One gardening phrase that has been around for decades, if not generations, is “one year’s seeding – seven years weeding,” and that is a conservative estimate actually. According to the Weed Science Society of America, there are plenty of weed seeds that can remain viable in the soil for decades!
Some easily are seen as they spread. Picture those white puffy dandelion heads blowing across the yard like mini tumbleweed or floating up in the wind one by one. Besides dandelion, sow thistle and groundsel also are windborne, according to the WSSA. Other weeds do not spread themselves around so obviously. For example, chickweed, a winter annual, happily produces seed for weeks and just drops them down to the soil below. Very few annual weed seeds move in other ways.
Given the opportunity, a gardener can reduce that seed bank, which is sitting there, just waiting for conditions to be right to sprout. More than 90 percent of our weed seed germinate in the spring, so we should focus on seed germination prevention, dealing with the weeds while they are just seedlings and easily removed.
Gardeners can create a stale seedbed by very lightly and shallowly working the very surface of the soil repeatedly, causing seed to germinate until the seed bank in that layer is exhausted. Others will use a quality mulch/compost to keep those seeds in the dark to prevent germination. If there is time and space to do soil solarization in your growing rotation, then that becomes a real option.
It’s amazing how prolific and hardy some of these weed seeds are. Dr. Mark Renz, working from literature from the California Weed Science Society’s “Principles of Weed Control,” third edition, compiled information of a number of weeds, providing the number of seeds produced by a single plant and how long we can expect them to be viable in the soil. Here are a few common weed plants we find every year in our beds:
Common purslane gives us up to 2 million seeds annually, but the good news is they may only last about 5 years in the soil. Compare that to Eastern Bitter Nightshade only giving up 825,000 seeds, BUT lasting 40 or more years. I pick the purslane!
That chickweed mentioned earlier can produce 25,000 seeds that immediately fall to the soil. University of Massachusetts lists our troublesome crabgrass as producing about 150,000 seeds every year. Crabgrass is not limited to growing in the lawn either.
In the end, gardeners should do what they can to limit seed production and over the seasons that weed seed bank will go down. Won’t you be surprised one spring when your plants win over the weeds?
About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.