Weed Seed Dispersal

The Muddy Boot Weed Seed Dispersal Method Tall waterhemp is one of the most problematic weed species throughout the Midwest and has now arrived and spread to eight counties in Upstate New York. Weed control, management, ecology, and minutia Vegetable integrated pest management weeds view page

The Muddy Boot Weed Seed Dispersal Method

Tall waterhemp is one of the most problematic weed species throughout the Midwest and has now arrived and spread to eight counties in Upstate New York. Waterhemp can spread from field-to-field and farm-to-farm on equipment, clothing, application equipment, or via water from over flooded ditches and rivers. Following a recent field day event we wanted to demonstrate the amount of weed seed that could travel back with you.

Boots that were considered “clean” were not as clean as we had thought (Figure 1). A knife was used to clean the boots and break up any hard clots that were present. Once the boots were clean, tweezers were used to separate the weed seeds from the dirt (Figure 2). The pigweed/waterhemp seed was then separated from other weed seeds that were present, and pigweed seeds were counted (Figure 3). The clods of dirt were also checked, and one pigweed seed was found stuck to a clay particle (Figure 4).

Figure 1: Muddy boots – Photo: Josh Putman Figure 2: Tweezers used to separate weed seed from dirt – Photo: Josh Putman Figure 3: Seeds were separated and counted; 17 total pigweed seeds – Photo: Josh Putman Figure 4: One pigweed seed hidden in a clay particle – Photo: Josh Putman

An estimate of a 3 year establishment of waterhemp assuming 50% of the seeds were waterhemp and 100% were waterhemp was then calculated, respectively. The calculations are seen below:

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16 pigweed seeds + 1 pigweed seed hiding in soil = 17 pigweed seeds from 2 boots.

Assuming only half of those are waterhemp and it can produce 250,000 seeds per female plant: 17/2 = 8.5 X 250,000 = 2.125 million seeds the following year in a field.

Assuming every seed on the bottom of the boots are waterhemp: 17 X 250,000 = 4.250 million seeds the following year.

Assuming 75% survival rate and reproduction in year 2: 4.250 million X 75% = 3.1875 million plants X 250,000 seeds per plant = |

**796,875,000,000 seeds going into the soil in year 3 (potentially)

In conclusion, correct and early identification is very important; learn the correct characteristics of the plants (Figure 5) and seeds. Proper cleaning and sanitation of equipment, clothing, and vehicles can help prevent spreading. Intense management and continuous scouting are vital to eradication of this weed species. Mechanical control such as plowing can bury the seed deep which might decrease seed bank numbers. And, if in doubt, contact your local CCE specialist for help with identification or other management practices.

Weed Seed Dispersal

I was out in the field today and drove by this fallow field and did two things. First, since I’m a weed scientist, I stopped to take a photo of this weedy streak. Second, because I know the site manager and know he hates weeds, I called to give him a hard time about “missing one”.

Any guesses on what happened here? It might help if I told you I’m facing south and this area often has a prevailing wind out of the northwest during winter and early spring. Another clue would be the construction site to my back with minimally managed weeds. One more clue might be the fence behind me still loaded up with Russian thistle carcasses and debris.

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Here’s a link to the UC-IPM Pest Note (Pub 7486) on Russian thistle. That document indicates ” Russian thistle normally matures in late summer. The seed is spread when mature plants detach at the base and are blown along by the wind. A large Russian thistle plant may produce more than 200,000 seeds. In spring, months after their dissemination, it is possible to trace the paths of tumbleweeds across plowed fields by the green trails of germinating Russian thistle seedlings .”

I think we have a winner! Looks like a handful of Russian thistles, one of our common “tumbleweed” species in California went rolling across this landscape earlier in the spring scattering seeds as it made its merry way to the next fenceline.

Good photo. Probably one of the “tumbleweeds” meandering across the field and spreading its seed on the way. FYI, Harry Agamalian has a similar great photo meandering across a seedling vegetable field in the Salinas Valley.

Weed Seed Dispersal

The tolerance for weeds in vegetable crops grown in this area is very low and some fields are kept almost weed free. It is hard to understand why so any weeds appear the next season in fields that are kept weed free. There is no simple answer to this question. Part of the reason lies in the fact that weed seed, unlike crop seed, does not germinate all at the same time. Some may germinate immediately and some may not germinate for years. Another reason is that weeds produce millions of seeds and these move from field to field. In an area like this where irrigation is so intensive, water is moving into and out of fields all of the time. Most weed seeds are very small and float. There is just no practical way to filter small seeds out of large volumes of water. Filters that are capable of removing small seeds restrict water flow too much. Water would have to be diverted from the ditch or canal to successfully filter the seed. After water, wind is probably the next major means by which weed seed is dispersed from field to field. Some weeds have developed structural characteristics that allow them to blow long distances.

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The composites like sowthistle, groundsel, prickly lettuce and others, have fuzzy seed heads that blow in the wind. Some seeds have spinners, gliders and other structures that help them travel. All of the tillage, cultivation and spraying we do provide a mechanism for weed seeds to be moved mechanically. Sandbur, puncturevine, clovers and other seeds are hard to keep off equipment, boots and clothes and are difficult to control. Birds can disperse weed seed, tubers and bulbs. Some eat and defecate seeds, fruit or pond sediment that can be deposited in other fields. Seeds can attach feathers, claws and beaks. The physical damage that birds do to lettuce and Cole crops is far worse than spreading weed seeds. They pull young seedlings out of the ground and cause direct crop loss.