"Are aster family weeds invasive or no?"
A new member in our Invasives ArMI names her arch nemesis, “I loathe japanese knotweed and want to eradicate it wherever I see it.” Who among us doesn’t share this sentiment, along with a profound sense of helplessness when it comes to contemplating removal.
The ArMI’s initial foray into knotweed removal is described here: https://tinyurl.com/y8hmt229 , and knotweed will be the subject of future journal entries. However, I wanted to turn to another concern of hers, aster behavior. She laments: "I struggle with identifying whether aster family weeds are invasives or no." Haven't we all had difficulty identifying asters at some point? As for their possible “invasiveness,” a quick web search reveals that others have this same concern. I have little experience on the topic and was intrigued to learn more.
But before we attempt an answer, let’s refine the question. Are asters (Symphiotrium genus) simply really good at growing in their native environment, making them good flowers to plant after invasives removal? Or are they "invasive" as some gardeners call aster reseeding and spreading behavior that's too robust for their garden esthetic.
How is aggressive growth different from "invasiveness?" What does it mean to be "invasive?"
According to a Nashua River Watershed Association piece entitled Protecting Native Ecosystems by Combating Invasive Species ... ( https://nashuariverwatershed.org/what-we-do/protect-water-and-land/invasive-species-overview/invasive-species-overview.html ), invasive plants are not defined by aggressive growth alone. It's important to examine a suspected invasive's behavior within in food webs and wildlife habitat systems.
Non-native (exotic) invasives:
• crowd out native species plants - they are strong competitors that displace native plants and create monocultures
• alter natural food webs - they don't support native pollinators, birds, and other wildlife
• destroy wildlife habitat. - they not only invade our woods, they are predatory. Think black swallow-wort killing monarch caterpillars with its poison, and oriental bittersweet strangling trees as they grow in its tight, twining grasp
Why don't invasives support food webs? University of Delaware entomologist, Doug Tallamy, noticed a distinct lack of something in his newly purchased acreage burgeoning with invasive plants. Remember, he's a bug guy, so he couldn't help but notice, “No caterpillars on the Oriental bittersweet, the multiflora rose, the Japanese honeysuckle, on the burning bush that lined his neighbor’s driveway.” (https://www.smithsonianmag.com/science-nature/meet-ecologist-who-wants-unleash-wild-backyard-180974372/ ). Lack of caterpillars is a key sign of altered food webs, because it’s the caterpillars and other insects that form the base of a healthy food web. Birds almost exclusively feed caterpillars to their young. So no caterpillars, no bird families. Tallamy's "No caterpillars!" observation caused him and his students to research which plants best support food webs by supporting the greatest variety of insects. Asters, it turns out are a keystone native!
Native aster behavior contrasts strongly with non-native invasives:
• Yes, some asters may grow aggressively and be difficult to remove, especially those with rhizomes, like the heath aster (A. ericoides).
• BUT unlike invasives, asters are a keystone native that support the eastern forest food web in a big way— 100 moth and butterfly caterpillars and 33 specialist bees thrive on asters in the eastern temperate forest.
(Keystone Plants Eastern Temperate Zone, Ecoregion 8, National Wildlife Federation https://www.nwf.org/-/media/Documents/PDFs/Garden-for-Wildlife/Keystone-Plants/NWF-GFW-keystone-plant-list-ecoregion-8-eastern-temperate-forests.ashx?la=en&hash=1E180E2E5F2B06EB9ADF28882353B3BC7B3B247D)
• Asters create wildlife habitat and are not predatory like black swallow-wort, which kills monarch caterpillars.
Garden.org describes three true-form (non-cultivar) native asters among a list that includes cultivars. Avoid the cultivar varieties if you want to boost caterpillar and bee activity in your garden. The genetically true species support more insects. Also avoid asters that spread via rhizomes (heath aster) if you want a more manageable species. But choose rhizome species if your goal is to replace weeded out garlic mustard! https://nativeplantherald.prairienursery.com/2016/05/chief-invasive-strategist-wild-ginger-repels-garlic-mustard/
Here's what garden.org says about the three asters (https://garden.org/learn/articles/view/283/) :
• New England asters (A. novae-angliae) are native through much of the country, from Vermont to Alabama, and west in North Dakota, Wyoming and New Mexico. They prefer moist soil and sun.
• White wood aster (A. divaricatus) is a native of eastern North American forests. Unlike most asters, it blooms well in dry shade. It's especially valuable in naturalistic woodland gardens, where it brightens dark corners with sprawling clouds of one-inch white daisies in September and October. The plants reach two to three feet. Their shiny, deep purple stems contrast beautifully with the flowers. Zones 3 to 9.
• Heath aster (A. ericoides) is native in the eastern United States, from Maine to Georgia, and west to Minnesota, South Dakota and New Mexico. Tiny flowers in white, pink or lavender bloom in great profusion, smothering dense, bushy plants to three feet. Flowers sprays are particularly good for cutting. The plants tolerate some drought, though in dry climates they need supplemental water. Plant in a sunny spot. Zones 4 to 9.
