Q: I don’t understand why I shouldn’t plant something considered invasive if I don’t see it spreading by seed on my own property. (Wouldn’t it start there first?) Are invasive plants truly that entirely evil? I see bees on my non-native flowers, for example.
A: This is a complex subject, but a plant’s status as “bad” is certainly at least somewhat subjective. On one hand, any plant species preventing soil erosion on disturbed land, producing oxygen and absorbing carbon, and feeding at least some animals could be considered to have redeeming qualities. In the case of invasive plants, though, their harmful impacts outweigh these bare-minimum perks. Invasives spread readily on their own, are hard to control once they escape our yards, and aren’t integrated into the ecosystem around them so wildlife interactions are very limited.
It may seem ironic that invasive species usually don’t rear their heads as weedy aggressors on our own properties, reinforcing their innocent appearance. Often, this is because the seeds they produce – including those that well-intentioned gardeners miss when trying to keep them deadheaded – wind-up being consumed by wild animals or whisked-off by wind or stormwater runoff. Transported out of sight into clearings, woods, or waterways, they do especially well out-competing native vegetation in sites that have disrupted plant communities or degraded soil.
Sadly, degraded habitat describes many of the “wild” areas around us these days. Invasives’ successful bullying of native plants is often enhanced by their inherent vigor, adaptability to adverse conditions, and unpalatability to deer and many of our native insect herbivores. Insects are a hugely significant component of the food web and support biodiversity, which is why lots of our focus on plant impacts on the ecosystem rests with them.
Typically, invasive species offer minimal wildlife benefits while displacing native species that would support a wider network of interconnected organisms. Pollinators are one of the wildlife groups that are key in this food webbecause they are often required for those plants to be able to reproduce themselves to sustain the population.
Many of our native bees, for instance, have relationships with just a handful of native plant species. They cannot use just any of the “convenience store” blooms that long-blooming non-natives provide to generalist bees, even though to us those plants look like they’re a bonanza of readily-available nectar and pollen. Generalist bees (which includes the non-native honey bee) visiting some of these blooms give us the impression that they’re popular and supporting a variety of bees when that’s not necessarily the case.
Let’s take the groundcovers periwinkle (Vinca minor) and English ivy (Hedera helix), used widely in gardens. Although they don’t generally pop-up uninvited on the other side of the yard, they do escape into local national parks and woods near neighborhoods, elbowing-out growing space for woodland natives like Dutchman’s-breeches, Trout lily, partridgeberry, trilliums, spring beauty, toothworts and creeping phlox.
The habitats in the Potomac River Gorge are some of the most plant-diverse in the mid-Atlantic (and even in the country at large), and yet to my dismay I still see many invasive and weedy non-native species that originated at some point in people’s gardens: vinca, ivy, burning bush, privet, barberry, perilla, lesser celandine, wintercreeper, wisteria, ornamental flowering cherries, and so on. You can explore our Invasive Species collection of web pages for more information about these ecological hazards, such as which plants to avoid gardening with and what impacts invasives have on our environment.
Q: I’m not sure what to do this fall…leave the leaves for the benefit of wildlife and soil health, or remove those that fell from diseased plants?
A: Generally, we advise gardeners to remove fallen leaves from plants with disease outbreaks to minimize how many infectious spores are lying in wait for next year’s growth. However, it’s not a foolproof prevention measure, and plenty of other factors influence how susceptible that plant will be any given year. In that sense, the benefits of keeping the leaves probably outweigh those risks.
It may also depend on how many leaves you have overall. If you have three big shade trees’ worth of leaves coming down, you can spare those that fall off the lone shrub or two with heavy leaf spot.
Keep in mind that the benefits fallen leaves provide to the organisms we consider beneficial are no different than for those we consider pests. Leaf litter is food or shelter to microbes or insects that cause plant damage as much as they are a safe haven for pollinators, predators of pests, and microbes that protect plants. Fortunately, there are way more of the “good guys” than there are problem species, so supporting biodiversity and healthy soil are going to work in your favor.
Given how many other negative impacts human development has on the environment, though, I for one would rather keep fallen leaves on-site and take my chances next year. Retaining plant debris also keeps any weed seeds, pathogens, pests and other undesirable leaf litter contaminants from being transported elsewhere, not only avoiding spreading that problem around, but also not wasting fuel energy or removing nutrients from your yard’s nutrient cycle. If you’re worried about renewing the same problem next year, hot-compost the infected leaves and use the finished compost to topdress the garden.
University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.