Common mullein flower. (Photo: Sharon Bokan/CSU Extension).

What characteristics make weeds so successful and able to annoy us and resist our attempts to eliminate them from our properties?

Weeds are non-native plants that have been brought here either intentionally as a landscape or food plant or unintentionally as a contaminate in crop seed, the gut system of animals or in some other material. Because they are not native, they do not have any natural controls to keep them in check and they became invasive. Scientists have worked to find their natural controls and bring them here but that hasn’t always been successful as the control in some cases affects our native plants. Before natural controls such as insects are released for sale, they are tested to make sure they won’t affect native species.

No native plant can be considered a weed even though we may think they are. Most of our weeds were brought here from the Mediterranean and Eurasia regions.

The first characteristic that make them successful is they are very adaptable plants that are capable of quickly adapting to growing in different soil and climate conditions, outcompeting native plants. They often survive and thrive in inhospitable conditions where other plants cannot survive.

Another characteristic is that weeds are very prolific in seed production or other reproduction methods such as roots or plant parts. Some plant seeds can last a very long time in the soil without germinating.

Here are the number of seeds produced by some of our common weeds and how long they can remain viable in the soil.

• Cheatgrass – 25 to 5,000 seeds per plant, viable for 3 – 5 years

• Dandelion – 2,000 seeds per plant, viable for 5 years

• Puncturevine – 200 to 5,000 seeds per plant, viable for 5 years

• Canada thistle – 1,500 to 5,200 seeds per plant, viable for 20 years

• Bindweed – 25 to 300 seeds per plant, viable for 50 years

• Common mullein – 100,000 to 240,000 seeds per plant, viable for upwards of 100 years

The last characteristic that makes them successful is that weeds establish very quickly is that they have mechanisms that allow them to germinate, grow quickly and spread easily, either by seed or other methods (i.e. roots) or other vegetative methods or methods for the seeds to be easily spread (i.e. cocklebur). Just in case you ever wondered why you can never seem to get all of the root on weeds like bindweed and Canada thistle, here’s why, bindweed roots can go 20’ deep in the soil although 70% of the roots are in the top 2 feet of soil. It has been estimated that bindweed produces 2.5 to 5 tons per acre of roots and rhizomes. Canada thistle roots can spread out 15 feet and go 6 to 15 feet deep. Tilling areas with bindweed and Canada thistle and other perennials that spread by the roots is counterproductive. As you cut up the root system, you are creating new starts that produce new plants.

So, what does all this mean to you? One of the best ways to help manage weeds is to eliminate or greatly reduce seed production. Just cutting off the seed head prior to the plant producing viable seeds reduces the weed problem you are going to have in the future. Don’t till the soil where you have perennials that spread by the root system. Tilling may bury weed seeds, but it also brings up other seeds that were previously buried. The once buried seeds are now at the right depth to be able to germinate.

Stopping seed production and not cutting up perennial root systems may not eliminate your weed problem but it’s a good place to start.

By Sharon Bokan, Colorado State University Extension, Boulder County. Sharon is the Small Acreage Coordinator at Colorado State University Extension Boulder County. For more information, call 303.678.6176, e-mail [email protected] or visit