“Haze” due to dormant seedpods of near monoculture of Garlic Mustard
I recently visited Ushers Road State Forest to collect some photos for my blogpage featuring that destination. As I was walking along the south loop toward the parking lot, I came upon the scene depicted above. From that distance, the dried-out upper stems of what appeared to be a monoculture of some plant cast a hazy appearance just above the forest floor, almost fog-like. The scene sickened me. It is continuing evidence of the success of one of the most invasive plants on the planet: Garlic Mustard (Alliaria petiolata). What I found particularly alarming about this scene is that the presence of Garlic Mustard along this trail segment is now a much higher density of this species than when I was compiling a wildflower inventory here in 2009-2011.
That scene prompted me to peruse information and research findings regarding the reasons behind this plant’s rampant invasion. What follows is a compilation of that information. Read on, if you dare.
Garlic Mustard is an invasive species currently invading the understory of North American woodlands where it is a serious threat to native flora. It is known as an allelopathic plant capable of exerting negative chemical effects on native plants and microbes in its environment. Those native plants include Bloodroot (Sanguinaria canadensis), Dutchman’s Breeches (Dicentra cucullaria), Hepatica (Hepatica spp.), Spring Beauty (Claytonia virginica), Thimbleweed (Anemone virginiana), Toothworts (Cardamine spp.), Trilliums (Trillium spp.), and Wild Ginger (Asarum canadense). However, even seedlings of oaks may be harmed by these same negative effects in these woodland settings. Together, this loss of plant diversity also threatens native insects, including butterflies, because egg-laying sites and food sources for their larvae may no longer be available.
What is allelopathy?
Coined in the 1930s from the Greek ‘allelon’ meaning “one another” and ‘pathos’ meaning “suffering,” it is a botanical term referring to harm caused to a plant by another plant through chemical substances that can inhibit germination or growth, or interfere with a symbiotic relationship (involving bacteria and fungi with a plant’s roots) that otherwise provides nutrients to the plant. It must be noted that a plant will have an allelopathic effect on only a few specific plants, but not universally against all other plants.
For a plant to exert allelopathic effects, it must produce compounds that are capable of entering the environment around the plant and persisting long enough to have biological effects on neighboring organisms. These chemicals can be released in many ways, including exuding from the root as well as volatilizing from decomposing leaves and other plant parts. In addition, other factors such as temperature, moisture and available nutrients can also interface with allelopathic chemicals to amplify their effects.
Garlic Mustard has long been used as a spice, having been found in 6,000-yr-old Neolithic cooking pots from northern Europe. Its utility as a spice is largely due to the presence of glucosinolates, a class of sulfur- and nitrogen-containing compounds derived from amino acids that provide the pungency to mustard and other food products from this family of plants. The primary glucosinolate in Garlic Mustard is sinigrin. Glucosinolates and their derivatives effectively render this plant to be unpalatable to herbivores, especially White-tailed Deer (Odocoileus virginianus), which, ironically, is itself a detriment to many native plants occupying the understory and forest floor of urban woodlands because of this animal’s frequent overpopulation within those urban settings. Studies suggest that these same compounds also affect seed germination and the growth of native plants, and alter the activity of soil biota (particularly bacteria and fungi otherwise fulfilling a mutually beneficial relationship (known as mycorrhizal associations) with the root systems of native plants such as tree seedlings), raising the possibility that these compounds contribute to Garlic Mustard’s overall success as an invader. The ability of Garlic Mustard to disrupt mycorrhizal associations illustrates an alarming pathway by which it may be altering forest regeneration.
Garlic Mustard was introduced to North America by early colonists, who valued it both as a medicinal plant and as a garlic substitute. By 1868, Garlic Mustard was found growing in native communities on Long Island, New York. As of 1991, the rate of expansion of Garlic Mustard was estimated to be faster than the expansion of Purple Loosestrife (Lythrum salicaria), a well-known and widely spread invasive plant. Once Garlic Mustard is established, it becomes a permanent member of the plant community and proliferates rapidly into adjacent habitats. Also, unlike many other herbaceous invasive plants, Garlic Mustard does not necessarily require a disturbance to become established or to proliferate.
Most of the plant diversity in eastern North American forests is found in understory forbs (vascular plants that are not trees, shrubs or grasses). These woodland herbs are predominantly deciduous perennials with limited seed production. The growth and reproduction of woodland herbs is primarily limited by understory light availability.
The phenology (study of cyclic and seasonal natural phenomena) and reproductive output of Garlic Mustard is substantially different from those of native species. Garlic Mustard seedlings begin emerging in early spring, a period when most native plants are still dormant. Initial growth is rapid until deciduous trees form leaves, after which growth slows. Garlic Mustard growing in more open areas may continue steady production of new leaves into summer.
Garlic Mustard first-year leaves
Seedlings develop into rosettes (compact arrangement of basal leaves) early during their first growing season. These rosettes may retain many green leaves throughout an entire winter. They are even capable of winter growth during periods of above-freezing temperatures and no snow cover!
Garlic Mustard leaves in snow
Photo Credit: https://myemail.constantcontact.com/March-Nature-Break–News-and-Events.html
Garlic Mustard plants that survive winter rapidly produces a flowering stem (called ‘bolting’) in early spring of the second year. It produces flowers from April through July and fruits from June through September. The second-year plant produces flowers, regardless of its size, and plants can continue to produce flowers even after a plant begins forming seed pods.
Garlic Mustard – note how different the shape and margins of these second-year leaves compared to those of first-year plants.
Following this period of rapid growth, most leaves quickly wither away. Nearly all two-year-old plants have no green leaves by early July. The fact that bolting occurs very early in spring is believed to contribute to Garlic Mustard’s competitive ability by limiting light and space that might otherwise be available to native forbs.
Three aspects of Garlic Mustard’s means of reproduction also appear to contribute to its success as an aggressive invader. First, flowers are adapted for generalist pollinators and have the ability to self-pollinate. Second, almost every pollinated flower develops viable seed, resulting in a profusion of seeds. The third aspect is that, because they attract a wide variety of pollinators (refer to first aspect), Garlic Mustard populations will cross-pollinate at a high level, ensuring a large amount of genetic variability and thereby avoiding the negative effects associated with inbreeding (which, interesting enough, may result due to self-pollination – again, refer to first aspect).
Yet another factor adding to the invasive success of Garlic Mustard is the absence of insects that would likely feed on this plant if it were still in its native range. As many as 69 insect species are known to consume Garlic Mustard in Europe; none of these species are present in North America.
West Virginia White (Pieris virginiensis), a butterfly vulnerable to extinction in New York, relies almost entirely on Toothwort (Cardamine diphylla) as its larval host plant. Unfortunately, these butterflies are often stimulated to deposit their eggs on Garlic Mustard because it is chemically similar to Toothwort; both contain sinigrin. Sinigrin is a strong attractant that these native butterflies use to identify the appropriate host plant. While these larvae will feed on Garlic Mustard leaves, most die just prior to becoming an adult because of the plant’s toxicity.
There is also concern that invasion by Garlic Mustard is decreasing the abundance of the native Mustard White butterfly (Pieris napi oleracea). In New York, it was formerly found as far west as Erie County. Presently, it is restricted to the most mountainous, forested parts of the state in the Adirondack Forest Preserve, Tug Hill Plateau, and the Catskill Forest Preserve. Field Mustard (Brassica rapa) and Wild Radish (Raphanus raphanistrum) are the primary hosts for this butterfly. Garlic Mustard can serve as an alternate host, but when these butterflies deposit their eggs on the leaves of Garlic Mustard, their larvae have a lower rate of survival when compared to native mustard plants. In a modeling study, researchers found that when Garlic Mustard dominates the herbaceous plant community (i.e., > 50% cover), populations of the Mustard White butterfly are predicted to become locally extinct within 50 years. The results of their modeling study suggest that the Garlic Mustard invasion is a primary factor driving the population decline of this native butterfly.
It appears that Garlic Mustard possesses a combination of traits, all slightly different from those of the surrounding native plants, which results in a highly successful species that is altering the composition and function of the native communities it invades.
It is one impressive plant, but a scourge, nevertheless.
Curious By Nature 
