By Jonathan Sciarcon
Based on online posts in the Colorado Native Plant Gardening room on Facebook and in-person conversations with Wild Ones members, it appears that most gardeners, even ones who advocate planting native plants, want well behaved flora in their yards or gardens. This makes sense as many gardeners’ aesthetic preferences tend towards some sort of order or design in the landscape. As someone who advocates for diversity when it comes to landscape/garden design, I am not trying to convince everyone to turn their yards into a Darwinian struggle between the most aggressive native plants, even if this is my own gardening preference. However, I would like gardeners in Colorado to consider some of the benefits of intentionally cultivating aggressive native plants in their yards.
Keystone Plants
Many of the most aggressive native plants are Keystone species in regional ecosystems and so are essential for insect and bird survival. For example, Prunus virginiana (Chokecherry, a shrub/small tree) and Prunus americana (American plum, also a shrub/small tree) are two of the most important host plants throughout much of the Front Range. Both plants support well over one hundred species of caterpillars in addition to providing early season nectar and pollen to insects and mid-season fruit for birds and other wildlife. Many gardeners are afraid to plant these shrubs/small trees because of their tendency to sucker and re-seed in the yard. It is true that these plants spread easily in many situations. Yet, allowing these plants to form colonies protects the watershed and enriches the ecosystem.
This past spring, I noticed a Spotted Towhee had taken up residence in a Prunus virginiana thicket in my backyard. In July, I noticed birds and squirrels spending a significant amount of time collecting fruit from this thicket. As Doug Tallamy has noted, around five percent of genera support approximately 70-75 percent of our native caterpillars and over 95 percent of native birds depend on caterpillars to rear their young1. According to current research, the Prunus genera is roughly tied with the Salix (Willow) genera when it comes to hosting caterpillars, trailing only the Quercus (Oak) family. On the Front Range, it is likely that Prunus and Salix are, in some order, first and second as our native Oaks are more prevalent in the Foothills and southern and western parts of the state. Considering that Prunus virginiana and Prunus americana can survive drier conditions than most native Salix species, it follows that we should be adding these plants to our xeric or near xeric landscapes to promote as much biodiversity as possible.
Using Helianthus to Your Advantage
Another native plant that is often considered to be problematic in the garden is Helianthus maximiliani (Maximillian Sunflower). This is an aggressive, low water perennial in the Sunflower family that spreads by both seed and rhizomes to form larger colonies. Many gardeners who recognize this plant’s importance as both a host plant and nectar source for mid to late season pollinators suggest placing Helianthus maximiliani in areas where it will not spread. However, I have used it throughout my yard to combat invasive or noxious non-native plants and to keep other aggressive native plants in check. For example, I have introduced Helianthus maximiliani, which has allelopathic properties, in concert with other aggressive natives (especially Solidago sp., Helianthus annuus, and Pascopyrum smithii) in parts of my yard that have Field Bindweed (Convolvulus arvensis), Creeping Bellflower (Campanula rapunculoides), and Prickly Lettuce (Lactuca serriola). Over the past three years the Helianthus and its companion plants have spread and either helped reduce or eliminate the problematic non-native plants in these areas. I was especially shocked but pleasantly surprised to see the reduction in Creeping Bellflower this year.
Groundcovers
Many native plant advocates have made the decision to reduce or eliminate their non-native lawns. For medium use landscapes, especially where dogs are not present, experts often recommend Bouteloua gracilis (Blue Grama) and Bouteloua dactyloides (Buffalo Grass). These are excellent, low water, warm season grasses that conserve water, host caterpillars, and add beauty to the landscape. However, they do have a downside in that they do not green up until mid to late May and, where they are not fully established, are often invaded by cool season exotic plants. For those of you who have low or no-use areas of your yard in full sun or part shade, you might consider two cool season alternatives: Pascopyrum smithii (Western Wheatgrass) and Artemisia ludoviciana (Prairie Sage). These are extremely aggressive xeric groundcovers that can rapidly colonize disturbed areas. In a planned, small garden environment, these are likely poor choices. However, if you have a yard or landscape with significant non-native plant pressure, these plants, which both enter active growth in late February or early March, can be used to figuratively fight fire with fire. Both plants are excellent hosts for numerous native caterpillars and provide seeds for birds. I started seeding both plants in 2020 in areas of my yard where Kentucky Bluegrass (Poa pratensis) had struggled to grow and where Field Bindweed was prevalent. Four years (five growing seasons) later, the Field Bindweed has been reduced by over 90 percent and neither plant has had trouble growing in what I had assumed was poor soil. Additionally, birds spend a lot of time in this section of my yard collecting seeds and hunting for caterpillars in autumn, winter, and early spring.
What Will the Neighbors Say?
I realize that it is not feasible for all homeowners or gardeners to embrace aggressive native plants, even if they would like to do so. Some homeowners live in HOAs while others reside in high fire danger areas. If these situations do not apply to you, you may still be reticent out of fear of what your neighbors will think. After all, even planned native gardens go against post-World War II American suburban and urban landscaping norms. Unplanned native landscapes that include aggressive, colonizing plants present an even more significant challenge to such entrenched societal norms. We cannot please everyone and, in my view (this could be the only child in me speaking), we should not aim to do so. However, if you are concerned with what your neighbors might think, please consider both using signage and spending time in your landscape, either weeding out non-native plants or even checking the spread of some of your more aggressive plants. Both activities will show your neighbors that you are working hard on something intentional and may also provide you with opportunities to explain why you are doing what you are doing.
Too Aggressive?
Although I have become a staunch advocate for planting aggressive native plants, I should share that some plants may be unsuitable for even wild landscapes. In my own yard, the one plant that has spread the most is Rosa woodsii (Woods’ Rose). Perhaps I should not have added 25 bare root specimens to a 7,000 square foot yard (yes, this is sarcastic). I was drawn to this plant as it performs four seasons of ecosystem services. Still, it has easily spread even without supplemental hand watering. It is possible that the spread of these plants will be checked when they come into contact with the various Solidago, Helianthus, Prunus, Quercus, Rhus, Shepherdia, and Artemisia in my yard. I am kind of looking forward to that battle, which is likely only one or two years away! Yet, I cannot in good conscience recommend this plant to others, unless they are truly ready to rewild their yard with what may be the most aggressive, colonizing plant with which I have experience. On the other hand, for anyone on a budget who wants to rewild their yard with a native shrub that provides excellent year-round bird habitat and can survive in dry sun and shade conditions, Rosa woodsii may be the ideal plant.
Benefits Overview
While aggressive native plants may not be appropriate in certain landscapes, they have a lot to offer. Homeowners on a budget can increase biodiversity via the addition of such plants to their yards with a small financial investment. Aggressive native plants are also often excellent host plants and several, such as Prunus sp., are Keystone species in many local ecosystems. Additionally, these plants can also often effectively compete against aggressive non-native plants, especially if they are given a helping hand now and then. These plants are also often xeric or near-xeric and their rhizomatous root systems do a tremendous job of preventing water runoff. Finally, many of these plants provide essential habitat, food, and cover for native insects, birds, and other fauna. If your goal is to promote as much biodiversity as possible in your yard, you should consider intentionally growing some aggressive native plant species. Please note that this column omitted many important aggressive native plants and focused on the ones with which I have experience.
Jonathan Sciarcon is a Board member of Wild Ones Front Range. He teaches in the Department of History at the University of Denver and is an avid native plant gardener.
- Doug Tallamy is a Professor in the Department of Entomology and Wildlife Ecology at the University of Delaware and author of many books on the importance of native plants in home landscapes. His often-cited research is conducted on the east coast of the U.S., but we don’t yet have enough research on all the specific plants that would qualify as the main keystone species in the Front Range. It is likely, however, that, as is the case in other parts of temperate North America, Prunus, Salix, Quercus, Solidago, and Helianthus would be on such a list. ↩︎