’s reserves and make some headway on our garden.
Solarization is one such method to trick weeds, where transparent plastic (2 to 6 mil) covers a garden bed and uses heat to kill seedlings. Common in southern states, colder areas such as Maine, Indiana, and Minnesota have found some use in this process.
The first few inches of soil must get to about 110-125 degrees F to kill weed seedlings. Having a clear barrier is key to getting these high temperatures, as letting light in brings more energy (think greenhouse effect). Black tarps may be hotter on the outside, but surprisingly they are slightly cooler underneath. Solarized beds are often heavily watered before, as the extra moisture pumps heat lower into the ground and can trigger weeds to start.
Equally important is a tight seal over the bed, if outside air gets in through the sides or corners it can make it not work. Researchers in Maine found burying the edges of the plastic was the most effective, compared to rocks being laid on top. The University of Maine also suggested that all the work you typically do in the garden (tillage, fertilization) should be performed before solarizing the area. Remember, we want to trick the weeds into germinating. A nice tilled bed and some food should help them start their doomed “investment”. The reason we do not want to rototill after solarization is that seeds deeper down could be dredged up to the surface and start germinating.
Before considering solarization, be aware of challenges using this method in the Midwest. Certain plants such as bindweeds and purslane are resistant to this method. You will also need a flat, full sun area that can be taken out of production for 4-6 weeks, typically between June to August. Experimenting on smaller areas could be a good idea before you go all out. Even after the tarp is removed, hand pulling/hoeing may still be needed. However, it should be reduced if the weather cooperated with enough sunshine and heat.
In conclusion, solarization is not a cure all, but it can be an option for beds with a persistent weed problem. Remember, we are whittling down the seed bank, but in some areas may feel like paying off a mortgage.