Ron Kujawski: Kill weed seed with help of sun


This is the time of year when my thoughts turn to things hot and steamy.

Yes, that's right; I'm thinking about sterilization via solarization. No, this is not about birth control. Specifically, I'm referring to sterilizing potting soil. I use a lot of compost in my potting mixes. Unfortunately, my compost contains a lot of weed seed. I'd like to kill those seeds without using herbicides or stinking up the house by sterilizing soil in the oven. Since summer is sunny and hot, why not let the sun do the work?

My plan is to place the potting mix into clear plastic bags. The soil is moistened and then the bags are sealed. The bags will be placed on a firm, level surface in full sun and flattened so that depth of the enclosed soil is an inch or two. The theory is that sunlight will heat up the soil and kill the weed seeds and perhaps any disease-causing bacteria and fungi. It is important that the surface of the soil be smooth and the plastic be firmly in contact with the soil. Otherwise, moisture from the soil will condense on the inside of the plastic bag and reflect the sunlight. Poke a soil thermometer through the bag to monitor the temperature within. Most weed seeds will be killed when soil temperature reaches 140 degrees. If no soil thermometer is available, leave the soil in the bags for six to eight weeks. Solarization may not kill all the weed seeds but I've had success killing most weeds using this technique.


Other tasks this week:

n Sow seeds of Chinese cabbage for fall harvest; the plants will mature in mid-October. Chinese cabbage grows best as a fall crop because it thrives as days get shorter and nights get cooler. Light frosts in early fall will not hurt the plants.

n Shred cabbage for sauerkraut within the first 24 hours after harvesting the heads. Waiting too long to shred cabbage will result in limp sauerkraut. Not many people make sauerkraut at home these days, but it's not difficult. Check any recent reference on pickling for an up-to-date recipe.

n Keep picking beans every other day. Don't worry if you yank off some immature bean pods. It's better to pick young beans than let them get to the stage where the pods are bulging with seeds. Once you leave too many mature bean pods on the plants, they'll stop producing. If you pick some immature beans, cook them the same day since the young bean pods tend to shrivel if they sit around for more than a day.

n Plant buckwheat or sudan grass as a cover crop between rows of remaining crops and in the vacant areas of the garden. Both of these are great for suppressing weeds and for contributing organic matter to soil. Mow down the buckwheat before it sets seed and the sudan grass when it is about 20 inches tall. Leave the mowed plants in place through the winter and then till them into the soil in early spring.

n Take a few moments to evaluate your perennial borders. Specifically, look for gaps in bloom. If there are few plants in flower, this is a good time to buy and plant August blooming perennials such as Russian sage, Oriental lilies, turtlehead (Chelone), Boltonia and the three H's -- Helenium, Helianthus and Heliopsis.

n Cut back the flower stems of perennial geraniums, or cut back entire plants if you see new growth occurring at the base of the plants. You should get another round of blossoms from these plants. This usually does not have to be done if you're growing the variety Rozanne since it blooms continuously through the summer.

n Deadhead garden phlox (Phlox paniculata) to keep the plants from reseeding. Plants that grow from the dropped seeds typically revert to pale mauve flowers.


If you'd like to leave a comment (or a tip or a question) about this story with the editors, please email us. We also welcome letters to the editor for publication; you can do that by filling out our letters form and submitting it to the newsroom.

Powered by Creative Circle Media Solutions