Posts Tagged: invasive weeds
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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
Scotch broom (Cytisus scoparius) and Portuguese broom (Cytisus striatus). Brooms were introduced as ornamentals, but also were used extensively for erosion control along roadsides and in mined areas.
Now growing profusely in California forests, on roadsides, and wildlands, brooms:
- Crowd out out desirable vegetation
- Form impenetrable thickets that limit access to some areas
- Shade out tree seedlings, and make reforestation difficult
- Burn readily, increasing the intensity of fire, and carry fire to the tree canopy
- Are toxic to cattle and horses and unpalatable to most wildlife
- Produce abundant, long-lived seed
- Are able to fix atmospheric nitrogen, giving broom a competitive advantage over native plants
Management of these and other weeds are presented in the recently published second edition of Forest and Right-of-Way Pest Control. Invasive species that create a dangerous wildfire hazard and crowd out desirable vegetation and wildlife are examples of why this book emphasizes vegetation management and pesticide handling, including correct equipment calibration and effective herbicide application. The second edition also provides broader coverage of insects, plant pathogens, vertebrate pests, and the various practices to manage them, recognizing that lands commonly have multiple uses and when and how pests are managed depends on many considerations with sometimes conflicting goals.
Experts with Cal-Fire, Caltrans, PG&E, USDA Forest Service, private industry, the University of California (UC) Berkeley and Davis campuses, UC County Cooperative Extension offices, and the California Department of Pesticide Regulation (DPR) contributed to Forest and Right-of-Way Pest Control, prepared by UC ANR's Statewide Integrated Pest Management Program.
Forest and Right-of-Way Pest Control is available for $35 online in the UC ANR Catalog. The table of contents and more information about the book are available on the UC IPM website. You can also preview and electronically search the contents on Google Books.
Some tips on grazing for invasive plant control
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 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.
You just KNOW that some plants are weeds. Their common names give it away. They sound awful. Giant hogweed (Heracleum mantegazzianum). Ripgut brome (Bromus diandrus). Devil's claw (Proboscidea lutea). Smellmelon (Cucumis melo). Itchgrass (Rottboellia cochinchinensis). Dog-strangling vine (Cyanthum rossicum).
Others...others seem more benign. Even sweet. For Valentine's day I present to you nine weedy plants with lovely names. Enjoy...
1. Tree-of-Heaven (Ailanthus altissima). How celestial. How divine. How invasive. Tree-of-heaven is a deciduous tree, native to China, in the Simaroubaceae. It has been used, extensively, as a street tree; in fact it is the subject of A Tree Grows in Brooklyn by Betty Smith. But it suckers. A lot. And it stinks. A lot. And it spreads, rapidly, by seeds (lots and lots of seeds!) and root-sprouts. Some urban areas have even renamed it 'Tree-of-Hell'.
2. Baby's breath (Gysophila paniculata). Baby's breath?!?!? Baby's breath?!?!? That delicate plant in the Pink family that is beloved by florists? You would have to work hard to make up a sweeter sounding name for a pest! Like 'Fuzzy kitten herb' or 'Baby giraffe weed'. Or 'Mouse ear chickweed' (wait a minute, that last one is real...). But baby's breath can become invasive in certain habitats, such as Sleeping Bear Dunes in Michigan, where it escaped from gardens.
3. Amaranthus spp. The origin of the genus is derived from the Greek words marantos (unfading) and anthos (flower). Unfading flower. A perfect metaphor for love. Well, I've got two words of my own for you: PALMER AMARANTH. Enough said. (To be fair, many Amaranths are used worldwide as food sources, either leaves or seeds...but my animosity isn't directed at them.)
4. Love-apple (Solanum capsicoides). Sounds delicious, right? If you are talking about a tomato (Lycopersicon lycopersicum), sure. If you are talking about red soda apple (i.e. love-apple, devil's apple and cockroach berry), not so much. The fruits are toxic and have been used in many countries for rodent and insect control.
5. Multiflora rose (Rosa multiflora). A 'rose with many flowers', how can that be bad? HA! The intentions were good when this species was being promoted in the 19th and 20th centuries (ornamental shrub, erosion control, living livestock fence, wild-life food source), but things have gone terribly wrong since then. This shrub forms dense stands that displace native vegetation.
6. Love leaves (Arctium minus). It sounds like you could brew up a nice aphrodisiac from this species. Although parts of the plant are said to be edible, love leaves, better known as common or lesser burdock, is also listed as being toxic in many weed guides. However, burdock doesn't make this list because of its questionable medicinal or culinary virtues, but rather for its seed. The tiny hooks on the seeds of this plant were the inspiration for velcro. Try pulling them out of the fur of a long-haired dog...
7. Heart's ease (Polygonum persicaria). Better know as smartweed or ladysthumb. This weed can grow everywhere, it seems. Seriously. It is even found in Greenland.
8. Love vine (Convolvulus arvensis). Love may keep us together, but you don't want to be in a relationship with this plant, which is better known as field bindweed. This perennial species is listed as one of the most noxious weeds in the WORLD. It has roots that can grow to depths of >10', it reproduces by seed and rhizomes, infrequent tillage just makes it mad, and repeated applications of herbicides are needed to suppress it. This plant also goes by the name possession vine...and you're nobody's property.
9. Bouquet-violet (Lythrum salicaria). People love getting flowers on Valentine's day, right? Maybe you shouldn't send this 'bouquet', though, which you probably know better as purple loosestrife. This plant was introduced, intentionally, to North America as an medicinal herb, but it has since escaped from our gardens and become naturalized. Large infestations can alter water flow in streams and rivers, reduce native plant species diversity and negatively impact macrofauna, such as amphibians and waterfowl, that rely on wetlands for food and shelter.