No Seeds, No Weeds

By Jane Rozum

I start each gardening season with the same mantra: I vow to do better this year in controlling weeds that inevitably invade my garden beds and landscape. I vow to not let the dandelions, crabgrass, purslane, etc. take over. I usually start the season with this mantra in my head, but at some point, I forget about area weeds or I just don’t get to them quickly enough before they flower and set seed. After attending a lecture by Dr. Tony Koski (CSU Extension Specialist) on weed management, I learned why one should never let the thugs of the garden take advantage of my garden space again.

During the lecture, Dr. Koski talked about the weed seed bank. A seed bank occurs when weeds are allowed to go to seed and drop seeds into the surrounding soil over many years. Many common species of weeds can produce thousands of seed in a season with just one plant. Take a look at the below table. Is there any doubt that many weeds are good seed producers?

                                                                                        Number of Seeds  

                                                     Weed                        Produced Per Plant

       Dandelion                15,000

       Canada thistle              680

       Curly dock                29,500

       Lamb’s quarter       72,450

       Mullein                     223,200

       Pigweed                   117,400

       Purslane                   52,300

 Think of what one weed is capable of producing as subsequent generations also produce seed, millions of seeds! The true horror, however, is allowing seeds to accumulate in the soil. What is the average viability of the seed; that is, how long is a seed able to persist in the landscape? The table below shows the average number of years that some common species of weed seeds can survive and still germinate and grow:

                                                     Weed           Viability of Buried Seed

Black mustard                  50 years

Curly dock                         80 years

Foxtail                               30 years

Mallow                             20 years

Plantain                            40 years

                                                    Shepherd’s purse            35 years

Say, what?! Not only are weeds proficient at growing and producing seed, the seeds they produce can still germinate and grow 50 years later! Is it any wonder why weeds are so prevalent in our landscapes? A weed seed bank can produce weeds for many, many years to come.

How can you decrease the weed seed bank in your yard? Here are some tips:

1.       The best weed control is applied when the weed is young, or even before seed germination. Crabgrass control in spring prevents the crabgrass seeds from germinating. That’s why it’s important to apply at the right time (like right now). Mechanical control is easier when weeds are young, and herbicides work best at this stage as well.

2.       Keep your garden and lawn healthy to promote weed growth competition. Deep, infrequent watering schedule works well for lawns and gardens, but shallow-rooted weeds don’t do well with this type of watering schedule. Healthy lawn and garden plants will crowd out and compete with weeds for light and nutrients, so keep nurturing your landscape throughout the gardening season.

3.       Apply mulch. CSU recommends 3-4 inches of weed-free mulch so weed seeds can’t germinate. Landscape fabric under the mulch is not necessary and can contribute to lower garden plant vigor.

4.       Most important of all: Do not let weeds contribute to the seed bank in your landscape. If weeds do pop up in your garden, pull or cut off the flower head. The weed may still grow and attempt to flower again, but if you never let a weed flower and set seed, it cannot contribute to the weed seed bank. 

So, as we head into the 2015 garden season, I now have a new mantra: NEVER let a weed set seed! At some point, maybe many years from now, I look forward to depleting the weed seed bank in my landscape.

  For more information, check out CSU’s GardenNotes #351 http://www.ext.colostate.edu/mg/Gardennotes/351.pdf