4 Environmentally Conscious Ways to Kill Invasive Plants

4 Environmentally Conscious Ways to Kill Invasive Plants

Natalie Chapman and Jackson Brooke hand pull weeds in the Enchanted Forest. Photo by Alix Soliman.

August 16, 2021

By Jenna Allred, Natural Lands Manager, and Jackson Brooke, Restoration Technician

There are numerous ways to treat invasive species in natural areas, however, not all methods leave the same mark on the landscape or can be applied everywhere. Here on the Santa Lucia Preserve, the Conservancy practices minimally intrusive methods for maximum effect. We use four tried and true methods to target invasive plants: hand pulling, spot-spraying herbicide, spot-mowing, and cut and daub.

Minimally intrusive methods are those that succeed in removing invasive plants without  impacting native species or disrupting the habitat. With an incredible attention to detail, our team spot treats individual plants rather than treating an entire area, making sure to factor in  environmental conditions such as terrain, proximity to water, soil conditions, and weather. Deciding which minimally intrusive methods we use can change daily, but we tend to stick to a seasonal schedule.

1. Hand pulling

2021 Field Crew members Natalie Chapman and Mary Snook take advantage of a rainy March day softening the ground to hand pull French broom.


Our go to minimally intrusive method is hand removal. This process allows us to move through the habitat while only affecting the invasive species and the effects are immediate. Once pulled, the plant is dead and we don’t have to worry about it producing seeds if we time it right. The habitat remains intact and wildlife are still able to utilize it. Hand-pulling works best when the soil is wet and soft. When the soil is hard, it requires a lot more effort and can cause erosion on steep terrain. As a result, you will mainly see us hand pulling in the winter and early spring to take advantage of the softer ground at that time of year.

2. Herbicide 

2021 Field Crew members Kevin Nolan and Mary Snook spray herbicide on a patch of invasive plants, including Italian thistle. Photo by Alix Soliman.

Spot herbicide treatment is another method that targets only the invasive species while keeping the rest of the habitat intact. However, timing the application is the most critical for this method. For best results, annual plants should be sprayed while in the rosette stage and perennials should be sprayed during the times of the year they are actively growing. Every spring, the Conservancy loads up our backpack sprayers and heads out into the grasslands. We walk through an area in a grid fashion to ensure we cover all of the ground, making sure to only spray invasive plants.

Weather heavily impacts our decision to start spraying each year, and influences when we can spray each day. We can’t spray when it is still raining as the rain can wash the herbicide off the leaves of the target plants, reducing the chemical’s effectiveness and potentially harming native species. The weather also affects when the invasive species start growing and enter the life stage where they are most effectively controlled, so we monitor the grasslands to make sure we don’t miss the start of our spray window. Once we’ve started to spray, we need to monitor wind speed and temperature as spraying when winds are over 10mph or the temperature is over 85 degrees can cause herbicide to unintentionally land on nearby desirable species. Spot herbicide treatments are also conditional on the proximity of the target population to bodies of water. Many herbicides require a buffer around bodies of water to minimize environmental impacts, so we must always be cognizant of the land features present in a target area when selecting a treatment method.

Spot herbicide treatments differ from broadcast treatments (which spray an entire area of land) in a few key ways. Our spot treatments allow for all other plants to remain untouched so we do not have to worry about native plants being accidentally sprayed. In a broadcast spray (which we do not do on The Preserve), any plants that are sprayed by the herbicide and affected by it will die. A broadleaf herbicide is most often used during those applications, meaning that grasses are not affected but any wildflowers or shrubs will be. Also, spot treating means we use less herbicide over an area. Ideally, we would like to get to the point someday where we no longer have to rely on spot herbicide spraying as a treatment method. However, many invasive species populations are too large to hand pull in a reasonable amount of time. When faced with these large populations, we use spot herbicide treatments to reduce the population to the point that hand pulling is an efficient and effective use of our time.

3. Mowing

Alex Wood, a member of the 2020 Field Crew, spot mows bull thistle growing under a willow on the edge of a grassland, taking care to only mow the invasive species and leave any native plants intact.

Spot-mowing is one of the least effective treatment methods we use, but when timed correctly, it can make a big impact. When the Conservancy utilizes the spot-mowing method, we time it after the invasive plant has bolted (when the plant begins its upward growth to produce a flower) but before it has gone to seed. Then, very similar to our spot herbicide treatments, we only go after the invasive plants while leaving the majority of the area untouched. We will later go back to an area after the plants have rebounded and begin to grow again. The goal of spot-mowing is to reduce the chance of the plant being able to set seed rather than killing the plant. While we always want to kill the invasive plants, when we implement spot-mowing it’s because the plants are beyond the point in their growth cycle that we can effectively control them with spot herbicide treatments and are more numerous than we can manage completely by hand pulling. Spot-mowing can be thought of as our last ditch effort to control as many invasives over as large of an area as possible.

Broadcast mowing invasives is a very different treatment method and takes out all the plants in the area. The habitat lost during this process causes cascading effects to wildlife as well as the establishment of additional invasive plants. It is not a rare occurrence to see raptors circling the area after a mowing event looking for either animals that were displaced by the mower or killed. Also, many invasive plants rely on disturbance to get established in an area. Native grasslands require various types of disturbance for their continued existence, but broadcast mowing indiscriminately kills or damages all of the plants in a field and many invasive species are able to recover faster from broadcast mowing than the natives can. Mowers that expose bare ground or accidentally cut into the ground have made the perfect area for these opportunistic invaders to establish themsleves. 

4. Cut and daub

Mary Snook of the 2021 Field Crew applies herbicide to a cut stump of French broom just seconds after it was sawed.

Another thing to consider when going after invasive plants is the terrain. On steep slopes we must factor in how the treatment method will affect the area and the safety of the applicator. Hand pulling plants can increase the risk of erosion and using a weedeater/mower on the slope can put the user at risk of falling. In these situations, the Conservancy uses the cut and daub method. The cut and daub method relies on the removal of the above ground vegetation while leaving the roots in the ground and then applying a small amount of herbicide to the cut area. 

Weather also impacts our decision to implement cut and daub. We don’t cut and daub if rain is in the forecast in the following two days to ensure that the herbicide is completely taken up by the plant and is not washed off the cut stump. When we are treating plants such as French broom after the ground has dried and hardened too much to hand pull, cut and daub is much more time efficient than trying to pry plants out of the tight grip of the dry soil.

We Want to Hear From You

There are many factors that play a role in our decision of what minimally invasive method we choose to use on an area in a given day. Each method has its limitations based on terrain, weather, proximity to water, soil conditions, life stage of the target species, and how time and labor intensive it is, so we’re lucky to have so many options in our toolbox. Regardless of what method we select after matching up the conditions of the site with the treatment that will best control the invasive plants present there, our goal is always to minimize our impact on native species while maximizing the effect on the invasive species.

Without active management, invasive plants have a knack for spreading and taking over native habitats. If you’re looking to remove weeds from your own property in the Central Coast region and need guidance on how to do it in the most ecologically sensitive way, reach out to Natural Lands Manager Jenna Allred at [email protected].