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The 10 Weeds You Don't Want To Know

December 2009
 
by Glenn McGourty
 
 

Definitions of weeds include “a plant out of place” or “a plant we haven’t figured out the value of.” These definitions are in some cases too kind, and doubtlessly the work of phyto-apologists. From my anthropomorphic perspective, some weeds are crafty, cunning and downright spiteful—not to mention potentially injurious (poison oak and Himalayan blackberry come to mind.)

Studies in my early career as a botanist convinced me that some weed species have extraordinary “fitness.” They show great agronomic vigor (grow readily and exuberantly from seed), prolific seed production and multiple strategies for reproduction (seeds, stolons, rhizomes, tubers). These weeds may also have amazing competitiveness for space, light, water and nutrients. Finally, complex genetics such as polyploidy (multiple chromosome numbers) impart to them extra vigor and adaptability, making them super plants.

Regardless, weeds are something that every agricultural crop producer must manage or control. More chemicals are used to control weeds than either insect pests or diseases, when you consider all the farms in the United States. This makes sense, since weeds are likely to show up every year whenever you have the conditions ideal for growing crops.

In most vineyards of the western U.S., the largest group of weeds by species and total biomass are winter annuals. These plants germinate in the fall, overwinter as low-growing basal rosettes of leaves, and then rapidly grow and bloom in the spring. By early summer, they are fairly well done growing, set seed and dry up.

Most of these plants are well enough behaved, and some winegrowers are happy to manage them as a self-reseeding cover crop simply by mowing whatever happens to grow between the rows of their vines. These species grow mostly when the vines are dormant, and they are quite good at preventing soil erosion in many cases. Weeds such as filaree, mustard, annual brome grass and fescue, bur clover and others come to mind as naturalized plants that for the most part are not difficult to manage in a vineyard and make good cover crops.

The real problems come when weeds grow at the same time as the crop, because then they become competitive. We mow, cultivate and spray weeds to keep them from growing at the same time as the vines. We also keep the weeds under control to reduce habitat, food and cover for rodents, such as voles and gophers. Finally, we like the clean look of a vineyard in which everything is manicured or cultivated.

Because the weed flora is so diverse and resilient, there always seem to be one or two species that manage to defy our efforts. Research has shown that multiple control strategies are best when it comes to suppressing weeds.

USDA scientist Dr. Kendra Baumgartner’s research in Napa Valley on sustainable approaches to weed control showed that a glyphosate (Roundup) spray followed by under-the-vine cultivation is more effective at controlling weeds than just spraying multiple applications of glyphosate or multiple passes of under-the-vine cultivation.

Similarly, continued use of the same herbicide combination invariably leads to one or two resistant or tolerant weed species gleefully propagating themselves. Before long, the neat tidy bare strips beneath your vines look like you didn’t even spray.

Effective herbicide applications
Choosing the right material, applying at the proper rate and putting on herbicides at the right time are all critical for effective weed control. Most growers prefer to use the lowest efficacious amount, as a matter of cost control and environmental stewardship. Remember that soils high in clay and organic matter need more pre-emergent materials for an effective application, since the herbicide is going to be bound more tightly by these soil components. Sandy soils require less material, and in fact, leaching may be a potential problem if you over-apply pre-emergents: You could unintentionally damage your vines if the material moves into the root zone.

My studies and experience have shown that you will get better results in high-rainfall areas by waiting until late in the winter to control most weeds—early applications of pre-emergent herbicides.

The downside of this is that you will most likely need a post-emergent herbicide in the spray as well, since the weed flora already has germinated. On the other hand, in low rainfall areas, early applications before the leaves fall off the vines may be more effective, since the pre-emergent herbicides are more likely to come into contact with the soil.

In fact, UC farm advisor and weed control specialist John Roncoroni has shown that blowing fallen leaves out of the way as you apply herbicides beneath the vines can improve control of many weed species. (Just borrow a front-mounted blower for your tractor from a friend who grows walnuts or almonds.) On the other hand, Roncoroni has seen that removing the leaves also allows some weeds to germinate better—especially if they are tolerant of the herbicides you are using (more on that later).

If you are having a problem with your weed-control program, it is an excellent idea to consult with a local pest control adviser, technical representative of the herbicide manufacturer of the products you applied—or a farm adviser to troubleshoot why your application didn’t work. Keep good notes on the application in terms of what you mixed into the tank and with how much water, how fast you went when you applied the spray, and the weather at the time of the application.

Nonchemical approaches

Organic and sustainable growers rely on tillage for weed control. Every piece of equipment has its own quirks and range of effectiveness in terms of the size and type of vegetation it can handle.

In general, you want to go after weeds when they are small. The problem is that sometimes the vineyard is too wet to run equipment without getting stuck, compacting soil or smearing the soil under the vines. Using Dr. Baumgartner’s principle of more than one control strategy, some growers have had good success using propane burners or flaming to sear the small weeds when they have barely emerged early in the winter. A lightweight tank pulled by a small ATV with low ground pressure can be very effective.

Relying on flaming alone is probably too expensive for a complete weed-control program, but it is quite effective to start a program when weeds are small and tillag e with larger equipment may be difficult. Remember, the object is to sear small weeds—this is flaming, not flame throwing. If you wait too long, you will either not control your weeds or set your vineyard on fire, and probably get a visit from the Air Quality Control Officer in your county, none of which are good options.

When weather conditions improve and the vineyard becomes dry, then under-the-vine tillage equipment will need fewer passes, and be more effective during each pass, since the weeds will be fewer and smaller in size.

Other growers have had great success employing sheep to graze the vineyards during the winter, when vines are dormant. They use portable electric fences to “mob stock” the sheep, confining the animals to a small area for a limited amount of time so that they munch on everything. If they are allowed free choice, the sheep become fussy and tend to leave plants that are less palatable.

When the sheep are removed from the vineyard following bud break, the weeds beneath vine rows are much easier to control with tillage equipment and don’t require the winter passes to go after the weeds when they are small and the ground is potentially too wet for ideal control.

Troublesome weeds

In the following paragraphs I’ve listed some of the most troublesome weeds that we see in California, descriptions about why they are a problem and what you can do to control them.

Johnson Grass Sorghum halepense: This weed is like crab grass on steroids. It is a perennial plant that reproduces readily from seed as well as coarse rhizomes. It grows to 6 feet tall and is tough when it gets mature. Seed may arrive by water through surface water irrigation systems, on machinery (especially mowers), manure or poorly made compost. It usually is a problem on better agricultural soils, especially in the Central Valley. It is reported to be resistant to glyphosate in some areas. The best control strategy is to hit it with everything you’ve got, including glyphosate (if your local population is still susceptible), selective grass herbicides, repeated close mowing or under-the-vine cultivation, weed eating and hand hoeing. Don’t let this one get away from you, or it will actually devalue your property during a bad infestation.

Bermuda Grass Cynodon dactylon: This is an example of a weed with great potential to spread—it grows by rhizomes, stolons and seed. It is a warm season perennial grass, so it tends to be a problem on better soils that are irrigated. It behaves much like Johnson Grass. It forms a dense turf and can be very competitive with vines. It may also harbor Pierce’s disease, so it is not something you want in your vineyard. It is spread around just like Johnson Grass.

Repeated tillage is an effective technique for nonchemical control, but the problem is that most under-the-vine implements leave patches of this weed around posts and stakes. Hilling the soil first, followed by a hoe plow is more effective than implements that just cut vegetation off below the surface. This weed can be effectively controlled by glyphosate, especially when applied to actively growing and flowering plants. Fall applications also are effective if the grass is not drought stressed. Repeated applications will certainly be required, due to this weed’s multi-faceted reproductive strategies.

Sharppoint Fluvellin Kickxia elatine: This summer annual is resistant to many pre-emergent herbicides, as well as glyphosate when it is more mature, perhaps due to its general hairiness, which makes it difficult to wet. It germinates over many months. Mostly it is a problem in vineyards utilizing herbicides to control weeds beneath the vines. It is not hard to control with cultivation when small. As this weed matures, it can form dense mats that get really balled up in tillage equipment. John Roncoroni found that removing leaves beneath the vines to increase herbicide effectiveness on other weeds made this one grow better, because there was better survival of herbicide-resistant fluvellin seedlings.

Horseweed Conyza canadensis: This biennial and summer annual is a prolific seed producer that is resistant to many pre-emergent herbicides. Seeds are dispersed by wind. It is also difficult to kill with glyphosate. It is not a problem when under-the-vine cultivation is used. Uncontrolled, it can grow up to 6 feet tall into the vine canopy and creates a real problem for vineyard operations. Newer herbicides such as glufosinate-ammonium (Rely) and flumioxazin (Chateau) keep it in check.

Tall Annual Willowherb Epilobium brachycarpum: This is a summer annual that is also a prolific seed producer. Its seeds disperse by wind. This weed is a problem because it is resistant to many of the widely used herbicides for vineyard weed control. It can be controlled easily with tillage and by newer herbicides such as glufosinate-ammonium and flumioxazin.

Rigid Ryegrass Lolium rigidium: Glyphosate-resistant strains of this species make beneath-the-vine herbicide applications ineffective. It also is quite competitive with vines if allowed to mature, and can produce copious biomass, making its presence a fire hazard for the area. It can also be a problem for under-the-vine cultivation, because the fibers in mature stems and roots are quite strong and don’t break easily. It remains susceptible to selective grass herbicides and can be controlled by tillage when small.

Shortpod Mustard Hirschfeldia incana: This is a biennial or short-lived perennial that is tolerant of herbicides (it looks like it is going to die and then recovers). A prolific seed producer, it has a tough taproot that makes it difficult to control with tillage. It is not super-competitive with grapevines, but it sure makes a mess out of your otherwise clean herbicide strips beneath the vines.

Puncture Vine Tribulus terrestris: This is a prostrate summer annual that thrives under tilled and irrigated conditions. The sharp, pointed seeds get into shoes, bicycle tires and pets’ paws, and are readily moved around this way. There is a seed weevil that can greatly reduce its populations, and it doesn’t do well in non-tillage conditions, as it is not highly competitive with more upright plants. It can be controlled effectively with tillage. Physically removing the weed and destroying it on sight is the best way to keep it from infesting your vineyard.

Yellow Star Thistle Centaurea solstitialis: The plant you love to hate, it starts as an innocuous looking basal rosette of leaves and morphs into a spiny, tough, dominating weed that creates a fire hazard and interferes with vineyard operations. It is a winter annual. If you mow it, it seems to just hunker down and continue to flower. When small, it is easily controlled with herbicides (glyphosate, 2, 4-D), grazing (especially sheep) and tillage. Once it bolts, you can still kill it with herbicides, but it remains standing and requires removal, so why bother? It is a prolific seed producer, the wind disperses the seed, and it germinates over a long period of time, just like fluvellin.

Spiny Cocklebur Xanthium spinosum: This is a summer annual with nasty thorns that grows best in places that have been tilled and irrigated. It has seeds with hooks that readily cling to your socks, pants and dog. It is easily controlled when small with herbicides and tillage. Treat it like puncturevine, and don’t let it get started.

Conclusion

I’m sure you have your own additions to this list of bad actors, but the same principles apply to controlling them:

• Try to keep them out of your vineyard to begin with.

• Find out about their life history and when they are most susceptible to control measures.

• Be eternally vigilant: Weed control is a continuous, ongoing task.

Glenn T. McGourty is a winegrowing and plant science adviser with the University of California Cooperative Extension in Mendocino County, Calif. To comment on this article, e-mail edit@winesandvines.com.

 
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