By Jon Healy, CPESC
Establishing native grasses, shrubs, and forbs can be challenging. Soil conditions, erosion, and weather can contribute to the challenges, although native plants are designed to succeed even when facing many of these environmental factors. Weed pressure is often the biggest threat to a successful, long-lasting native planting.
Weed pressure can have a negative impact on a native seeding primarily due to germination speed and strong spreading traits. This can occur either through rhizome/stolon growth or through well-designed seed delivery systems. In comparison, most native plants grow and spread more slowly making them susceptible to competition from weeds.
There are several options for controlling weed growth in native plantings and there are many factors to consider when determining the right control for a project. There are many variables in site and environmental conditions that may require different, site-specific strategies, but the basic concepts include burning, mowing, herbicide application, and spot control. Another essential method—starting with a clean slate —should be used in conjunction with one or more of the other options.
Burning a native seeding can be one of the most effective means of controlling weeds in some climates and environments. Native plants are well-adapted to burning, while many weeds are not. In fact, native plants tend to grow back faster and stronger after burning. A controlled burn will eliminate shallow-rooted introduced grasses, broadleaf weeds, and juvenile woody plants. At the same time, seeds of these undesirable species that exist on or near the surface of the soil will be eliminated, which reduces weed pressure in future years. Burning in dry/windy conditions or rough terrain can be difficult to control and extremely dangerous. A burn should be coordinated with local government offices, forest service, and fire departments to ensure appropriate permits are obtained and support is available if the fire gets out of control.
Mowing a native grass planting can be an effective means of controlling weeds. Mowing minimizes weed pressure in a few ways. Most native plants incur little stress from being mowed while the growth of many weeds will be stunted, giving the intended species an opportunity to thrive. Another benefit is many of the common weeds are annual species that rely on dropping seed to propagate. Mowing prevents seed from forming. No seed means no plants next year. Additionally, mowed material creates a natural mulch that can help retain moisture, block sunlight from juvenile weeds and minimize erosion. Terrain or accessibility may make mowing impossible, but mowing can be the ideal combination of affordability, safety, and effectiveness.
Using herbicide to control weeds in native plantings can be very effective. There are many options intended for use on native plantings that fall into three broad categories: selective pre-emergent, selective post-emergent, and non-selective.
Pre-emergent herbicides target plants that have not yet emerged from the soil. Plants are easiest to eliminate when they are young so a pre-emergent used at the right time of year —usually early spring—can be very effective. Pre-emergents can be used on new seedings or existing stands, but typically require that the product reaches the soil to be effective, so applying a pre-emergent to a well-established stand can be difficult.
Post-emergent selective herbicides are only effective on actively growing weeds. Most products work through foliar absorption. Timing of post-emergent applications is essential since many weeds become difficult to control at full maturity. Generally, selective post-emergent applications are most effective in late spring to early summer, but can be used with good success throughout the growing season.
There are several products on the market with both pre- and post-emergent value that can help eliminate growing weeds and prevent new weeds from growing for up to several months.
Non-selective herbicides are the “nuclear” option of herbicides. These products such as glyphosate (e.g. Roundup®) are intended to kill everything they touch. Non-selective products are best used for spot spraying or pre-seeding weed control.
Severity of weed pressure, primary type of weed present, stage of weed growth, and species present in the intended vegetation all need to be considered when selecting herbicide. Product selection and application planning are crucial to achieving the results you want. If you are not certain, consult an expert.
When weeds are sparse, on small sites, or in areas where weeds are concentrated, spot spraying or hand pulling can be an effective method of controlling weeds. This is potentially the most labor-intensive method, but can also have the highest rate of success. Many wildflowers cannot tolerate any type of herbicide application. If burning or mowing are not feasible, spot control may be the only option. In locations where weeds are sparse, re-seeding is typically not required after eliminating the weeds. However, large, concentrated areas of weed growth will need to be re-seeded after weed elimination. If re-seeding is not done, these open areas will invite new weed growth.
Eliminate Weeds Before Seeding
The most effective and important method for controlling weeds in native plantings is to start with a clean slate. Eliminating weeds before seeding will give the planting its best chance for success. Project schedules, seasonal limitations, and site conditions can limit the ability to control weeds before seeding. Ideally, a site should receive at least two applications of non-selective herbicide to eliminate as many weeds as possible. Tilling or ripping the soil between applications can expose weed seeds that might not otherwise start growing for a couple years. Another challenge of this prolonged pre-seeding period is exposing the site to weather and erosion, which can result in costly repairs. One of the most cost-effective ways to battle this is to use cover crops.
Using inexpensive small grains or other annual plants to cover bare ground between final grading and seeding during this weed control period can cost as little as a few dollars per acre for seed and can provide several benefits without the need for thousands of dollars in erosion control products. These cover crops can prevent or minimize erosion, provide natural mulch and add organic matter to the soil. Seeding as little as 20 lbs. of small grain per acre (22kg/ha) can help keep the site intact between herbicide applications. Typically, this would be an herbicide application followed by a cover crop seeding, wait two to four weeks, till or rip the soil, wait another two weeks, apply herbicide and do the final seeding.
Blending a nurse crop of the same small grain or annual plants with a permanent seed mixture can provide some of the same benefits as a pre-seeding cover crop. In most cases, native seedings are planned to result in 30 to 80 seeds per square foot (325-860 seeds per square meter). Eighty native plants per square foot sounds like a lot, but the reality is that not all seeds will germinate simultaneously and some will not germinate at all. A typical native seeding will have bare ground between plants for at least the first two seasons. Bare ground has the potential for growing weeds or allowing erosion. Adding a nurse crop can achieve quick growth to fill in this bare space between permanent plants. For sites that may be sensitive to potential regrowth of annual nurse crops, there are options such as sterile triticale that are not capable of regrowth.
There are several options available to control weeds in native plantings that will achieve a permanent, durable, and attractive stand of native plants. The right option for your site is going to vary depending on the species of weeds, geography, site conditions, terrain/accessibility, and many other factors. There is not a one-size-fits-all option, but a thorough assessment of the site and potential unintended consequences of various weed control options will help identify a workable solution.
About the Expert
Jon Healy, CPESC, is the commercial product manager for Millborn Seeds, Inc., with a primary area of expertise for the Great Plains and northern Rocky Mountains. He has over 25 years of experience in the construction industry and has served as the South Dakota State Representative on the IECA Mountain States Chapter Board for the past three years.