Nita removes bur clover from around purple needle grass.
Photo by Caroline Christman

Choosing which non-native plants to remove

Caroline Christman

As the wet weather fades away and days get longer, our gardening efforts shift from planting to weeding. While the soil is moist, we pluck out small, young weedy plants before they establish and set seed. Well, at least that’s the goal, but it’s not possible to get everything. And it’s not really necessary to remove every non-native plant from the garden. Which brings us to the all-important question – how do we decide where, when, and what non-native plants to control

This spring I’ve worked with gardeners on weeding projects in several native habitat gardens in Marin. On one sunny hillside in Marin City we focused on bur clover (Medicago polymorpha) and vetch (Vicia sp.) competing with purple-needle grass (Stipa pulchra) and soaproot (Chlorogalum pomeridianum). On the north shore of Tiburon we scoured the small coastal prairie for bristly ox-tongue (Helminthotheca echioides) and Italian thistle (Carduus pycnocephalus) and dug-out the taproot of small fennel plants (Foeniculum vulgare) whenever we saw them. At the Marin CNPS Bay Model Pollinator Garden we focused on panic veldtgrass (Ehrharta erecta) hiding in the shelter of native shrubs and making its way out to fill in all the open soil.

In each garden mentioned the weeding focus was different. We made choices of where and how to work based on multiple factors including: native species or plant community to protect; abundance of particular non-native species; likelihood for a non-native species to crowd out natives and alter the ecosystem; manner and speed at which the species spreads (seeds, rhizomes); most effective methods for control and removal; and difficulty of removing a mature plant.  

Laura Weeding in the Bay Model Pollinator Garden
Photo by Caroline Christman

Protecting Native Plants:
In the spring, new plantings will benefit greatly from controlling weeds near their base.  If irrigation is directed to new plants, weeding is especially important to ensure the weeds are not gaining all the benefits of late-season water.  Some plant communities and locations are vulnerable to invasion even after plants are established. Grasslands, for example, contain many low-growing forbs and have spaces between native bunch grasses that fill with annual wildflowers in the spring. Maintaining a native grassland requires on-going invasive species control to keep weeds from filling in the spaces and towering over the smaller plants. Areas adjacent to roadways, paths, or other sources of weed seeds, are also likely to need continuous attention.

Assessing Potential Weedy Species:
Some non-native species are higher priority for control than others, and this varies from place to place. In wildland settings, some species are well-known for spreading quickly and creating dense monocultures (a large area of a single plant species). For example, French broom (Genista monspessulana) and jubata grass (Cortaderia jubata) are highly invasive non-native species which drastically alter ecosystems. In a garden setting, however, highest priority weeds may not be the same as in natural areas. 

Getting to know the garden and watching it change over time is helpful to understand potential invasive issues. In the Tiburon garden, for example, several years of intensive French broom removal had allowed the remnant grassland to regenerate. We chose to focus on Italian thistle and bristly ox-tongue because we had seen these species spreading into the grassland from a nearby construction site. Both of these species are in the aster family and produce thousands of wind-blown seeds; a few Italian thistles left to go to seed one year may result in hundreds of thistles the following year. We also made sure to remove any small fennel or broom plants we came across because these are much easier to remove when small, and we didn’t want to let even one plant flower and seed. In the Marin City garden, Nita (the owner) had seen the bur clover and vetch populations expanding, also in a grassland. Both are in the pea family (Fabaceae) and are nitrogen-fixers like many legumes. In fact, bur clover is planted for forage and to increase nitrogen in the soil in agricultural settings.  In a native grassland, however, both species can alter the soil and make it more hospitable to invasives. 

Walking the garden regularly, observing new invasive species making an appearance and those trying to take center stage, will help us in recognizing new threats. Learning about the way invasive species alter the habitat, how they spread (seeds, berries, rhizomes) and how quickly they multiply, and how difficult they are to remove is the next step.

Tools for Weed Assessment:
Links for invasive species above are to the California Invasive Plant Council (Cal-IPC) plant profile. Cal-IPC has many resources to help assess and prioritize invasive species. They are focused on wildland rather than garden settings; however, the information in the plant profiles is still helpful in assessing invasives and choosing the best methods for control.

The UC Weed Research and Information Center (WRIC) website has lots of useful information on weeds and management methods. The site has a Weed ID tool, as well as information presented by weed species.

Cal-IPC, Cal EPA, and the UC Statewide Integrated Pest Management Program worked together to create WeedCUT (Weed Control User Tool). WeedCUT has an overview of different management practices, best management practices by species, and a decision-support tool to help users determine the best method to control different weeds. This is also geared towards wildlands but can still be helpful for us native habitat gardeners.

Another important tool is our own observations in nearby gardens and parks. Gardeners can’t help looking at plants, so we often have a good idea of what weeds are problematic in our area, how they move, and how quickly they spread. This type of local knowledge is often the most useful!

How Weeds Spread:
An important part of deciding which weed to focus on is knowing how species spread. For many species, this is knowing when it will flower and seed.  Timing can vary from year to year, and from place to place, based on weather patterns, so we need to keep an eye on the garden as soon as sunny days start to spur flowering. Ideally, we want to catch plants before they go to seed. If plants are starting to seed, it is possible to put flower heads into bags and remove them from the site, but it’s much easier to remove plants before they seed rather than trying to contain tiny seeds that want to get free. Keep in mind that some species’ seeds are known to persist in the soil for decades (broom, again!), so getting a plant out before it sets seeds is crucial. There are some invasive species which can flower and seed multiple times each year, panic veldtgrass being a notable one, and a different strategy is needed for these species. They can be worked on throughout the year to gradually reduce and control the population. 

English ivy smothering native ferns
Photo by Caroline Christman

Some of the most challenging invasive species spread vegetatively through rhizomes or stolons, such as English ivy (Hedera helix), periwinkle (Vinca major),  and Bermuda grass (Cynodon dactylon). These plants flower and spread via seed as well, but their vegetative spread may be the hardest to control. Because they are difficult to remove once established, it is important to remove young plants when they are first seen in the spring. Removing a mat of English ivy is a large task we can undertake later in the year once the spring and summer seeders are under control. 

Weed Control Methods:
There are numerous methods used for control depending on species, location, slope, soil type, time of year, resources available and more. The WeedCUT tool mentioned above is a good place to see a broad range of options. For the smaller native garden, some of the easiest methods available to us are hand removal; brushcutting; and covering areas with mulch or tarp. There is some overlap between these methods and many, many variations on how to use them.

Weed Wrenches

Hand or Manual Removal. This is the method we were using, as described above, to remove thistles, bur clover, and panic veldtgrass. In winter and spring, when the soil is still moist and easy to dig, this approach works well. This is the most targeted method and the most certain, as we can ensure that the root is removed and the plant will not return. However, hand removal is time consuming when there is a lot of ground to cover.  Any number of hand-held tools are available; we can choose between hoes, shovels, spades, handpicks, digging knives, weed wrenches, etc. Different tools are good for different species and soils: a handpick or digging knife is good when it’s important to get out a long tap root, and a weed wrench is needed for getting larger species such as broom or cotoneaster (Cotoneaster pannosus). (Marin CNPS Chapter lends out small and large weed wrenches.) Pruning shears can be used to cut herbaceous plants, such as annual grasses – this is a nice option to use around new plantings where many small weeds come up and digging each one out is overkill. Hand weeding is also fun to do together- we can grab a few gardening friends and weed together!

Brushcutting. A brushcutter or weed whip can be a great way to cut back larger areas of weedy grasses and forbs. This method allows control over how much material to leave above the soil surface, so we can leave 3 or 4 inches of stem to act as erosion control. When using power-tools it is important to keep in mind user safety, fire safety, and the potential to harm animals hiding in the vegetation. Before brush cutting an area, walk through and make sure there are no bird nests or wildlife. See WeedCUT for more information: Cutting: String Trimmers / Brush Cutters

Sheet mulching

Covering. Covering areas with mulch, sheet mulch, or tarp is another option to consider. On a small scale, mulch can be used to cover areas around new plantings and reduce the amount of hand weeding needed while also holding in soil moisture. There are many potential mulches to use, wood chips being the most common. When choosing mulch, make sure you are not introducing weed seeds (choose rice straw rather than hay) and consider fire safety.  Sheet mulching (placing cardboard or heavy-duty paper covered with a layer of mulch) and tarping can also be used on a larger scale in areas dominated by weedy plants. We start this process by cutting plants back to the soil surface. For annual or biennial plants, covering for a few months during the growing season may be enough. For perennial plants with more energy stored in their roots, covering will probably need to be maintained for a year or longer, and still may not work depending on the weed species. When choosing to cover an area, we need to keep in mind that on-going maintenance is required to keep the cover in place and intact, and to remove any new growth coming up around the edges. Also note that long-term coverage will prevent the growth of any native plants in the seed bed. Tarping, which may be more effective to kill weeds than mulching, will also increase soil temperatures and may harm soil biota. See WeedCUT for more information on tarping: https://weedcut-new.ipm.ucanr.edu/management-practices/tarping/

Disposing of Weeds:
If weeds are removed prior to setting seed, we can keep them in the garden, adding them to a compost pile or bin, cutting them up to be used for mulch, or allowing them to decompose on site.  Home compost piles are not hot enough to kill seeds, so any seed-bearing material should be added to the green bin to be composted at an industrial facility and kept out of the garden. Some plants are able to sprout from stems, think periwinkle and Cape ivy (Delairea odorata), and the biomass should be taken out of the garden to reduce the chance of starting a new infestation.

This is just the tip of the weed control iceberg, and there is no one right answer for any weed management issue. Weed control, like gardening, is a process which takes time and experience, and lots of trial and error. It can be a lot of work, so don’t go it alone! Join us in the Marin CNPS Gardening Group and look for upcoming Marin CNPS or other gardening events to meet other gardeners.  Gardening together gets more than twice as much as done and is more than twice the fun!

Please share your stories, tips, and tricks for weed control to help other habitat gardeners! Send them to Ann Elliott.