Tilling spreads mint, other invaders

Question: I have a big problem with wild mint every year invading my sweet corn patch. It grows, we dig it out, it comes back, we dig it out again -- the cycle never stops. I do not use chemical weed killers in the garden and would like an idea on how to kill this stuff. Last fall, I placed a sheet of black plastic over a piece of the garden and left it there until it was time to plow. Now, that section of ground has very little weeds. Am I on to something or was that just luck?

Answer: By placing the black plastic over a section of the garden you did what is commonly called soil solarization. When snugly in place, the plastic serves to "bake" the soil, killing shallow weed seeds, some pathogens, soil-dwelling pests, and also plant roots in the upper few inches of soil. Ideally, solarization should take place during the summer months and the plastic should remain in place for six weeks for maximum effect. Several studies have shown that thick (1 to 4 mils), clear plastic works better than black. The plastic must be snug against the soil to avoid the presence of air pockets, and it must be anchored securely around the entire perimeter. Watering the soil first is helpful, too, as the water will conduct the heat further down into the soil.

Solarization, however, also may kill certain microscopic beneficial soil life in the top few inches of soil, though their populations rebound fairly quickly after the plastic is removed. Larger soil organisms, like worms, often migrate out of the area as the temperatures rise. As a result, they remain largely unaffected by the solarization process.

Part of the problem you face in controlling the mint is your attempts to dig it out and till the area. Each time the mint is dug out, small pieces of roots remain in the soil; each one will sprout into a new plant, expanding the population and the problem. The same goes with the tilling process. It chops each root into many small pieces which will then go on to grow lots of new plants. Tilling also offsets many of the benefits of solarization, as it brings nonsolarized soil to the surface and introduces a whole new population of weed seeds. If you go the solarization route, you'll have to avoid deep tilling (a maximum post-solarization tilling depth of 2 inches is recommended) or switch to no-till methods of growing (which, by the way, is an excellent way to garden).

Another option would be to use an organic weed killer to help control the mint as well as other weeds. Based on clove oil and acetic acid, these products work best when temperatures are in the 80s and should be applied in summer (brand names include BurnOut, Perfectly Natural Weed and Grass Killer and others). They are non-selective and care must be taken when applying them to avoid contact with the corn.