Is this plant really dangerous?

Dame’s rocket, Hesperis matronalis. Also called sweet rocket and many other pretty names. Delightful biennial. Brought to America from Europe in about 1600 and much loved by gardeners for its color, its fragrance and for being so easy to grow. Now naturalized in most US states.

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.

Comments

[Jane]

Hang on, I think I introduced that into my woodland garden thinking it was a wild phlox. I'm looking at a big patch of it right now - it looks beautiful. It doesn't behave like an invasive species, more like other woodland plants such as wild geranium or mayapple. It appears to coexist with other plants quite happily.

Given that it's hard in the Midwest to find plants that look this good in the shade and are disliked by deer, I think I will try to file your post under "disregard" and get back to enjoying my pretty flowers.

[Graham Rice]

Just to be clear, Jane, wild phlox has five petals and is a hardy perennial; dames rocket has four petals and is a biennial. And it is beautiful... And the guys at the US Geological Survey seem to give it the OK. You're right, it does seem to coexist with other plants quite happily. So enjoy its color, enjoy its fragrance.

[The Giving Garden]

I would think since it was introduced so long ago (1600's) that it would have noticeably disrupted our native ecosystems by now if it was going to be a big problem. It seeds around in my area of SE PA with our native wood poppy and makes for a lovely combination. People do often confuse it with Phlox from a distance (but it blooms earlier than Phlox). Hopefully, we pay more attention to it, but for now I would focus on weeding out the nasty invasives - canadian thistle, multiflora rose, garlic mustard (this one is edible by the way), japanese stilt grass, and the list goes on.........

[Graham Rice]

You're right, we should concentrate on the real enemies (add, of course, Japanese knotweed to your list) and not get worked up about plants like Dames Rocket which are now as good as native anyway.

[Yvonne @ country gardener]

I'm glad you wrote about this. I really like this plant, which grows wild in a patch on our 10 acre property. I was amazed and enthralled again by its perfume last night. In the 12 years that we have been here, it has not spread from the little patch where it grow under a grove of sumacs.

[Graham Rice]

Thanks Yvonne, interesting that yours has spread so little.

And Diane from Victoria B.C. emails with this great thought: "I encourage it (Dames Rocket) in my garden for its own beauty and scent and also because it is a good nectar provider for swallowtail butterflies. I make sure to have fennel growing nearby as it provides larval food for them."

[tracey]

We had our garden open here , in UK and EVERYONE LOVED Sweet rocket and wanted to buy plant! I use it alot in bouquets,as the scent is amazing! We could say the same for Buttercups here,but hey ,they look stunning too!!

[Graham Rice]

Tracey, try to get hold of some of the double flowered forms. The scent is just as amazing - but the flowers last much longer.

[mary]

I have seen this growing wild around my part of NJ (Somerset County). I have lived here all my life, and it does not seem to be invasive at all, unlike the rosa multiflora which has really taken over...

[Graham Rice]

I think some people just don't like non-native plants, Mary, however attractive they are and however little damage they do.