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Solarization in the Garden

Photo by: University of California Riverside

Last week, I was helping a client reclaim a weedy patch that once held an old grain silo. Years of the silo loafing near a farm field edge led to 12-foot high giant ragweed (Ambrosia trifida) that towered over me. Thankfully, the size of the area was about a 20-foot wide circle (instead of the acres I have seen in soybean). The homeowner also wanted to avoid using herbicides to treat the ragweed when they emerged next spring. Understandable, it was possible that the weeds were resistant to RoundUp (glyphosate) due to their farm field proximity. If true, using other herbicides could have been more dangerous, expensive, or susceptible to drift. I cautioned her that this would be a long war against the weeds, but a mixture of hard work and wits can help us win a few battles.

When planning a strategy against weeds, it is important to think of the soil as the weed’s bank account. The number of seeds that are alive is the ragweed’s cash balance. If some manage to germinate and reproduce, it is like a credit to the account. However, every year fungus or other pathogens can debit the weed’s resources by killing germinating seeds. In addition, if the seeds fall on the soil surface, mice, insects or birds can further drain the bank account by feeding on them. Occasionally, our efforts can help reduce the number of seeds in the ground by cultivating or spraying.

One tactic, especially common in organic agriculture systems, is to create an environment enticing for weeds to germinate in, and then eliminate them in a single control. You can think of it as swindling ragweed 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 (and some diseases). Common in southern states, more temperate areas such as Maine, Indiana, and Minnesota have found some use in this process and are continuing to explore how to make it more effective.

The first few inches of soil has to get to about 110-125 degrees F to kill weed seedlings. Having a clear barrier is key to getting these high temperatures, as trapping light’s radiation puts more energy into the ground (think greenhouse effect). To aid in this process, the solarized bed is heavily watered before, as the extra moisture pumps heat lower into the ground. In some cases, producers even hook up irrigation under the plastic to keep the soil wet during the whole process.

Equally important is a tight seal over the bed, as the heat and moisture treatments can be neutered if outside air gets in through the sides or corners. Researchers in Maine found burying the edges of the plastic was the most effective, compared to pipes or rocks being laid on top of the tarp. University of Maine also suggested that all the prep 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 who avoided death could be dredged up to the surface and start germinating.

Before considering solarization, be aware of the realities of using this method in the Midwest. Certain plants such as bindweeds, nutsedges, 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 from June to July (the hotter months in Minnesota). Experimenting on smaller areas could be a valuable option, but be aware that 4-6 weeks is approximately a quarter of the growing season. Garden favorites such as tomatoes or peppers will not work in that system, so using solarization will force you into planting other vegetables/cover crops suitable for fall or early spring growing. Even after the tarp is removed, hand pulling/hoeing may still be needed. However, it should be reduced if solarization was properly executed and 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 some areas may feel like paying off a student loan to an Ivy League school.

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