Is this plant really dangerous?
May 25, 2010
Banned in Colorado, Connecticut, and Massachusetts as an invasive species.
And here it is in north east Pennsylvania (left, click to enlarge), a delightful image of color and fragrance glimpsed in the distance through the remains of some natural deciduous forest of a bright and blazing afternoon. True, there are quite few plants here – though very few if you think of the forest as a whole. It’s not a very likely candidate as a dangerous invasive, is it. Yet some people want it ripped out.
It doesn’t make a smothering mass of foliage like vinca or Japanese knotweed. In flower, each plant often sends up just one vertical flowering stem. Not much smothering there. (In gardens it tends to be bushier.) True, in a mass they might shade out other plants. But for how many weeks in the year?
With some plants it’s their winter rosettes which are the problem – dandelions, and other similar members of the daisy family, often develop broad, leafy, ground-hugging rosettes which very effectively crowd out neighboring plants. Some of these are important weeds of lawns, crowding out the lawn grasses. But the rosette of dame’s rocket is a relatively puny thing (right, click to enlarge).
And it’s not even a long-lived, ever-increasing perennial. It’s a biennial so plants don’t get bigger and bigger each year – they mostly die in their second summer.
But even the agencies warning us of its terrors don’t seem convinced. The Invasive Species pages of Wisconsin’s Department of Natural Resources, for example, tell us: “The effects of dame's rocket invasion are not known, but it may compete with native species…. Dame's rocket has not been studied extensively. In fact, it is not yet widely recognized as an invasive plant in the Midwest.” So why do they insist that “any plant whose seed may escape to roadsides or woodlots should be eradicated or prevented from going to seed by cutting the flower heads after they bloom”.
And let’s take a look at one interesting piece of research undertaken by the US Geological Survey and presented at their 93rd annual meeting just a couple of years ago – as it happens, in Wisconsin. In summary: They marked out twenty plots in which dames rocket was growing; they measured the cover for every plant species present in each plot; they pulled the dames rocket out of half the plots; then for three years they assessed the flora in both sets of plots.
The result: “Removal did not significantly affect species richness and species diversity”. “In the three years, neither native nor exotic forbs, nor native woody plants, significantly responded to the removal of H. matronalis.”
OK, got that? Pulling out the dames rocket made no difference. You couldn't make it up, could you. So enjoy their attractive flowers (left, click to enlarge).
Here in Pennsylvania, the situation I examined the other day gave a fat clue as to why some non-native plants proliferate. Many of the trees in the area of forest where the dame’s rocket was growing had been felled a few years ago so the tree canopy was very thin and gappy. This is also an area with a very high deer population. The most widespread plant in this patch of forest was a more troublesome non-native plant, garlic mustard (Alliaria petiolata). There were also quite few plants of multiflora rose (Rosa multiflora). Dames rocket, garlic mustard and multiflora rose are all non-natives and all, to varying degrees, deer resistant.
The one striking native perennial I came across was Erigeron philadelphicus: one plant, surrounded by tree trunks – and so protected from the deer.
My point? If the area had been fenced to keep the overabundance of deer out after the trees were felled then the diverse local native flora – which the deer love to eat – would have established itself and the deer-resistant non-native plants would have been much less likely to take hold.
But hey, let’s rip out those nasty non-natives (sorry, alien invasives) anyway.