Master Gardener Column 

November 2019 

By Jan Phipps 

“One year’s seeding – seven years weeding”. Now there is a depressing thought. It is one of the many gardening adages; this one referring to weed seeds. Actually, seven years is very conservative. Many annual weed seeds last for decades. If you are consistent and tenacious, you can decrease the seed bank in your gardens.  

The mainstream thinking is to deny weed seeds light, preventing them from germinating. Maintaining an adequately thick layer of mulch works, but the seeds are still there, lurking just under the surface of the mulch just waiting for their chance in the sun.  Anything that disturbs the protective layer becomes problematic. The mulch decomposes and becomes too thin. Seeds come to the surface if planting in that bed. Footsteps push the mulch to the side. Animals and birds rooting around for insects uncover the soil. The list goes on.  

Another theory relies on letting the seeds germinate and then removing the plants. According to Richard Hentschel, Extension Educator, “Gardeners can create a stale seedbed by very lightly and shallowly working the very surface of the soil repeatedly, causing weed seed to germinate until the seed bank in that thin layer of soil is exhausted.”   

I’m doing something similar this fall. I divided and transplanted my iris to a new place. In the process a host of cleome seeds germinated in the new spot creating a carpet of unwanted plants between each iris. With the first hard freeze all those little seedlings will perish, and that bed should be free of cleome next spring.  

If you have space, you can add solarization to your rotation, especially in the vegetable garden. Solarization utilizes the sun to “cook” the soil, killing all the weed seeds. Unfortunately, it kills everything else including the beneficial organisms living in the soil. The fastest way to solarize involves covering the soil with clear plastic during the hottest days of summer. The sun does the rest.  

We are up against tough foes. Common purslane produces 1,800,000 seeds from one plant annually; however the seeds only last for 5 years. Hentschel noted eastern black nightshade drops 825,000 seeds per plant per year, but they can last 40 or more years.   

Good luck, gardeners. I know you are up to the task of reducing the annual weed seed bank in your landscape.  

For questions, contact University of Illinois Extension Master Gardeners of Edgar County at 217-465-8585.