How far can a weed seed travel?

Remember that trip some months back, or even last year, when you went out of state in your pickup truck.  Very uneventful; everything was fine when you returned home. And the next day you continued with your daily chores and drove your truck to several different locations on the farm.  There is no difference in what you did and the reason that bio-security measures are taken on many livestock farms.  Your truck was probably the carrier of a seed, which was picked up many miles away, then germinated on your farm, and suddenly you find a strange patch of weeds you have never seen before that is hard to control.  What you did is akin to downloading a new computer virus.

If you have a new weed appear on your farm, and have no idea what it is and where it came from, you might either blame your pickup truck, or blame yourself for not thoroughly washing your truck before coming home from that last road trip.  You probably are told to take off your boots before coming in the house.  You probably don't have to be told to wash up before dinner.  And washing your pickup truck falls into the same category if you have been out of your normal territory.  You will never know what you are bringing home, and thanks to your truck and some periodic mud, many different weed species are spread around the country without you knowing it.

Several researchers who know all too well what you are doing are from Montana State University, and have published a fact sheet on weed seed dispersal by vehicles that has some eye opening data that will make you head to the truck wash several times a day.  They do well beyond the sins of your pickup truck and also address the use of all terrain vehicles, which can do a much better job of unknowingly spreading unwanted weeds.  They report that passenger vehicles can carry anywhere from 3 to 135 seeds per vehicle, and have spread over 500 different plant species in their research.

Weeds can be carried from remote locations to populated areas, and can be carried farther down the road than where they once were limited.  They report researchers in Chile found that the further roads were extended the more weed species that were spread.  Here in the US, there are 4 million miles of roads, half are unpaved, 80% are rural, and many pickup trucks and outdoor recreational vehicles are likely spreading weeds where they have not previously been.

The researchers conducted several different studies to prove their point, washing dirt and mud off vehicles, cultivating weed seeds in a greenhouse, and then identifying them and where they originated.  One study used a National Guard Humvee, which was washed spic and span at the start, allowed to accumulate dirt and mud on its exercises, and also outfitted with a GPS to record specific locations.  That allowed weed species to be identified per mile traveled, and even a comparison of wet and dry conditions, since there was rain in the middle of the exercises.  The results indicated an average of 1,700 seeds were picked up every mile, and nearly all were exotic species, when the tests were conducted in the spring.  Fall tests gathered over 5,000 seeds per mile driven, about half of them exotic species.  In both cases the seeds were gathered from off-road conditions.  The Humvees picked up 14 times more seeds when conditions were wet than dry. 

The researchers found that over 90% of the weeds stayed attached to the vehicle after traveling 160 miles in dry conditions.  On unpaved roads the weed seed retention was 86% due to rocks knocking them off.  Only 37% of the seeds remained on the underside of the vehicle in wet conditions after an 80 mile distance.  Seeds of different sizes and shapes were retained or lost at the same rate.  When weed seeds are picked up, coated with mud, they can travel for indefinite periods unless water becomes involved or the vehicle is washed.

So if washing your vehicle is the key to preventing it from bringing home unwanted passengers, how should that happen with the greatest degree of success?  The Montana State University researchers have created another factsheet outlines such methods, and makes some recommendations.  In fact a Presidential Order directs federal agencies to take whatever means to minimize transportation of non-indigenous species, but does not indicate best management practices.

The researchers sprayed a known quantity of wet soil onto the underside of a clean vehicle and washed it for various lengths of time. The tests were repeated and remaining soil analyzed each time after the test wash.  Numerous variables entered into the tests and those were also evaluated.  They concluded that the longer the period of washing the more soil that was removed.  Because of the additional soil that was physically removed after the test wash, they found that three washes would remove nearly all of the dirt.  Commercial washers all perform about the same.

Summary:weed seeds from sources either known, unknown, desired or undesired can easily become attached to a vehicle and remain with the vehicle for great lengths of time.  Weeds have been known travel great distances while attached to vehicles.  Mobility and durability make them ideal for traveling great distances in small areas

Source: the FarmGate blog