Issue 11, September 13, 2019

Seedy Facts

One of the best management practice in the home landscape is not allowing annual weeds to flower and set seed.  The story goes a bit deeper than that in terms of why that is so important.

One gardening phrase that has been around for decades, if not generations is “One Year’s Seeding – Seven Years Weeding” and that is a very conservative estimate.  According to the Weed Science Society of America and other Weed Science Societies, there will be plenty of weed seed viable in the soil for decades!

Some are readily seen as they spread like dandelion seeds blowing across the yard like mini tumbleweed or floating in the wind. Dr. Robert Norris, Ph.D. and fellow of the Weed Science Society of America notes that besides dandelion, sowthistle and groundsel are also windborne. Others do not care to spread themselves around so obviously. 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 waiting to sprout when the conditions are right.  Over 90% of our weed seed typically germinates in the spring, focusing on seed germination prevention and dealing with the weeds while they are just emerging and easily managed.

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. Others will use a quality mulch/compost to keep those seeds in the dark to prevent germination.  If there is time and space, putting soil solarization in your rotation for vegetables for example, becomes a real option.

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 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 giving us 1,800,000 seeds annually with the good news they may only last about 5 years in the soil.  Compare that to Eastern Black Nightshade only giving up 825,000 seeds, BUT lasting 40 or more years.   I pick the purslane!

Chickweed mentioned earlier can produce 25,000 seeds that immediately fall to the soil.  The 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.  Wonder why we never seem to get rid of purslane, if one plant can give us 1,800,00 seeds, how many do we pull or hoe out in one year avoiding seed set?

In the end, gardeners should do whatever they can to limit seed production and over the seasons that weed seed bank will go down and won’t you be surprised one spring when your plants win over the weeds.

Richard Hentschel, Extension Educator

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