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| Siberian Squill (Scilla siberica) is an introduced plant that blooms in early spring. Leaves are basal, up to six inches long and 1/2 inch wide. Flowers are blue on leafless stalks. This ephemeral plant dies back in late spring. |
Siberian squill is a beautiful, eye-catching
plant. Its bright blue flowers are among the first to
bloom in spring, providing a pop of early color in gardens, lawns and
woodlands. To many who are winter-weary, it's a welcome sight.
Lately, though, the plant is getting attention for a
different reason. The same qualities that have long made Ssquill popular – its
profusion of flowers, its tendency to spread into masses, its easy propagation
– have also raised suspicions that it can be invasive. In some places, squill
has jumped the garden border or the naturalized planting to grow where it
wasn’t intended. That includes deciduous forests, where there is rising concern that
it can displace native woodland plants.
It has taken a long time, and some long-distance travel, to
get here. Squill is thought to have arrived in the United States in
the late 1700s, when it was imported from its native range in southwest Russia
and the Caucuses [1]. Promoted as an
ornamental, it reached the Midwest by the mid-1900s. The first record of squill
in the Wisconsin State Herbarium [2] is a plant collected in 1955 from
Hortonville, in eastern Wisconsin. Later herbarium records find it at Point
Beach State Forest near Manitowoc in 1964, the campus of UW-Oshkosh in 1966, and
Turville Woods, west of Milwaukee, in 1975.
Squill reached Minnesota sometime later. The garden census
for the Eloise Butler Wildflower Garden has counted squill among its numbers since
1985 [3]. The earliest herbarium records from the Minnesota Biodiversity Atlas [4]
are dated 1997, when squill was found in Forestville State Park, and 1999, when
it was found in Frontenac State Park, both in southeast Minnesota. Today squill
is found throughout much of the state.
Judging from records of its distribution, squill can live quite
well outside tended plantings. Freed of any apparent constraints, it can reproduce
quickly by bulbs and seeds to form potentially large populations. A sample of
EDDMapS reports [5] gives some idea of its ability to spread: 7 square feet of cover in a
Rochester park; 800 square feet in a park in Maple Grove; an acre – about
43,000 square feet – in the Roberts Bird Sanctuary in Minneapolis, where squill escaped from a
garden into an adjacent woodland reserve.
According to these reports, not all wild
populations are (yet) large and dense, but there are more clues this can happen. In fact, some people want it to happen. For example, many gardening websites and advertisements picture broad expanses of squill, wide carpets of blue below an overstory of
handsome trees. Far from being
a red flag, squill’s habit of spreading is viewed as positive. If the pictures don't say it, the words that accompany them do: “Great naturalizer,” proclaims one advertisement. “Best when planted in large drifts,” advises another. One more takes that advice a step further: “Mass in sweeping drifts in woodland, wild or naturalized areas.”
The fear, again, is that sweeping
drifts of squill could broom other plants out of existence. Several native
spring wildflowers – bloodroot, wild ginger, Trillium, Hepatica, spring beauty,
and others – share a similar woodland habitat and phenology, and they may be
unable to compete with an aggressive introduction like squill. Proof of harm needs formal
research, but until any such studies are complete, informal studies, reports, and
observations are raising concerns.
Suspicions are now great enough
that leaders in the gardening world are urging caution. The University of Minnesota Extension
Service [6] warns that although squill is beautiful, it may be harmful.
“Because of its rapid spread and condition tolerance,” they write, “this
non-native species has the potential to become an invasive plant.”
At the Minnesota Landscape
Arboretum, managing squill has become a struggle. Gardeners there are frustrated by
the difficulty of establishing other plants where squill has spread. One gardener tried digging out squill and planting wild ginger. Unfortunately, two years later squill had rebounded and the wild ginger was all but gone [9].
The Southeast Wisconsin Invasive Species
Consortium [7] is also wary. Calling squill “a classic case of gardening
gone awry,” the group is alert to the probability of trouble. Citing the
plant’s cold hardiness and unpalatability to deer and other herbivores, they lament the consequences of its popularity: “Sadly, the same traits that make it attractive as a garden
plant . . . are also what make it invasive.”
Understandably, fans of squill find this hard to
accept. A lively debate on the Minnesota
Wildflowers website includes defenders who like the plant
for its beauty, its role as a harbinger of spring, and its sentimental value.
Some assert that the plant is well-behaved and is a food source for
pollinators. Others want proof that squill displaces native plants.
The debate goes to the heart of invasive species
biology. Many plants are introduced because, like squill, they're beautiful. People plant them because they admire them. Most of these species cause no harm to natural areas. Marigolds, petunias and impatiens, for example, usually stay put. A few introduced species, though, have it in them to spread. They might limp along for a while, but
eventually they gain enough of a toe hold to dominate a landscape, whether it’s a
garden, a yard or a natural area.
Increasingly, squill appears to be one of those runaway species. Some gardeners and natural area stewards have spent years trying to remove squill from garden plots, lawns and woodlands, and their frustration is clear. So are their warnings. They advise removing squill when its populations are small and easier to take out. Better yet, they say, avoid the problem by not planting it at all.
Proof of harm to native plants and pollinators will take time to complete. If studies confirm that squill is invasive, it will be
years before that evidence is published. In the meantime, it's clear from a growing number of
observations that squill as an emerging concern. As one gardener at the Arboretum
put it, “It’s beautiful, but it’s very
invasive. When it naturalizes too much, it’s difficult to get rid of.” [8]
Squill is indeed beautiful. This unique plant reminds
us of spring and perhaps of those who planted the bulbs with good
intentions of creating a pleasing landscape. However, if we’re on the cusp of recognizing that squill can be
harmful, now is the time to act. In some places, at least, this pretty
plant has become a big problem.
Squill Removal and Look-Alikes
Removing Squill can be a tough job, especially
if it’s growing in masses. For tips, scroll through the discussion on Minnesota
Wildflowers and see the advice from the University of
Minnesota Extension Service [6].
Squill is easiest to recognize when it’s
flowering, but it has some look-alikes.
Harebell (Campanula rotundifolia) has bell-shaped,
purple flowers on leafy stems reaching 20 inches tall. Typically, it grows in
dry, open places. It blooms in summer.
Blue-eyed Grass (Sisyrinchium species)
has grass-like leaves and dark to pale purple flowers with yellow centers and
yellow anthers. Squill has blue anthers. Three species grow in Minnesota, all
in sun. They bloom from May to July.
Like Squill, Spring Beauty (Claytonia
virginica) is a woodland spring ephemeral. It blooms a little later than
Squill and has pink-veined flowers. Flowering stems bear long, narrow leaves.
 |
From left: Spring Beauty, Harebell, and Mountain Blue-eyed Grass, Sisyrinchium montanum.
Harebell photo copyright 2008 Katy Chayka. Blue-eyed Grass photo copyright 2012 Peter Dzuik. Both photos are from Minnesota Wildflowers. |
References
[1] Flora of North America.
Scilla siberica. Website accessed May 2021.
[2] Consortium of Midwest Herbaria, midwestherbaria.org. Accessed May 2021.
[3] The Friends of the Eloise Butler Wildflower
Garden. Blue
Squill, by G.D. Bebeau. 2015. Accessed May 2021.
[4] Minnesota Biodiversity
Atlas. University of Minnesota Bell Museum. Accessed
May 2021.
[5] EDDMapS. 2021. Early Detection & Distribution Mapping
System. The University of Georgia - Center for Invasive Species and Ecosystem
Health. Available online at http://www.eddmaps.org; last accessed May 7, 2021.
[6] University of Minnesota Extension Service. Squill, by Angela Gupta, Amy Rager and Megan Weber. Reviewed 2021.
[7] Southeastern Wisconsin Invasive Species
Consortium, Inc. (SEWISC). Siberian
Squill. Website accessed May 2021.
[8] Pretty,
but aggressive squill, by Erin Buchholz. Nature Notes. News from the
Minnesota Landscape Arboretum. May 13, 2020.
