Book Review: Trees and shrubs - a new edition of the essential reference
Book Review: A brilliant new month-by-month bird book

Purple loosestrife - is it really that bad?

Purple loosestrife (lythrum salicaria) in northern New Jersey. Image ©GardenPhotos.com
It’s purple loosestrife season here in Pennsylvania. Swamps and other wet habitats are vivid in its purple coloring (above, click to enlarge), in some places it looks as if it’s smothered everything. This colourful European native is generally viewed as a destructive menace and many millions of dollars are spent every year in a futile attempt to eradicate it.

In Britain, by the way, where purple loosestrife (Lythrum salicaria) originated, it’s far less common and is a popular plant for bog gardens with over a dozen named varieties, two of which have been awarded the prestigious Award of Garden Merit by the Royal Horticultural Society.

In North America, purple loosestrife is generally regarded as evil but part of its reputation derives from the simple fact that it’s so colorful, it’s so obvious that it’s there. If it had dull brown flowers no one would notice. Ken Thompson, in his important book that I reviewed here recently, put it this way: “Very tall people with red hair, big tattoos and conspicuous facial scars rarely have successful careers as bank robbers, and purple loosestrife has a similar problem: it’s just too conspicuous for its own good.” He goes on to emphasize: “Recognition of loosestrife as a problem was largely based on anecdotal observations, which are likely to be particularly unreliable in the case of a tall species with such bright, obvious flowers. This is a well known problem that standard textbooks warn against: it’s easy to conclude that an otherwise rather dull wetland has been completely taken over if you look at it when loosestrife is in flower…, even if a more careful examination would reveal no such thing.”

My local experience indicates that it does not necessarily spread once the first plant arrives, and that it can also decrease over time. On the lake where we live, I spotted two plants growing together about ten years ago. I checked the whole lake last week and those two plants are still the only ones present in spite of there being many suitable habitats all along the margins. Native swamp loosestrife, Decodon verticillatus, is far far more aggressive.

And in the swamp where I first saw it flowering colorfully it has declined, as it has elsewhere, and a striking feature is that native shrubs including dogwood (Cornus) and arrowwood (Viburnum) are establishing themselves on individual clumps of purple loosestrife.

Monarch butterfly on Purple Loosestrife (Lythrim salicaria). Image © Liz West (EWestPhotos.com)And it’s not as if purple loosestrife is entirely useless for wildlife. It’s been shown that a number of native species are more likely to grow in habitats containing purple loosestrife and in a study of over 250 plots it was found that there were more birds in those with purple loosestrife than in those without. I’ve seen around thirty individual butterflies on one plant and over sixty insect genera use it, including adult monarchs (left, click to enlarge). Birds even nest in it. Another significant study that looked carefully at this issue found that there was no difference in species richness between plots with and those without purple loosestrife.

Ken Thompson says, as he concludes his discussion of the issue: “Persecuting loosestrife is, and always has been, a waste of time.”

I was tempted to pull out those first two plants that appeared at the far end of our lake but I just left them and kept an eye on the situation. Ten years later they’re doing no harm.

Don’t get me wrong. I’m not saying that anything and everything that could be, and is, invasive should just be left alone. What I am saying is that it’s not as simple as “native good, non-native bad”. Especially when you look at how fast US native swamp loosestrife (Decodon verticillatus) can spread and the monocultures that result.


Monarch butterfly on Purple Loosestrife (Lythrim salicaria). Image © Liz West (http://www.EWestPhotos.com). Used here under Creative Commons Attribution 2.0 Generic License.

Comments

Feed You can follow this conversation by subscribing to the comment feed for this post.

T. P. O'Rourke

I told you! Just look, the purple loosestrife is swamping everything. How you can say it's not doing any harm - you are an idiot.

Graham Rice

But that's the point, T. P., research shows that although it looks as if purple loosestrife smothers everything - it doesn't.

Beth

Don't bother the Native-Only crowd with your non-PC facts, Graham, they know what's right for your garden and everyone else's. I myself don't plant purple loosestrife in my garden, simply because my husband's mother has bemoaned its aggressiveness (not invasiveness) in her garden, which is not far from my garden. (And purple loosestrife is actually illegal to sell here in Iowa.) However, I am known to plant some other spreaders and self-seeders that also aren't invasive, that have been condemned by the N-O evangelists and the "invasive" species hysterics. Thanks for letting me know about purple loosestrife's actual tendencies (although it's possible that in my area it is far more prone to spread). It just shows that most gardening knowledge is local. But be careful of the moral outrage that may follow your post.... :-) -Beth

Graham Rice

Thanks for your support, Beth. Much appreciated. One of the things that Ken Thompson points out in his book is that what seems to happen over the decades is that purple loosetrife arrives, sometimes it spreads a lot and may look dominant - but then it may well decline significantly to the extent that no one would think it a pest at all.

OK, this may take a few decades but a few decades is nothing in biological time. That's what happened with Canadian pondweed in Britain: it arrived, it spread quickly, everyone decried it as a menace - now it's uncommon.

The comments to this entry are closed.