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Weed Research & Information Center
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Weed Research & Information Center

Posts Tagged: invasive weeds

(Weed) Science and Replication

I have had the pleasure to conduct a variety of experiments, watch numerous management talks and take many classes on the scientific method. During these adventures I've noticed that people seem to misunderstand (or not comprehend the full power of) replication.


Replication is the repetition or duplication of an experiment. On the surface this seems pretty straightforward; why would someone want to replicate or duplicate an experiment? They do it to double-check that the results are correct.


If I want to test to see if hummingbirds prefer red or yellow feeders, I can put out two colored feeders and count the number of visits. Great! Right? Not exactly.


The experiment is not replicated. What if one feeder is closer to something the hummingbirds do not like? They would avoid that feeder and prefer the other no matter what the color. Replication solves this problem.


Professionals get it wrong sometimes too. I just visited a colleague who has a growing facility with 3 replicate greenhouses (3 repeated greenhouses in a row). One of the researchers using his facility had all the trials in one greenhouse. It was the south-facing greenhouse. What if the south-facing greenhouse was slightly warmer than the average greenhouse? They will not know if this influenced the result.


How many weed trails have we seen that have been conducted at a site, spray herbicide A here, herbicide B there. Done. Great! Right? Not exactly. It certainly helps to have the initial trial, and its even more helpful to replicate the experiment.


Replications can occur across space or time (this sounds like Star Wars), and on the ground it means experiments can be repeated over a variety of distances, or over different seasons.


Replications Across Space

Conducting an experiment is good; conducting the same experiment (especially when it comes to weed management) in multiple field sites, counties or states is even better. There are more herbicide resistant weeds in the Central Valley than outside that area. Same species, different resistance. How do we know? Replicated experiments across the state.


Replication Across Time

Weed scientists like to conduct herbicide trials, and I do them myself, and how many of them are conducted every year for 5 years at the same site? This type of replication allows one to determine if the result is the same during different years or seasons. Some populations of hairy fleabane (Conyza bonariensis) are resistant to the herbicide glyphosate during the summer, (they will not die when sprayed) but when those same resistant plants are sprayed during the winter they are susceptible (see footnote*). This is fascinating! Spray in summer and it grows like a weed, spray in winter and growth is reduced. The only way this was discovered was by replicating experiments across time; applications were made in spring, summer, fall and winter. On a side note the researchers in charge of this project thought they mixed the resistant with susceptible seeds and repeated (i.e. replicated) the experiment several times until they understood the results!


Replication Across Space and Time

It's the gold standard; the researcher conducts an experiment at multiple sites and repeats it over different durations. It also takes a lot of work. One simple experiment now has multiple sub-experiments. But you know how well it really works.


In Conclusion

Just because we see weed trials doesn't mean it has been fully vetted. Simple trials are good, they teach us a lot. Multiple trials are better. Control plot, treated plot, good. Control plot, treated plot, repeat, repeat, repeat here, repeat there, repeat next year, and repeat in the winter, ...even better.




*Note: I am not advocating spraying resistant hairy fleabane with glyphosate, you should consider an IVM (or IPM) program to tackle this problem. 

Posted on Monday, August 25, 2014 at 2:28 PM

Cal Ag article: Predicting invasive plants in California (by Brusati et al.)

Link to a recent (July 2014) research article in California Agriculture (link to table of contents for volume 68)


The article by E.D. Brusati, D.W. Johnson, and J.M. DiTomaso is entitled "Predicting Invasive Plants in California" focuses on risk assessment modeling of plants under consideration for importation through the horticultural industry.


Preventing plant invasions or eradicating incipient populations is much less costly than confronting large well-established populations of invasive plants. We developed a preliminary determination of plants that pose the greatest risk of becoming invasive in California, primarily through the horticultural industry. We identified 774 species that are invasive elsewhere in Mediterranean climates but not yet invasive in California. From this list, we determined which species are sold through the horticulture industry, whether they are sold in California and whether they have been reported as naturalized in California. We narrowed the list to 186 species with the greatest potential for introduction and/or invasiveness to California through the horticultural trade. This study provides a basis for determining species to evaluate further through a more detailed risk assessment that may subsequently prevent importation via the horticultural pathway. Our results can also help land managers know which species to watch for in wildlands.

Link to full article in California Agriculture


arundo donax from CalAg68 pg89 DiTomaso
arundo donax from CalAg68 pg89 DiTomaso

Posted on Tuesday, August 5, 2014 at 3:40 PM

The Cooperative Mule

From the Invasive Plants in Southern California blog :: Nov. 22, 2013


Hi All. In my last blog the subject was about what I had learned regarding the use herbicides as a tool for effective passive restoration of CSS and native grassland habitats. As an Extension Advisor my job is to not only develop new information, but to also try to move it forward into practice.

In this case that means educating land managers and related professionals on how to efficiently and accurately apply herbicides to large areas. That's what the Cooperative Mule is all about, so sit back, I hope you enjoy the show. By the way, Ryan Krason, our Digital Media Specialist produced this video and came up with the name; I really think he did a great job, so thanks Ryan.

The video [CLICK HERE] is just a general introduction to the potential to use an UTV or ATV for restoration work. I have captured the same information in a PDF that is available on my website at -- look for UTV Sprayer System. As always, if you have questions or comments, please post them below or email them to me at And if you are finding this blog useful, please subscribe and please send me ideas for blog topics.

Please follow me on twitter @CarlEBell.

Posted on Saturday, June 21, 2014 at 11:14 AM

Weed Seed Germination

I'm sure you've noticed that different years have dominant weed species. For example this year in Southern California Russian thistle (aka tumbleweed Salsola tragus among other Salsola species) is abundant in many areas. Other years it is only moderately abundant and some years it seems to hardly hang on. The obvious question is why? Why is there so much variation in abundance between years?


Fortunately for us people have been working on this issue for a while and have a few general answers for us. Since the bulk of weeds in California are annuals I'll limit my discussion to those plants.


The seeds of annuals germinate only under a certain range of environmental conditions. For example, many winter annuals in California germinate when temperatures are cool or cold. Sahara mustard (Brassica tournefortii) does not germinate well when the temperatures are above about 85F. Other plants do not germinate when the temperatures are cold. You can see this pattern on THIS page of common garden vegetables by Cooperative Extension in Alabama. There are cold season plants (onion) and warm season plants (tomato, melon).


Although we have only one rainy season in California, and sometimes less than that, we still have cool season weeds and warm season weeds. Most of our weedy grasses (like bromes) are cool season, and our late season weeds (like tumbleweed) are warm season. Of course there can be overlap, warm temperatures in the fall can provide suitable conditions for warm season weeds, even though they ‘normally' germinate in the spring.


Our ability to predict which weeds will be abundant is hampered by many things; most of all is insufficient data. We know plants respond to soil moisture, soil temperature, oxygen levels in the soil, light levels (seeds can ‘see' light!), scarification (abrasion of the seed coat), soil salinity, age of seed, age of parent plant, and many other factors.


Imagine we tested to the germination conditions of 3 weed species for 3 factors (moisture, salinity and light) and each factor at just 3 different levels (high, medium or low), and we replicated this test in 3 different soil types (sandy, loam, silt). In the end, we would have 81 different trials for this simple experiment (weed #1 at high light in loam = 1, …). Barring a large infusion of money to conduct experiments to predict weed germination, and thus examine one factor in the spread and increase in weeds, we will be using coarse measures to figure out the population of weeds.


Back to my original question, why was there a lot of tumbleweed this year? Because tumbleweed germinates better in conditions of high light or disturbed soil, with relatively warm temperatures and the seed germinates rapidly, effectively using small amounts of moisture. Given there was limited rainfall early in the winter, which limited production of many cool season weeds, and moderate moisture in the mid spring coupled with a relatively warm winter, conditions were rife for the establishment of tumbleweed.

UC IPM page for tumbleweed is HERE

Posted on Wednesday, June 18, 2014 at 11:29 AM

Tips on grazing for invasive plant control

Some tips on grazing for invasive plant control

Using livestock for controlling invasive plants has a lot of appeal; the animals seem like a natural, green method; they're cute; and at times they can be a very inexpensive way to do some weed control. But there are also various difficulties and issues with using livestock that should be understood before you jump into a grazing program, I've discussed some below. 

Livestock have different eating preferences and needs; Cattle (photo of cattle courtesy of Jack Kelly Clark, UCANR) like grass, sheep like grass and forbs, goats like browse (foliage on stems of woody shrubs, young stems and bark, like photo), and horses like grass.

They eat differently; cattle pull up plants with their tongues, sheep and horses clip off plants close to the ground, and goats pull foliage off of stems with their tongues.

The owner of the livestock (called the grazier) usually has a different goal for the animals then you do. The grazier wants the animals to fatten up for the market. You want them to eat weeds. If the weeds are what the animals want to eat and they are nutritious, then your goals are compatible. It is more often the case that the weeds are not desirable and the animals will try to eat everything else instead.

The grazier wants the animals to wander freely over a large area. You probably want the animals to be confined to a smaller area to maximize weed control. This requires fencing, which is a cost to the grazier. It also means the animals are being forced to eat plants they may not want to eat (Kind of like being forced to stay at the table and finish all of your lima beans when you were a kid).

 The grazier wants to turn the animals out to graze when she/he arrives at your site. You want the animals to stay in a penned area for 2-4 days eating hay and voiding all of the weed seed from their last site. You also want the pen in case of rain, rather than allowing the animals to wander around in the mud and trample small plants.

Livestock do not often eradicate weeds; they just remove above ground stems and foliage. The San Diego Weed Management Area tried goats for control of perennial pepperweed (Lepidium latifolium) a few years ago. The grazier promised that the goats would eradicate the weed if they were allowed to graze it twice in the summer for two weeks each time. They rented the goats to the WMA for $300/acre. The goats removed about ½ of the stems and foliage and a year later it looked like it had never been grazed.

Graziers are used to driving hard bargains, so make sure you know what you want and are ready to insist on it.

Don't get me wrong, livestock can be very effective and useful in large-scale weed management if they are suitable consumers of the target weed(s) and they are managed properly. I collaborated with several UC scientists to study sheep grazing as a weed control practice in newly planted alfalfa in the low desert. The sheep were very successful, as good as the herbicide alternative. Citations for three of our papers are on this website page (near the bottom). This short blog only presents some of the issues, especially those I have personal experience with. After spending eight years studying sheep grazing, I like to say that I know far more about sheep than I ever wanted to.

And they really are cute.
Posted on Monday, February 24, 2014 at 3:36 PM

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