You can tell how long someone has been trying to restore native sites by whether or not they use herbicides. Some people have never had to deal with hundreds or thousands of acres of pampas grass (Cortaderia jubata), veldt grass (Ehrharta calycina) or Brome (Bromus spp.). You can't control weeds in a weedy hillside by talking about environmental sensitivity and using volunteers. You'll be out there by yourself within a week or so. The weeds (We call them the “Borg”) will win. Look at herbicides as a possible antibiotic for an ecosystem. If the ecosystem has a staph(weed) infection, a small infection is tolerated; a large infection needs antibiotics (herbicides). Some parts of California look like a rest home in a leper colony for Aids victims; the ecosystem has one native plant left, and no one wants to use chemicals. When it comes to weeds, everything is lesser of evils, treat the ecosystem with the most specific antibiotic (herbicide), bandage (mowing), surgical removal (grazing), skin grafting (native planting) you can. A grass- killer for weedy grasses, a star thistle- killer for star thistle (Centaurea spp.), some pre-emergent herbicides for early weeds; all have their place in the war against weeds. If you want to save the world, protect the clean native sites from mismanagement and any, any, introduction of alien species. If the 'infection' is small, KILL it!
Note: A very important reminder: know the plants you are trying to remove. If you do not know the plants, find someone that does, or you may be killing the exact species you are trying to save!
Trappe et. al., lists a lot of different sprays and their effects on mycorrhizae; most herbicides are not toxic to the soil microorganisms. The most commonly used spray in restoration is probably Round-up. Round-up (glyphosate) works by limiting enolpyruvylshikimate phosphate synthase (EPSP), an enzyme in the shikimic pathway (Levesque & Rahe). The shikimic pathway produces precursors for lignin (Lehninger), anthocyanins, flavonoids, and isoflavonoids. Lignin is produced by the roots of plants to protect them from pathogenic invaders (Allen, et. al., 1992). Our experience has been that some plants that are more community-dependent will tolerate some low doses of Round-up. We know now that mycorrhizal plants do not need to generate as much lignin to combat invaders because the associated microorganisms protect the roots. Other studies showed low doses of Diphenamid (Enide), Napropimide (Devrinol), Oryzalin (Surflan), and MCPA did not have negative effects on native plants. Just think before you spray. We have heard of lots of problems with people spraying Roundup and killing the weeds and the native plants because they did not spray very carefully, because they had no experience with spraying, or sprayed on a windy day, etc. We've generally had no problem with pre-emergent herbicides, but major problems with insecticides, fungicides and soil sterilants. Pre-emergent herbicides can do wondrous things to a declining ecosystem. Spraying herbicides is one good method of removing the weeds from a native plant habitat.
If you have a large area, that you cannot control with sprays and the area is flat and sunny, grazing is another tool you can find useful (though horses do ravage the Needle Grass (Nassella spp.) and the Venus Thistle (Cirsium occidentale var. venustum), but they love the filaree (Erodium spp.). Many areas that we have seen grazed, though, in terms of native plants, are highly disturbed and look poorly, with a lower diversity of native plants and introduction of certain ruderal weeds (Carduus pycnocephalus comes to mind). If the animals are removed, the weeds return with a vengeance, because the soil disturbance caused by the animals tromping around, makes a perfect seed bed for the weeds, the manure encourages weeds and no animals are there to eat the weeds. The cycle of grazing is this: animals-soil disturbance-weeds-animals, etc. The animals from Europe and Asia (cattle and horses) did not evolve here, and are too large and overpowering for this delicate ecosystem with the Mediterranean climate. Before the invasion of weeds, California's vegetation did not support large herds of massive grazing mammals such as cattle and horses. Central and southern California's vegetation did support herds of smaller hoofed mammals that grazed in the spring and browsed in the summer and fall and winter. The grasses and the wildflowers were grazed in the spring and the shrubs were browsed the remainder of the year. But: if the weeds are already established in the area, and you have a large area to manage, the grazing animals can be very helpful in keeping the weeds trimmed so that a fire danger is lessened and you will have some wildflowers. Long term, though, you will have degradation of the site.
In your area you will have some of the same and some different invasive weeds and, if you carefully learn the biology of the plants, you will learn how to manage your area, learn the best method for killing the weeds with minimal effect on the native plants. Before you spray anything, however, know the plants in your area, their life cycle, when they germinate, the conditions under which they grow (sun or shade, for example), whether they are annual or perennial, etc., so you do not inadvertently kill a plant you are trying to save! Like a patient with fever and pneumonia after an antibiotic treatment, a few correctly applied treatments with specific herbicides, under specific conditions, and the results have been astounding! If you still are worried about herbicides there are alternative ways to remove/control weeds such as mowing, grazing, or pulling by hand.
Note: we have heard of farmers that have sprayed herbicides WAY ABOVE the recommended dose on hundreds of acres for many years and now the water table is contaminated. The moral is: the ecosystem can break down these compounds into simpler, natural compounds, up to a point; go beyond that point, and that is when you are doing damage to the ecosystem. For a few years, applied only at the RECOMMENDED DOSE, herbicides are useful and helpful, in the war against weeds. If you overdose the area with herbicides and and apply them indiscriminately, they are a problem, like anything else.
1. Do a survey of the area first; is there shade or sun, trees or annuals?, etc.
2. Identify which plants to save and which plants to remove.
3. Target specific weeds, in your management plan.
4. Remove the weeds with no soil disturbance if possible.
Mowing- best for sunny, flat areas (not under trees)
Grazing- best for already disturbed and invaded (by weeds), large, flat areas
Specific Herbicides- best results have occurred where there were small areas of infection, where there was still a partial plant community present (for example, under oaks and pines.)
Removing weeds by hand below the crown with pruning shears (on older weeds), or pulling carefully by hand (for those of you with small areas, and/or lots of determination and time).